Download Higher Human Biology Physiology and Health

SCHOLAR Study Guide
SQA CfE Higher Human Biology
Unit 2: Physiology and Health
Authored by:
Eoin McIntyre
Reviewed by:
Sheena Haddow
Previously authored by:
Mike Cheung
Eileen Humphrey
Eoin McIntyre
Jim McIntyre
Heriot-Watt University
Edinburgh EH14 4AS, United Kingdom.
First published 2014 by Heriot-Watt University.
This edition published in 2014 by Heriot-Watt University SCHOLAR.
Copyright © 2014 Heriot-Watt University.
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All rights reserved. No part of this publication may be reproduced, stored in a retrieval system
or transmitted in any form or by any means, without written permission from the publisher.
Heriot-Watt University accepts no responsibility or liability whatsoever with regard to the
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Distributed by Heriot-Watt University.
SCHOLAR Study Guide Unit 2: SQA CfE Higher Human Biology
1. SQA CfE Higher Human Biology
ISBN 978-1-909633-17-9
Printed and bound in Great Britain by Graphic and Printing Services, Heriot-Watt University,
Edinburgh.
Acknowledgements
Thanks are due to the members of Heriot-Watt University's SCHOLAR team who planned and
created these materials, and to the many colleagues who reviewed the content.
We would like to acknowledge the assistance of the education authorities, colleges, teachers
and students who contributed to the SCHOLAR programme and who evaluated these materials.
Grateful acknowledgement is made for permission to use the following material in the
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i
Contents
1 Reproductive organs, gametes and fertilisation
1.1 Gametes . . . . . . . . . . . . . . . . . . . .
1.2 The male reproductive system . . . . . . . .
1.3 The female reproductive system . . . . . . .
1.4 Learning points . . . . . . . . . . . . . . . . .
1.5 Extended response question . . . . . . . . .
1.6 End of topic test . . . . . . . . . . . . . . . .
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1
2
3
7
11
12
12
2 Hormonal control of reproduction
2.1 Introduction . . . . . . . . . . .
2.2 Onset of puberty . . . . . . . .
2.3 Control of sperm production . .
2.4 Control of the menstrual cycle .
2.5 Learning points . . . . . . . . .
2.6 Extended response question .
2.7 End of topic test . . . . . . . .
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15
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27
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3 The biology of controlling fertility
3.1 Introduction . . . . . . . . . . .
3.2 Fertile periods . . . . . . . . .
3.3 Treatments for infertility . . . .
3.4 Contraception . . . . . . . . . .
3.5 Learning points . . . . . . . . .
3.6 Extended response question .
3.7 End of topic test . . . . . . . .
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5 Patterns of inheritance
5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2 Genetic terms and their meanings . . . . . . . . . . . . . . . . . . . . .
5.3 Pattern of inheritance of a pair of alleles - one dominant, one recessive
5.4 Dominant and incompletely dominant alleles . . . . . . . . . . . . . . .
69
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72
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4 Antenatal and postnatal screening and care
4.1 Introduction . . . . . . . . . . . . . . . . .
4.2 Antenatal care . . . . . . . . . . . . . . .
4.3 Antenatal screening . . . . . . . . . . . .
4.4 Postnatal screening . . . . . . . . . . . .
4.5 Learning points . . . . . . . . . . . . . . .
4.6 Extended response question . . . . . . .
4.7 End of topic test . . . . . . . . . . . . . .
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ii
CONTENTS
5.5 Sex-linked inheritance . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.6 Learning points . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.7 End of topic test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
77
82
83
6 Blood vessels
6.1 Why have a cardiovascular system? . . . . . . . . . .
6.2 Blood vessels . . . . . . . . . . . . . . . . . . . . . . .
6.3 Exchange of materials between the blood and the cells
6.4 Learning points . . . . . . . . . . . . . . . . . . . . . .
6.5 Extended response question . . . . . . . . . . . . . .
6.6 End of topic test . . . . . . . . . . . . . . . . . . . . .
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7 Structure and function of the heart
7.1 Introduction . . . . . . . . . . . .
7.2 The structure of the heart . . . .
7.3 The human circulatory system .
7.4 The control of heart rate . . . . .
7.5 The cardiac cycle . . . . . . . . .
7.6 The cardiac conducting system .
7.7 Blood pressure . . . . . . . . . .
7.8 Learning points . . . . . . . . . .
7.9 Extended response question . .
7.10 End of topic test . . . . . . . . .
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103
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8 Cholesterol and cardiovascular disease
8.1 Introduction . . . . . . . . . . . . . . . . .
8.2 Cholesterol . . . . . . . . . . . . . . . . .
8.3 Atherosclerosis and associated diseases
8.4 Learning points . . . . . . . . . . . . . . .
8.5 Extended response question . . . . . . .
8.6 End of topic test . . . . . . . . . . . . . .
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131
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9 Pathology of cardiovascular disease
9.1 Introduction . . . . . . . . . . . . . . . . .
9.2 Regulation of blood glucose levels . . . .
9.3 Blood glucose levels and diabetes . . . .
9.4 Blood glucose levels and vascular disease
9.5 Obesity . . . . . . . . . . . . . . . . . . .
9.6 Learning points . . . . . . . . . . . . . . .
9.7 Extended response question . . . . . . .
9.8 End of topic test . . . . . . . . . . . . . .
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149
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10 End of unit test
171
Glossary
179
Answers to questions and activities
1
Reproductive organs, gametes and fertilisation
2
Hormonal control of reproduction . . . . . . . .
3
The biology of controlling fertility . . . . . . . .
4
Antenatal and postnatal screening and care .
190
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194
200
206
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© H ERIOT-WATT U NIVERSITY
CONTENTS
5
6
7
8
9
10
Patterns of inheritance . . . . . . . . .
Blood vessels . . . . . . . . . . . . . .
Structure and function of the heart . . .
Cholesterol and cardiovascular disease
Pathology of cardiovascular disease . .
End of unit test . . . . . . . . . . . . . .
© H ERIOT-WATT U NIVERSITY
iii
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211
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1
Topic 1
Reproductive organs, gametes and
fertilisation
Contents
1.1 Gametes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2 The male reproductive system . . . . . . . . . . . . . . . . . . . . . . . . . . .
2
3
1.3 The female reproductive system . . . . . . . . . . . . . . . . . . . . . . . . . .
7
1.3.1 After ovulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.4 Learning points . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10
11
1.5 Extended response question . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.6 End of topic test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
12
12
Learning Objectives
By the end of this topic, you should be able to:
• explain the origin of gametes;
• describe the role of the seminiferous tubules and the interstitial cells;
• describe the role of the prostate gland and seminal vesicles;
• describe the development of the ova in the ovary;
• describe the functions of the follicle in the ovary;
• describe the process of fertilisation.
2
TOPIC 1. REPRODUCTIVE ORGANS, GAMETES AND FERTILISATION
As all organisms die, so all organisms must reproduce for their species to survive.
Success in evolutionary terms is measured by the numbers of offspring which survive to
breed.
In animals, the cells which will become the gametes are segregated from the other body
cells very early in the development of the embryo, but they only become mature when
the body of the individual reaches a stage in growth at which it can support the offspring.
We find that in nearly all human societies there is a gap between the age of attainment
of sexual maturity and the recognition of that individual as an adult, which we know as
adolescence.
1.1
Gametes
Learning Objective
By the end of this section, you should be able to:
• describe the production of germline cells;
• explain how an individual's sex is determined;
• compare male and female sex cells.
In Unit 1 of this course, the two basic types of cell in the body are identified as the
germline cells which give rise to the gametes, and the somatic cells which produce all
the other cells of the body. A key difference between these types of cell is that while
both can undergo mitosis, only germline cells can divide by meiosis.
Somatic cells
Germline cells
In all mammals, the development of the germline cells in the testes or ovaries is
determined by the presence or absence of a single gene which is carried on the Y
chromosome. You will have learned how the sex of an individual is inherited in your
previous course.
The male gametes (sperm) are produced in the testes; they are very small and thin (the
head being 5 × 3 microns and the tail 41 microns). They are active swimmers with a
compact nucleus, many mitochondria, but no energy stores. The female gametes (ova)
are produced in the ovary. In contrast to sperm, they are very large, round cells (200
microns across) with a yolk which are not capable of moving by themselves.
© H ERIOT-WATT U NIVERSITY
TOPIC 1. REPRODUCTIVE ORGANS, GAMETES AND FERTILISATION
3
Gametes: Questions
Q1: Why is it necessary for germline cells to divide by meiosis?
..........................................
Q2: Suggest an explanation for the differences in size and structure of the sperm and
ovum.
..........................................
1.2
The male reproductive system
Learning Objective
By the end of this section, you should be able to:
• describe that sperm are produced in the testis;
• describe where testosterone is produced;
• explain the function of the secretions of the prostate gland and the seminal
vesicles.
The following diagram shows the male reproductive system.
© H ERIOT-WATT U NIVERSITY
4
TOPIC 1. REPRODUCTIVE ORGANS, GAMETES AND FERTILISATION
The male reproductive system
The testes are the male reproductive organs. They have two functions:
1. the production of sperm in the seminiferous tubules;
2. the production of testosterone in the interstitial cells, which are found between
the seminiferous tubules.
Each testis contains several intensely folded seminiferous tubules within which germ
cells divide, first by mitosis and then by meiosis, to produce immature sperm. These
then undergo differentiation, developing the tail and the thickened mid-piece that
contains many mitochondria. The DNA of the (now haploid) nucleus becomes highly
condensed and inactive. In this condition, the sperm are further matured under the
influence of testosterone, e.g. by the removal of excess cytoplasm, before being
transported to the epididymis where they finally become motile and capable of
fertilisation.
© H ERIOT-WATT U NIVERSITY
TOPIC 1. REPRODUCTIVE ORGANS, GAMETES AND FERTILISATION
Between the seminiferous tubules lie the interstitial cells (also known as Leydig cells
after the German anatomist who first discovered them in 1850). These cells release
testosterone when they are stimulated by luteinising hormone (LH) released by the
pituitary gland. As a result of the action of LH in the male, it is also referred to as
Interstitial Cell Stimulating Hormone (ICSH).
Also associated with the male reproductive system are two other organs: the prostate
gland and the seminal vesicles. These produce fluids which are collectively known as
seminal fluid, the role of which is to:
• maintain the mobility of the sperm by providing a liquid medium at optimum
viscosity for the sperm to swim in;
• supply nutrients (e.g. fructose) for the sperm, which use a lot of energy but carry
no energy reserves.
The male reproductive system: Questions
Q3: Complete the diagram using the labels provided.
..........................................
© H ERIOT-WATT U NIVERSITY
5
6
TOPIC 1. REPRODUCTIVE ORGANS, GAMETES AND FERTILISATION
Q4:
Complete the diagram using the labels provided.
..........................................
Q5:
Where are sperm produced?
..........................................
Q6:
Where is testosterone produced?
..........................................
Q7: Describe the functions of seminal fluid and suggest why each function is
important.
..........................................
..........................................
© H ERIOT-WATT U NIVERSITY
TOPIC 1. REPRODUCTIVE ORGANS, GAMETES AND FERTILISATION
1.3
7
The female reproductive system
Learning Objective
At the end of this section, you should be able to:
• describe the development of ova in the ovaries;
• state the roles of the follicle and the corpus luteum;
• describe the possible fates of ova released from the ovary;
• describe the early development of the zygote after fertilisation.
The following diagram shows the female reproductive system.
The female reproductive system
The ovaries are the female reproductive organs. They have two functions:
1. the production of ova. Each ovary contains on average 300,000 immature follicles
at birth. This number is reduced at an accelerating rate towards the menopause
when all ovulation ceases (often between the ages of 45-55). Therefore, in
each ovary there are many follicles containing immature ova in various stages
of development. The follicle both protects the developing ovum and secretes
hormones
Once a female begins ovulating (at puberty), an ovum matures inside a follicle
approximately every 28 days. The ovum is released from the ovary when the
follicle ruptures at the surface of the ovary (ovulation).
After ovulation, the ovum passes into the oviduct where it may be fertilised.
The follicle then develops into a corpus luteum (literally 'yellow body') which
also secretes hormones. If fertilisation does not take place, the corpus luteum
degenerates after about 14 days and ceases to release progesterone; if an egg is
fertilised, the corpus luteum enlarges and continues to secrete progesterone until
that function is taken over by the placenta.
2. the production of the female hormones oestrogen (from the follicle) and
progesterone (from the corpus luteum). Further discussion of the production,
functions and feedback control of these hormones is found in the next section.
© H ERIOT-WATT U NIVERSITY
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TOPIC 1. REPRODUCTIVE ORGANS, GAMETES AND FERTILISATION
Maturation of an ovum: Steps
5 min
The following shows the stages in the maturation of an ovum inside its follicle, ovulation,
and the development of the follicle into a corpus luteum. If the ovum is not fertilised, the
corpus luteum degenerates.
1. In response to increasing levels of FSH, a dormant 'primordial' follicle begins to
grow.
2. The follicles matures.
3. At the point of maturation it is known as a Graafian follicle.
4. The Graafian follicle bursts, releasing the ovum from the ovary. This is ovulation.
5. The Graafian follicle develops under the influence of LH into the corpus luteum.
6. If the ovum is not fertilised, the corpus lutem degenerates.
7. In a few days, another follicle will begin to mature and the cycle will start again.
..........................................
© H ERIOT-WATT U NIVERSITY
TOPIC 1. REPRODUCTIVE ORGANS, GAMETES AND FERTILISATION
The female reproductive system: Questions
Q8: Complete the diagram using the labels provided.
..........................................
Q9:
Match the developments at each of the steps labelled on the diagram to the following
descriptions:
•
•
•
•
•
•
The Graafian follicle bursts, releasing the ovum from the ovary. This is ovulation.
If the ovum is not fertilised, the corpus lutem degenerates.
The follicles matures.
The Graafian follicle develops under the influence of LH into the corpus luteum.
At the point of maturation it is known as a Graafian follicle.
In response to increasing levels of FSH, a dormant 'primordial' follicle begins to
grow.
..........................................
© H ERIOT-WATT U NIVERSITY
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TOPIC 1. REPRODUCTIVE ORGANS, GAMETES AND FERTILISATION
1.3.1
After ovulation
Once in the oviduct, the ovum is carried down towards the uterus. On this journey it
may fuse with a sperm (fertilisation) to form a zygote. This cell divides repeatedly to
form the ball of cells known as a blastocyst which will eventually implant itself in the
endometrium of the uterus.
If the egg is not fertilised by the time it reaches the uterus, it ceases to be receptive to
sperm and passes out of the body.
During the fertilisation process, only the mitochondria of the sperm do not pass into the
ovum; it is the fusion of the sperm nucleus with the ovum nucleus that is technically the
moment of fertilisation. Thus, while the inherited information in the nucleus of the zygote
is an equal mixture from the male and female parents, the structures in the cytoplasm
are all derived from the female parent. In particular, the mitochondria, with their own
DNA, come only from the mother.
Mitochondrial DNA is always passed down through the female line and is not subject
to any of the gene mixing processes of meiosis. Mutations to its genes take place at
a slow rate (approximately one every 3,500 years). These have allowed the analysis
of the evolutionary relationships between different groups of humans. In the same way,
and for the same reasons, the DNA of the Y-chromosome has been used to study such
relationships based on inheritance through the male parent.
After ovulation: Questions
Q10: Suggest an explanation for the fact that the dry mass of the blastocyst when it
implants is less than the dry mass of the zygote.
..........................................
Q11: Explain why meiosis does not cause exchange of alleles in:
a) mitochondrial DNA;
b) Y-chromosome DNA.
..........................................
© H ERIOT-WATT U NIVERSITY
TOPIC 1. REPRODUCTIVE ORGANS, GAMETES AND FERTILISATION
1.4
Learning points
Summary
• Sex cells called gametes.
• Gametes are produced from germline cells.
• Germline cells first divide by mitosis.
• Gametes are later produced by meiosis in the testis or ovary.
• Sperm are produced in the seminiferous tubules of the testis.
• Testosterone is released from the interstitial cells of the testis.
• The prostate gland and the seminal vesicles secrete fluids collectively called
seminal fluid.
• Seminal fluids maintains the mobility and viability of the sperm.
• The ovaries contain many immature ova in various stages of development.
• The ova are contained within follicles.
• The follicles protect the ovum and secrete hormones.
• The release of an ovum from the ovary is called ovulation.
• From puberty to the menopause, ovulation takes place every 28 days on
average.
• At ovulation, the ovum is released into the oviduct.
• After ovulation, the follicle develops into the corpus luteum.
• The corpus luteum secretes hormones.
• After ovulation, the ovum travels down the oviduct where it may be fertilised
to form a zygote.
• Fertilisation takes place when a single sperm fuses with an ovum and its
nuclear material joins with that of the ovum.
• The zygote undergoes a series of divisions as it passes down the oviduct
into the uterus as a blastocyst.
© H ERIOT-WATT U NIVERSITY
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TOPIC 1. REPRODUCTIVE ORGANS, GAMETES AND FERTILISATION
1.5
Extended response question
The activity which follows presents an extended response question similar to the style
that you will encounter in the examination.
You should have a good understanding of the development of ova before attempting the
question.
You should give your completed answer to your teacher or tutor for marking, or try to
mark it yourself using the suggested marking scheme.
Extended response question: Development of ova
Describe:
15 min
A) the development of ova in the ovary; (6 marks)
B) and their possible fates after ovulation. (2 marks)
..........................................
1.6
End of topic test
End of Topic 1 test
Q12: Complete the paragraph using the words from the list. (12 marks)
cells which are found in the testes and ovaries.
Gametes are produced from
and then by meiosis. Sperm are produced in the
These cells divide first by
tubules of the testes and ova within the
in the ovaries. Seminal
gland and the
vesicles which
fluid contains secretions from the
of the sperm. Hormones are released from the
maintain the motility and
cells, the follicles and the corpus
.
takes place
approximately every 28 days, releasing an ovum into the oviduct. Fertilisation takes
, forming a zygote which then undergoes a series of divisions to
place in the
.
form the
Word list: blastocyst, follicles, germline, interstitial, luteum, mitosis, oviduct, ovulation,
prostrate, seminal, seminiferous, viability.
..........................................
Q13: Complete the table showing the differences between germline and somatic cells
in relation to how they divide and the types of cell into which they can develop. (4 marks)
Cell type
Difference
Germline
Somatic
Division
Develop into
Word list: all other cells of the body, gametes, mitosis, mitosis and meiosis.
© H ERIOT-WATT U NIVERSITY
TOPIC 1. REPRODUCTIVE ORGANS, GAMETES AND FERTILISATION
..........................................
The following diagram shows a section through part of a testis.
Q14: Which cells on the diagram have been produced by mitosis? (1 mark)
..........................................
Q15: Name the cells labelled A. (1 mark)
..........................................
Q16: State the function of the cells labelled A. (1 mark)
..........................................
The following diagram shows an ovary in cross-section, labelled to identify significant
stages in the development of an ovum.
Q17: State what is taking place at stages X, Y and Z. (3 marks)
..........................................
Q18: Name structure W. (1 mark)
..........................................
© H ERIOT-WATT U NIVERSITY
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14
TOPIC 1. REPRODUCTIVE ORGANS, GAMETES AND FERTILISATION
Q19: State the function of structure W. (1 mark)
..........................................
Q20: State two possible fates of the ovum once it is in the oviduct. (2 marks)
..........................................
Q21: How is a blastocyst formed? (1 mark)
..........................................
Q22: What does a blastocyst do? (1 mark)
..........................................
In fertility clinics, men provide samples of seminal fluid which are tested in various ways.
The following table shows the analysis of such samples taken from five men.
Man
1
2
3
4
5
Number of sperm in sample
(millions/cm3 )
40
19
25
45
90
Active sperm (%)
65
60
75
10
70
Abnormal sperm (%)
30
20
90
30
10
A man is fertile if his seminal fluid contains at least 20 million sperm/cm 3 , at least 60%
of the sperm are active, and at least 60% of the sperm cells are normal.
Q23: Express the lowest number of sperm in a sample as a percentage of the largest.
(1 mark)
..........................................
Q24: Express the Active Sperm (%) of man 1 and man 4 as a simple whole number
ratio. (1 mark)
..........................................
Q25: How would the clinic ensure that the results for each man were reliable? (1 mark)
..........................................
Q26: Which of the men produced samples containing sufficient sperm to be classed as
fertile? (1 mark)
..........................................
Q27: Which men would be classed as infertile on the basis of the whole analysis? (1
mark)
..........................................
..........................................
© H ERIOT-WATT U NIVERSITY
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Topic 2
Hormonal control of reproduction
Contents
2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2 Onset of puberty . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
16
16
2.3 Control of sperm production . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.4 Control of the menstrual cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . .
18
19
2.4.1 The menstrual cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
19
2.4.2 The follicular phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.4.3 The luteal phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
21
23
2.5 Learning points . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.6 Extended response question . . . . . . . . . . . . . . . . . . . . . . . . . . . .
25
27
2.7 End of topic test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
27
Learning Objectives
By the end of this topic, you should be able to:
• explain how hormones cause the onset of puberty;
• describe the influence of the pituitary hormones (follicle stimulating hormone and
luteinising hormone/interstitial cell stimulating hormone) on the testes and the
ovaries;
• describe the influence of testosterone on the testes and the negative feedback
control of its production;
• describe the influence of the ovarian hormones (oestrogen and progesterone) on
the uterus and the pituitary gland;
• explain the changes which take place during the menstrual cycle and the control
of these changes through the interaction of various hormones.
16
TOPIC 2. HORMONAL CONTROL OF REPRODUCTION
2.1
Introduction
Communication between cells in the body is carried out by the production of particular
chemicals which act as signals. This communication may be between cells that are:
• touching, as in the developing embryo;
• separated by a very small gap, e.g. the synaptic cleft between nerve cells;
• widely separated, e.g. the adrenal glands on the kidneys and the muscles
controlling the opening of the pupil in the eye.
The control of reproduction belongs to this third category, in which the signal chemicals
are hormones. These chemicals are released into the blood and detected by their
target cells through receptors on, or inside, the cell membrane. Hormones control both
the initiation of the production of gametes at puberty and their continued production
throughout the fertile life of the individual.
The production of sperm and eggs are both regulated by a system of negative feedback
control, in which the production of a hormone by one organ, or gland, is linked with
the production of a second hormone by an another organ. For example, the production
of testosterone by the interstitial cells of the testis is stimulated by LH/ICSH released
from the pituitary. As the concentration of testosterone in the blood rises, the pituitary
reduces its production of ICSH, so the level of testosterone will be reduced in response.
By this process, the concentration of testosterone in the blood will be maintained within
a narrow range.
The same pituitary hormones are involved in both the male and the female,
and, in general terms, their effects are similar as well.
Thus, FSH
(follicle stimulating hormone) stimulates the germline cells to produce gametes and
LH/ICSH stimulates the release of hormones from the ovary and testis. The roles of
these hormones are examined in more detail in the following sections.
2.2
Onset of puberty
Learning Objective
By the end of this section, you should be able to:
• state that the hypothalamus triggers the onset of puberty by passing a releaser
hormone to the pituitary gland;
• describe how the pituitary responds to this releaser hormone by, in turn,
releasing follicle stimulating hormone (FSH) and luteinising hormone/interstitial
cell stimulating hormone (FSH/ICSH);
• describe how FSH and LH control the production of gametes throughout the
reproductive life of the individual;
• state that FSH and LH production form part of a negative feedback cycle.
© H ERIOT-WATT U NIVERSITY
TOPIC 2. HORMONAL CONTROL OF REPRODUCTION
17
Puberty is the sequence of physical changes by which the human body develops from
that of a child into that of an adult capable of reproduction. The process is initiated by
a releaser hormone that is secreted by the hypothalamus, which is a part of the base of
the brain. It is located centrally and adjacent to the pituitary gland. Quite what triggers
this secretion is still uncertain.
The releaser hormone stimulates the pituitary gland to secrete FSH and LH. FSH
initiates gamete production by means of the development of follicles in the ovaries, and
sperm production in the seminiferous tubules. FSH and LH act together to stimulate
the production of oestrogen from the ovarian follicles, although LH prompts the corpus
luteum to release progesterone after ovulation. LH in the male stimulates the release of
testosterone from the interstitial cells of the testis.
It is the oestrogen and testosterone which, when detected by the receptors on the cells
of their target organs, trigger the physical and psychological changes that are typical of
puberty. These changes principally involve the growth and transformation of bones,
muscle, skin, hair, breasts, sexual organs and brain, leading to the development of the
secondary sexual characteristics that are typical of men and women.
Once gamete production has become established, the various controlling hormones
interact in negative feedback cycles, ensuring constant sperm production in the male
and regular ovulation in the female. Details of these interactions are given in the
following sections.
Onset of puberty: Question
Q1: Complete the sentences by matching the parts on the left with the parts on the
right.
Hypothalamus
LH causes the release of progesterone.
Pituitary gland
releases progesterone.
FSH acts on the ovaries
produces releaser hormone.
FSH acts on the testes
secretes FSH and LH.
Follicles release oestrogen
follicles develop.
After ovulation
causing release of testosterone.
The corpus luteum
seminiferous tubules start to produce
sperm.
LH acts on the interstitial cells
under the influence of FSH and LH.
..........................................
© H ERIOT-WATT U NIVERSITY
10 min
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TOPIC 2. HORMONAL CONTROL OF REPRODUCTION
2.3
Control of sperm production
Learning Objective
By the end of this section, you should be able to:
• state that FSH stimulates the cells lining the seminiferous tubules to produce
sperm;
• state that LH stimulates the interstitial cells to release testosterone;
• state that testosterone stimulates the production of sperm by the seminiferous
tubules;
• state that testosterone activates the prostate gland and the seminal vesicles;
• describe how high levels of testosterone levels inhibit the production and release
of FSH and LH;
• explain why this is an example of negative feedback control.
Sperm production is controlled by the pituitary gland directly, as a result of the secretion
of FSH, and indirectly, through the secretion of LH/ICSH. FSH stimulates the cells lining
the seminiferous tubules to divide to produce sperm, and LH causes the interstitial cells
between the tubules to release testosterone, which then plays a major role in promoting
sperm production.
High levels of testosterone inhibit the production of both FSH and LH by the pituitary
gland. Decreased concentrations of LH lead to a decrease in the secretion of
testosterone and, therefore, reduced concentrations reaching the pituitary which
increases its production of FSH and LH as a result. The production of testosterone
is an example of negative feedback control.
Regulation of male hormones
As well as promoting the production of sperm, testosterone also activates the prostate
gland and the seminal vesicles which produce their secretions that contribute to seminal
fluid.
© H ERIOT-WATT U NIVERSITY
TOPIC 2. HORMONAL CONTROL OF REPRODUCTION
2.4
19
Control of the menstrual cycle
The three sections here describe the menstrual cycle, generally, followed by detailed
descriptions of the early follicular phase and the later luteal phase.
2.4.1
The menstrual cycle
Learning Objective
By the end of this section, you should be able to:
• state that the first day of menstruation marks the start of the menstrual cycle;
• state that a menstrual cycle lasts 28 days on average;
• describe the part of the menstrual cycle up to the point of ovulation, which is
known as the follicular phase;
• describe the part of the menstrual cycle after ovulation, which is known as the
luteal phase.
All female mammals have fundamentally the same reproductive system that includes the
pituitary gland, which secretes FSH and LH, and ovaries, which release oestrogen and
progesterone. There are, of course, significant variations in the manner in which they
function, the most basic being timing and frequency of ovulation. Most mammals show
an oestrous cycle, ovulating and being sexually receptive only at one time of year which
is known as the mating season. Such cycles are timed to ensure that the young are
produced when they will have maximum chance of survival, often many months after
mating. These species are only sexually active around the time of ovulation and the
endometrium is reabsorbed if fertilisation does not take place.
Humans (and some other higher primates) show a menstrual cycle in which ovulation
takes place at regular intervals; the female may be sexually active at any time during the
cycle and the endometrium is shed (menstruation) if fertilisation does not take place.
The menstrual cycle in humans lasts approximately 28 days with the first day of
menstruation being counted as day 1. Menstruation usually lasts about four days (but
anything from 2-7 days is considered normal) and ovulation typically occurs at day 14.
The first part of the cycle, from menstruation to ovulation, is known as the follicular
phase; the second part, from ovulation to the start of menstruation, is the luteal phase.
In the following diagram, the changes taking place during the menstrual cycle are
summarised and shown together so that the inter-relationship between the various
processes can be more easily appreciated. It is important to remember that this picture
is artificial to an extent, in that each cycle does not take place in isolation; therefore,
events at the start of the cycle are strongly influenced by what was happening at the
end of the previous cycle.
© H ERIOT-WATT U NIVERSITY
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TOPIC 2. HORMONAL CONTROL OF REPRODUCTION
Summary of the menstrual cycle
The menstrual cycle: Questions
Q2: Suggest some environmental conditions which would maximise the chance of a
young mammal, such as a roe deer, surviving its first year.
..........................................
Q3:
Suggest why humans do not have a specific breeding season.
..........................................
© H ERIOT-WATT U NIVERSITY
TOPIC 2. HORMONAL CONTROL OF REPRODUCTION
2.4.2
The follicular phase
21
Learning Objective
By the end of this section, you should be able to:
• describe the release of relatively high levels of FSH by the pituitary in the first
few days of the menstrual cycle;
• state that FSH stimulates the development of follicles in the ovary;
• state that FSH stimulates the release of oestrogen by the follicle;
• explain that oestrogen stimulates the proliferation of the endometrium in
preparation for implantation;
• state that the high oestrogen levels around the time of ovulation cause the
production of cervical mucus, which is more watery so more easily penetrated
by sperm;
• explain that the high oestrogen levels of the late follicular phase causes the
pituitary to release a surge of LH into the blood;
• state that the high level of LH around day 14 triggers ovulation.
In the first few days of the menstrual cycle, the influence of the previous cycle is clearly
seen. Oestrogen and progesterone levels are low and, as a result of negative feedback
control, the pituitary is a releasing a high level of FSH. This prompts the development of
a follicle in the ovary; as it grows, it releases increasing quantities of oestrogen, which
depress the release of FSH by the pituitary.
Oestrogen has other target organs. The endometrium of the uterus proliferates,
becoming much thicker and vascularised with a dense system of blood vessels. These
changes adapt it to supporting the blastocyst if fertilisation occurs. In addition, the cervix
alters the viscosity of its mucus lining, making it more watery and so easier for sperm to
swim through.
As the concentration of oestrogen in the blood rises, it eventually reaches a critical level
at which the pituitary responds by releasing a surge of LH. This sudden increase in LH
concentration causes the follicle, which has moved to the surface of the ovary, to rupture
and release the ovum into the oviduct. This marks the end of the follicular phase of the
menstrual cycle.
© H ERIOT-WATT U NIVERSITY
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TOPIC 2. HORMONAL CONTROL OF REPRODUCTION
The follicular phase: Questions
20 min
Q4: Complete the sentences by matching the phrases on the left with the phrases on
the right.
Low oestrogen and progesterone levels
increasing oestrogen levels suppress
FSH release.
High level of FSH causes
under the influence of oestrogen.
Developing follicle releases oestrogen
causing proliferation.
Oestrogen acts on the endometrium
development of follicle.
Follicles release oestrogen
pituitary releases high level of FSH.
..........................................
Q5:
Place the following into the correct order of development:
• pituitary releases surge of LH;
• oestrogen reaches critical level;
• oestrogen level rises;
• follicle releases egg into oviduct;
• LH acts on mature follicle.
..........................................
© H ERIOT-WATT U NIVERSITY
TOPIC 2. HORMONAL CONTROL OF REPRODUCTION
2.4.3
The luteal phase
23
Learning Objective
By the end of this section, you should be able to:
• state that after ovulation, the high level of LH causes the follicle to develop into
the corpus luteum;
• state that the corpus luteum secretes progesterone and oestrogen;
• state that progesterone causes further development and vascularisation of the
endometrium;
• explain how this provides an optimum environment for the implantation and
growth of the blastocyst;
• state that during this phase, the secretion of oestrogen and progesterone rise
to a maximum and then decline;
• state that FSH and LH production form part of negative feedback cycle with
oestrogen and progesterone;
• describe how the high levels of oestrogen and progesterone inhibit the pituitary
from secreting FSH and LH;
• state that these low levels of FSH and LH suppress the development of further
follicles;
• explain that the low level of LH causes the corpus luteum to degenerate and
progesterone secretion to fall to a minimum;
• state that the falling level of progesterone at the end of the cycle triggers the
start of menstruation;
• state that the low level of oestrogen at the end of the cycle causes the pituitary
to increase secretion of FSH;
• explain that if fertilisation occurs, a hormone from the implanted embryo causes
the corpus luteum to continue producing progesterone for another eight weeks
until this function is taken over by the placenta.
After ovulation, under the influence of the high level of LH, the follicle develops into
the corpus luteum, which then grows and becomes a dense yellow structure up to 5cm
in diameter. As it develops, it releases increasing quantities of both progesterone and
oestrogen, reaching peak production about halfway through the luteal phase.
The progesterone causes the continued development of the endometrium in anticipation
of the implantation of a blastocyst, causing the pituitary to gradually release less LH.
The oestrogen suppresses the production of FSH by the pituitary so preventing the
premature development of further follicles.
In the absence of high levels of LH, in the final days of the cycle the corpus luteum begins
to degenerate and progesterone production falls. As a consequence, the endometrium
is no longer maintained and sloughs off to be lost as the menstrual flow. In the same
© H ERIOT-WATT U NIVERSITY
24
TOPIC 2. HORMONAL CONTROL OF REPRODUCTION
way, the low level of oestrogen stimulates the pituitary to increase its secretion of FSH,
thus starting the cycle again.
If the ovum is fertilised, it develops into the blastocyst as it is carried down the oviduct.
About nine days after fertilisation, the blastocyst implants in the endometrium and
begins to secrete a hormone which causes the corpus luteum to continue to release
progesterone until this function is assumed by the placenta some two months later.
The luteal phase: Questions
20 min
Q6: Use the statements to complete the diagram showing the events in the order in
which they occur after ovulation:
• progesterone suppresses release of LH by pituitary;
• follicle becomes corpus luteum;
• corpus luteum releases progesterone;
• lack of FSH means no follicles develop;
• progesterone stimulates continued development of endometrium;
• oestrogen suppresses release of FSH.
..........................................
Q7:
Select the correct organ response to each of the hormone changes described.
Hormone changes
Organ responses
Low level of LH secreted by pituitary:
Progesterone production falls:
Low level of oestrogen in blood:
Organ responses: endometrium breaks down; pituitary increases secretion of FSH;
corpus luteum degenerates.
..........................................
© H ERIOT-WATT U NIVERSITY
TOPIC 2. HORMONAL CONTROL OF REPRODUCTION
2.5
Learning points
Summary
Onset of puberty
• The hypothalamus triggers the onset of puberty by passing a releaser
hormone to the pituitary gland.
• The pituitary responds to this releaser hormone by, in turn, releasing
follicle stimulating hormone (FSH) and luteinising hormone / interstitial cell
stimulating hormone (LH/ICSH).
• FSH and LH control the production of gametes throughout the reproductive
life of the individual.
• FSH and LH production form part of a negative feedback cycle.
Control of sperm production
• FSH stimulates the cells lining the seminiferous tubules to produce sperm.
• LH stimulates the interstitial cells to release testosterone.
• Testosterone stimulates the production of sperm by the seminiferous
tubules.
• Testosterone activates the prostate gland and the seminal vesicles.
• High levels of testosterone inhibit the production and release of FSH and
LH.
• This is an example of negative feedback control.
Control of the menstrual cycle
• The first day of menstruation marks the start of the menstrual cycle.
• Aa menstrual cycle lasts 28 days on average.
• The part of the menstrual cycle up to the point of ovulation is called the
follicular phase.
• The part of the menstrual cycle after ovulation is called the luteal phase.
Follicular phase
• In the first few days of the cycle, the pituitary releases relatively high levels
of FSH.
• FSH stimulates the development of follicles in the ovary.
• FSH stimulates the release of oestrogen by the follicle.
• Oestrogen stimulates the proliferation of the endomentrium in preparation
for implantation.
© H ERIOT-WATT U NIVERSITY
25
26
TOPIC 2. HORMONAL CONTROL OF REPRODUCTION
Summary Continued
• High oestrogen levels around the time of ovulation cause the production of
cervical mucus, which is more watery and more easily penetrated by sperm.
• The high oestrogen levels of the late follicular phase cause the pituitary to
release a surge of LH into the blood.
• The high level of LH around day 14 triggers ovulation.
Luteal phase
• After ovulation, the high level of LH causes the follicle to develop into the
corpus luteum.
• The corpus luteum secretes progesterone and oestrogen.
• Progesterone causes further development and vascularisation of the
endometrium.
• This provides an optimum environment for the implantation and growth of
the blastocyst.
• During this phase, the secretion of both oestrogen and progesterone rise to
a maximum and then decline.
• FSH and LH production form part of negative feedback cycle with oestrogen
and progesterone.
• The high levels of oestrogen and progesterone inhibit the pituitary from
secreting FSH and LH.
• These low level of FSH suppresses the development of further follicles.
• The low level of LH causes the corpus luteum to degenerate and
progesterone secretion to fall to a minimum.
• The falling level of progesterone at the end of the cycle triggers the start of
menstruation.
• The low level of oestrogen at the end of the cycle causes the pituitary to
increase secretion of FSH.
• If fertilisation occurs, a hormone from the implanted embryo causes the
corpus luteum to continue producing progesterone for another eight weeks
until this function is taken over by the placenta.
© H ERIOT-WATT U NIVERSITY
TOPIC 2. HORMONAL CONTROL OF REPRODUCTION
2.6
27
Extended response question
The activity which follows presents an extended response question similar to the style
that you will encounter in the examination.
You should have a good understanding of negative feedback control before attempting
the question.
You should give your completed answer to your teacher or tutor for marking, or try to
mark it yourself using the suggested marking scheme.
Extended response question: Negative feedback control
Give an account of negative feedback control under the headings:
15 min
A) testosterone production; (3 marks)
B) the luteal phase of the menstrual cycle. (7 marks)
..........................................
2.7
End of topic test
End of Topic 2 test
Q8: Complete the sentences by matching the parts on the left with the parts on the
right. (10 marks)
Hypothalamus:
menstrual cycle up to ovulation.
Pituitary:
FSH and LH form part of it.
FSH and LH:
stimulates the cells lining the seminiferous tubules.
Negative feedback
cycle:
mestrual cycle after ovulation.
FSH:
high levels of testosterone production.
LH/ICSH:
controls the production of gametes.
Testosterone:
responds to the releaser hormone by releasing other
hormones.
Inhibit FSH and LH
release:
a releaser hormone released from here triggers puberty.
Follicular phase:
activates the prostate and seminal vesicles.
Luteal phase:
stimulates the interstitial cell.
..........................................
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Q9:
Select the option which correctly completes each of the sentences. (10 marks)
At the start of the menstrual cycle, the pituitary releases high levels of FSH / LH /
oestrogen / progesterone.
The hormone which stimulates the development of follicles FSH / LH / oestrogen /
progesterone.
Proliferation of the endometrium is stimulated by FSH / LH / oestrogen / progesterone.
High levels stimulate the surge in the release of LH FSH / LH / oestrogen / progesterone.
High levels stimulate the development of the corpus luteum FSH / LH / oestrogen /
progesterone.
Hormone which causes further vascularistation of the endometrium FSH / LH /
oestrogen / progesterone.
High levels oestrogen and testosterone inhibit the production FSH and LH / FSH and
progestrogen / LH and progesterone.
Degeneration of the corpus luteum is caused by low levels of FSH / LH / oestrogen /
progesterone.
Falling levels trigger the start of menstruation FSH / LH / oestrogen / progesterone.
Stimulates the increased production of FSH by the pituitary FSH / LH / oestrogen /
progesterone.
..........................................
Puberty marks the start of the fertile period of an individual's life.
Q10: How is puberty initiated? (1 mark)
..........................................
Q11: The pituitary gland releases FSH and LH during puberty. Complete the following
table to show their first effects in the male and the female. (4 marks)
Hormone
Effect on male
Effect on female
FSH
LH
Phrase list: Follicles release oestrogen; Follicles start to mature in the ovary and release
oestrogen; Interstitial cells start to release testosterone; Seminiferous tubules start to
produce sperm.
..........................................
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TOPIC 2. HORMONAL CONTROL OF REPRODUCTION
The diagram represents gamete production in an ovary.
Q12: Identify structure B. (1 mark)
..........................................
Q13: Identify structure C. (1 mark)
..........................................
Q14: Name the hormone released by structure B. (1 mark)
..........................................
Q15: State its effect on the pituitary gland. (2 marks)
..........................................
Q16: Describe what happens to structure B after C has been released from it. (1 mark)
..........................................
Q17: During the luteal phase of the menstrual cycle, how is the development of
immature follicles suppressed? (2 marks)
..........................................
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TOPIC 2. HORMONAL CONTROL OF REPRODUCTION
The diagram represents a section of part of a testis.
Q18: Name the cells labelled D. (1 mark)
..........................................
Q19: Name the structure which contains cells E. (1 mark)
..........................................
Q20: Which cells in the diagram release a hormone? (1 mark)
..........................................
Q21: What is the hormone that is released called? (1 mark)
..........................................
Q22: Explain how the level of this hormone in the blood is controlled. (3 marks)
..........................................
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31
The graphs below show the changes in concentration of four hormones in a woman’s
blood during one menstrual cycle.
Q23: Complete the table of hormones A-D. Some of the words may be used more than
once. (4 marks)
Hormone
Name
Produced
A
B
C
D
Word list: Corpus luteum, Follicle, Follicle stimulating hormone, Luteinising hormone,
Oestrogen, Pituitary, Progesterone.
..........................................
Q24: On which day would ovulation have taken place? (1 mark)
..........................................
Q25: What causes ovulation? (1 mark)
..........................................
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Q26: Describe and explain the change in concentration of hormone C during the first
12 days of the cycle. (4 marks)
..........................................
Q27: What is the effect of the high level of hormone A between days 18 to 26? (1 mark)
..........................................
Q28: In what way does hormone B contribute to the survival of the blastocyst? (1 mark)
..........................................
Q29: Explain the changes in the concentration of hormone B during the cycle. (3 marks)
..........................................
..........................................
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Topic 3
The biology of controlling fertility
Contents
3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2 Fertile periods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
34
34
3.3 Treatments for infertility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.1 Causes of infertility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
36
36
3.3.2 Artificial insemination (AI) . . . . . . . . . . . . . . . . . . . . . . . . . .
37
3.3.3 Stimulating ovulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.4 In vitro fertilisation (IVF) . . . . . . . . . . . . . . . . . . . . . . . . . . .
38
39
3.3.5 Intracytoplasmic sperm injection (ICSI) . . . . . . . . . . . . . . . . . .
3.4 Contraception . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
40
41
3.4.1 Calendar-based methods . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4.2 Barrier methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
41
42
3.4.3 Intra-uterine devices (IUDs) . . . . . . . . . . . . . . . . . . . . . . . . .
3.4.4 Sterilisation procedures . . . . . . . . . . . . . . . . . . . . . . . . . . .
42
42
3.4.5 Chemical contraceptives . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.5 Learning points . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
43
44
3.6 Extended response question . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.7 End of topic test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
46
46
Learning Objectives
By the end of this topic, you should be able to:
• explain the reasons for infertility treatments and contraception;
• contrast the fertile period of males and females;
• describe and explain the various treatments for infertility;
• explain the basis of the different methods of contraception.
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TOPIC 3. THE BIOLOGY OF CONTROLLING FERTILITY
3.1
Introduction
Human fertility is a complex subject, attitudes to which vary immensely between different
cultures. The perspective of this course is very much that of contemporary 'Western'
society where the standard of living for the majority of the population is high, birth rates
are low, and infertility is seen as a more important problem than fertility. In less affluent
countries where infant mortality is high, advanced medical provision is rarely available,
and children represent a much needed addition to the workforce, priorities and practice
are very different.
It is also important to remember that many of the world's, and humanity's, problems stem
from the fact that the human population is growing at such a pace that the resources of
the Earth are being exploited as never before. Whether it be the deforestation of the
Amazon basin to provide additional farmland, the diversion of major rivers to supply
industry, cities and agriculture with water, or the depletion of the fish stocks of all of the
oceans, the ultimate driver of these changes is the burgeoning number of human beings.
In this topic we will look at the various causes of infertility in men and women, and the
ways in which it can be addressed. These techniques provide the possibility of having
a child of their own to potential parents, one, or both, of whom are unable to produce
sufficient gametes of a quality or quantity necessary for effective fertilisation by normal
sexual intercourse.
Contraception in our society is seen as an artificial means of preventing a pregnancy,
but it should be remembered that natural methods, such as extended suckling to
suppress ovulation, have probably been used for thousands of years.
3.2
Fertile periods
Learning Objective
By the end of this section, you should be able to:
• describe continuous and cyclical fertility;
• explain why men are continuously fertile;
• explain why women are cyclically fertile;
• describe how a woman's cyclical fertility may be used as a contraceptive
measure;
• describe how a woman's cyclical fertility may be used to increase the chances
of conception.
Men
Men are potentially capable of fathering a child from puberty until they die, although
sperm production does decrease with age. Under the influence of the negative feedback
between testosterone and ICSH/LH, males produce sperm constantly and so are
continuously fertile.
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TOPIC 3. THE BIOLOGY OF CONTROLLING FERTILITY
Women
Women only release eggs from puberty until the menopause at age 45-55. However,
as there is only a window of a few days either side of ovulation during which intercourse
can result in pregnancy, there is only a brief interval during each menstrual cycle when
a female is capable of conception. The regular recurrence of this fertile period means
that such fertility is cyclical.
The fact that a woman can only conceive for a few days during each cycle can be used to
reduce the chance that intercourse will lead to pregnancy. This is often loosely referred
to as 'the rhythm method' of contraception; in fact, there are several calendar-based
contraceptive methods, some being more reliable than others. A woman's fertile period
is determined by noting changes in the consistency of the cervical mucus and her body
temperature. Just before ovulation occurs, the cervical mucus becomes thin and watery
while body temperature rises slightly (by about 0.5 ◦ C).
By tracking these changes over the course of several cycles, a woman can predict fairly
accurately when she is going to ovulate. Therefore, a couple who do not wish to have
a child will not have sexual intercourse during the few days before and after the fertile
period. This method is not very reliable if a woman's cycle varies in length from month to
month. However, for many people, this is the contraceptive method of choice for either
personal or religious reasons.
A couple who want the woman to become pregnant would use this method to work out
when the woman was fertile and would have intercourse during this period while the
chance of fertilisation is at its greatest.
Fertile periods: Questions
Q1: Use your knowledge from the previous topic to explain the link between ICSH
levels and sperm production.
..........................................
Q2: Suggest what triggers the changes to cervical mucus and body temperature.
..........................................
Q3: Suggest some reasons why all calendar-based contraceptive methods have a
significant failure rate (at least a 4% chance of pregnancy).
..........................................
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TOPIC 3. THE BIOLOGY OF CONTROLLING FERTILITY
3.3
Treatments for infertility
This introduction gives a summary of the biology of infertility, which will help you
understand why the techniques that follow are necessary. This material is not
examinable.
3.3.1
Causes of infertility
Learning Objective
By the end of this section, you should be able to:
• state some of the effects on fertility with relation to:
– age;
– genetics;
– disease;
– lifestyle.
Fertility problems affect about one in seven couples in the UK. The cause of this inability
to conceive may lie with the male or the female partner, and there are many causes.
Men
The WHO (World Health Organisation) considers a sperm count (strictly 'sperm
concentration') below 15 million per ml to be significantly low (the average being around
60 million per ml). The motility and health of sperm are other factors: do the sperm swim
well enough to make good forward movement and do many sperm have an abnormal
morphology, such as small or large heads, double heads, or a double tail? Other
inherited and environmental factors must also be considered:
• age - although they usually remain fertile from puberty onwards, men's fertility
does gradually decline with age;
• genetics - including Y chromosome microdeletions, where one or more genes
are missing from the Y chromosome, and Klinefelter's syndrome, where nondisjunction in the formation of the egg has resulted in a zygote (and hence a man)
with XXY sex chromosomes;
• disease - mumps is a viral disease that is typically contracted during childhood
although when suffered by adult males it can infect the testes and lead to a
reduction in size with possible sterility. Sexually transmitted infections, such as
chlamydia or gonorrhoea (caused by bacteria), can result in sterility or testicular
cancer if left untreated; where the treatment involves the removal of a testis then
the associated chemo- or radiotherapy may lead to reduced or complete infertility;
• lifestyle - smoking tobacco, stress, obesity, drug and alcohol abuse and
medications (e.g. anabolic steroids) all significantly reduce fertility.
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TOPIC 3. THE BIOLOGY OF CONTROLLING FERTILITY
37
Women
Infertility in women is generally the result of an interaction of inherited and environmental
factors:
• age - a woman's age is a major determining factor, with a peak in fertility during
the early to mid-twenties and an accelerating decline after 35, with cessation of
ovulation usually between 45 and 55 at the menopause;
• genetics - chromosomal abnormalities, such as Turner syndrome (in which one of
the X chromosomes is missing or abnormal in some way), and a whole host of
genetic mutations may cause a woman to be unable to produce viable ova;
• disease - as with men, sexually transmitted infections, e.g. gonorrhoea, can result
in infertility by causing inflammation and scarring of the Fallopian tubes. Chronic
kidney or liver diseases may also cause infertility. Eating disorders, causing a
woman to be either under- or over-weight, both tend to disrupt the menstrual cycle
by affecting oestrogen production;
• lifestyle - smoking tobacco reduces the body's ability to produce oestrogen,
affecting the development of follicles and also causing the earlier onset of the
menopause. Cannabis likewise interferes with fertility by reducing the chances of
implantation of the blastocyst. Repeated courses of chemotherapy pose a high
risk of infertility.
Causes of infertility: Question
Q4: Although this material will not be tested in the SQA exam, to help ensure that
you have understood the biological basis of infertility, create a table in which you give
examples of the factors which can affect the fertility of men and women, under the
headings of age, genetics, disease and lifestyle.
..........................................
3.3.2
Artificial insemination (AI)
Learning Objective
By the end of this section, you should be able to:
• describe what is involved in the process of AI;
• describe the situations in which AI is appropriate.
Artificial insemination, where semen is injected by syringe into the vagina or by
catheter (a thin flexible tube) into the uterus, has been widely used in the livestock
industry for many years. The procedures were adapted for use in humans where
there are problems of a physical or psychological nature which prevent a man from
fertilising an ovum by sexual intercourse. In particular, it is useful where the sperm
count (concentration) is too low.
Sperm samples are usually obtained by masturbation and may be used directly or
stored. In the case of low sperm count, repeated samples are collected over a period of
days, 'washed' and concentrated by separating them from the bulk of the seminal fluid
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TOPIC 3. THE BIOLOGY OF CONTROLLING FERTILITY
(e.g. by centrifugation), and then frozen after being mixed with a chemical to assist the
freezing and thawing processes.
In the case of the husband or partner being able to provide the sperm, the procedure is
known as artificial insemination by the husband (AIH). If another sperm donor is used
(e.g. if the husband is sterile), it is known as artificial insemination by a donor (AID).
Artificial insemination: Question
Q5: Suggest reasons why the use of AI is particularly prevalent in the livestock
industry.
..........................................
3.3.3
Stimulating ovulation
Learning Objective
By the end of this section, you should be able to:
• describe the situations in which ovulation may need to be stimulated;
• describe the action of drugs that may be used to increase FSH levels in the
blood;
• describe the action of synthetic drugs that mimic the action of FSH and LH;
• state that these drugs may cause superovulation, providing several ova for IVF;
• state that superovulation may result in multiple pregnancies.
There are two situations in which it is necessary to stimulate ovulation artificially:
1. if a woman is not ovulating or is ovulating irregularly, then the artificial stimulation of
ovulation will increase the likelihood of sexual intercourse resulting in conception.
Similarly, it makes conception more likely after intrauterine insemination, where
sperm from the partner or donor is introduced artificially directly into the uterus;
2. if IVF is to be carried out, ovulation is artificially stimulated so that ova may be
collected in order to be fertilised in a culture solution in the laboratory.
Ovulation is under the control of the negative feedback loops involving FSH, LH and
oestrogen. One way of stimulating ovulation is to block the oestrogen receptors so that
secretion of FSH by the pituitary remains high, and the development of follicles continues
to be stimulated.
Alternatively, synthetic forms of FSH (and to a lesser extent LH) can be used to raise the
concentrations of these hormones in the blood, stimulating the development of several
follicles during a single menstrual cycle. This is known as superovulation, which is
particularly valuable in making ova available for IVF. However, it is not desirable when
the intention is to achieve fertilisation through sexual intercourse because the outcome
is likely to be a multiple pregnancy, the survival rate in higher order multiple pregnancies
being low.
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TOPIC 3. THE BIOLOGY OF CONTROLLING FERTILITY
39
Stimulating ovulation: Question
Q6: Explain why blocking oestrogen receptors, or artificially increasing FSH levels, will
stimulate ovulation.
..........................................
3.3.4
In vitro fertilisation (IVF)
Learning Objective
By the end of this section, you should be able to:
• describe the situations in which IVF is appropriate;
• describe the process of IVF;
• state the uses of pre-implantation genetic screening.
When is IVF appropriate?
The use of IVF is considered when a woman is infertile due to problems with the fallopian
tubes, e.g. blockage, or where a man produces healthy sperm but in low numbers.
In addition, IVF provides the opportunity to test embryos for a wide range of genetic
diseases or chromosomal abnormalities before implantation.
The process of IVF
After the induction of superovulation, ova are retrieved using an ultrasound-guided
needle which is passed through the vaginal wall to reach the ovary. Typically 20-30 ova
are removed and the procedure lasts about 20mins.
To promote fertilisation, sperm and ova are incubated together in a culture medium for
about 18 hours, with a ratio of about 75,000 sperm per ovum. The sperm are 'washed'
and concentrated by centrifugation before being mixed with the ova.
The zygotes are then incubated in special culture media until they have divided to form
an embryo of 8 or 16 cells, which takes about three days. Some systems continue
incubation for another two days so that the blastocyst stage has been reached. The
embryo is then transferred to the woman's uterus using a catheter, which is introduced
through the vagina and cervix. In most cases, two embryos will be transferred in this
way to increase the chance of implantation, but without risking the problems of a multiple
pregnancy.
Pre-implantation genetic screening
Pre-implantation genetic screening (also known as pre-implantation genetic diagnosis
- PGD) is used when there is a significant possibility of an embryo inheriting a genetic
disorder caused by a single gene or a chromosome abnormality. Following an analysis
of the family histories of the parents, the likelihood of such a disorder being present in
the offspring can be estimated and, if the expectation is unacceptably high, PGD can be
carried out.
In the most common screening procedure, two cells are removed from the 8-cell stage
of the embryo (at which point all cells are still totipotent, i.e. capable of developing into
any cell type), their chromosomes are examined and their DNA analysed. Transfer to the
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TOPIC 3. THE BIOLOGY OF CONTROLLING FERTILITY
uterus is delayed until the fifth day, thereby allowing time for the analysis to be completed
before the final choice of embryos is made.
As well as chromosomal abnormalities, the list of genetic disorders which can be
detected is considerable and includes: cystic fibrosis, sickle-cell disease, Huntingdon's
disease and Duchenne muscular dystrophy.
In vitro fertilisation: Question
10 min
Q7: Re-arrange the following steps into the correct order in which they would occur in
PGD:
• blastocyst at eighth day;
• harvesting of ova;
• woman given FSH injection;
• chromosomes and DNA analysed;
• sperm are washed;
• superovulation;
• two cells removed from embryo;
• embryo transferred into uterus;
• ova incubated with sperm;
• eight cell stage at 3 days.
..........................................
3.3.5
Intracytoplasmic sperm injection (ICSI)
Learning Objective
By the end of this section, you should be able to:
• describe the process of ICSI;
• state when ICSI is appropriate.
ICSI is a form of IVF which differs from the usual procedure in that a single sperm is
injected into an ovum. Several ova are collected as a result of superovulation and mature
sperm are selected from the donor sample by only using those which have adhered to
a microdot that carries some of the chemical which surrounds an ovum.
The procedure is normally carried out in a petri dish of culture medium using a
microscope and various micromanipulation devices to stabilise the ovum and inject the
sperm. A single sperm, which has been immobilised by removal of its tail, is drawn into
a micropipette and introduced into a mature ovum. The ovum is then incubated and
checked for fertilisation (i.e. the fusion of the nuclei of the sperm and the ovum).
ICSI is particularly appropriate where the donor sperm are defective in some way, e.g.
they have low motility (i.e. they are not very active) or the sperm count is low.
© H ERIOT-WATT U NIVERSITY
TOPIC 3. THE BIOLOGY OF CONTROLLING FERTILITY
41
Treatments for infertility: Question
Q8: Use the phrases below to complete the following table which summarises
information about treatments for infertility.
Treatment
Full name
15 min
Appropriate for
AI
IVF
ICSI
PGD
Phrase list: artificial insemination, blocked fallopian tubes, chromosome abnormality,
conception not possible by sexual intercourse, inherited genetic disorder,
intracytoplasmic sperm injection, in vitro fertilisation, low sperm count, low sperm
motility, pre-implantation genetic diagnosis (some may be used more than once)
..........................................
3.4
Contraception
Learning Objective
By the end of this section, you should be able to:
• explain the principle of calendar-based methods;
• describe how barrier methods work;
• explain the how IUDs work;
• describe sterilisation procedures in men and women;
• describe the action of the various types of oral contraceptives.
Contraception is the prevention of fertilisation and may be achieved by natural or artificial
means. Natural methods include extended suckling of infants and calendar-based
methods (e.g. 'the rhythm method'). Artificial methods are many, ranging from simple
barriers, such as the condom, to the complex control of hormonal interactions by the
various contraceptive pills.
3.4.1
Calendar-based methods
The fact that a woman can only conceive for a few days during each cycle can be used to
reduce the chance that intercourse will lead to pregnancy. This is often loosely referred
to as 'the rhythm method' of contraception; in fact there are several calendar-based
contraceptive methods, some being more reliable than others. A woman's fertile period
is determined by noting changes in the consistency of the cervical mucus and body
temperature. Just before ovulation occurs, the cervical mucus becomes thin and watery
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TOPIC 3. THE BIOLOGY OF CONTROLLING FERTILITY
while body temperature rises slightly (by about 0.5 ◦ C).
By tracking these changes over several cycles, a woman can predict fairly accurately
when she is going to ovulate. Therefore, a couple who do not wish to have a child will
not have sexual intercourse during the few days before and after the fertile period. This
method is not very reliable if a woman's cycle varies in length from month to month.
However, for many people, this is the contraceptive method of choice for personal or
religious reasons.
Calendar-based methods: Question
Q9: When would a woman be expected to ovulate (on average) and why is it necessary
to abstain from intercourse for days before and after ovulation to minimise the chance
of conception?
..........................................
3.4.2
Barrier methods
Barrier methods prevent sperm entering the uterus. This may be achieved by means
of a male condom placed over the penis or a female condom, cap or diaphragm, all of
which prevent sperm reaching the cervix. These methods are made more effective by
being used in conjunction with spermicidal jellies.
It is a controversial point that the condom is the only form of contraception currently
available for use by men. It is also the only form of contraception which also gives a
measure of protection against the transfer of sexually transmitted infections.
3.4.3
Intra-uterine devices (IUDs)
IUDs are flexible plastic structures (often T-shaped) which are placed within the uterus.
One form contains copper and works by reducing the motility of sperm, stopping their
progress towards the ovum. It is also possible that the presence of the IUD in the uterus
irritates the endometrium, thus preventing implantation. A side effect is the increased
loss of blood at menstruation.
The second type of IUD releases a low dose of progesterone, which works more like the
contraceptive pills.
3.4.4
Sterilisation procedures
Sterilisation is any medical technique which renders a person unable to reproduce, the
most common methods involving the cutting and closing tubes. In women the tubes are
the two fallopian tubes down which ova travel from the ovaries to reach the uterus and
in which fertilisation must take place. In men the tube is the vas deferens in each testis,
which carries sperm from the testis to combine with the secretions of the prostate gland
and seminal vesicles to form the semen.
The operation for women is called 'tubal ligation' and the operation for men is a
'vasectomy'. In both cases the operation is effectively irreversible.
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TOPIC 3. THE BIOLOGY OF CONTROLLING FERTILITY
3.4.5
43
Chemical contraceptives
Chemical contraceptives are currently only available for use by women. They are usually
taken orally as a daily pill. These pills are of various formulations, but all contain
synthetic forms of oestrogen and/or progesterone:
• the pill - the standard contraceptive pill contains a combination of synthetic forms of
oestrogen and progesterone which must be matched to the woman's natural level
of hormone release. The effect is to adjust the negative feedback mechanism
involving FSH and LH which controls the menstrual cycle so that ovulation is
suppressed but menstruation still takes place;
• mini-pill - these contain synthetic progesterone (hence 'progesterone-only pill'),
which causes a thickening of the mucus on the cervix that prevents the passage
of sperm into the uterus and hence fertilisation;
• morning-after pill - also known as emergency contraceptive pills (ECPs), these
contain high levels of synthetic oestrogen and progesterone and are taken only
after sexual intercourse has taken place. They work either by suppressing
ovulation or by inhibiting implantation. Despite their popular name, they are
effective for two or three days after intercourse, dependent on the formulation of
the pill.
Contraception: Question
Q10: In this exercise you are to complete statements about different forms of
contraception by choosing the correct options from the bracketed lists.
Sterilisation is available to men / women / men and women and is reversible/irreversible.
The mini-pill causes thickening of cervical mucus / failure of implantation/ failure of
ovulation and contains oestrogen / progesterone / oestrogen and progesterone.
Barrier methods kill sperm / stop ovulation / stop sperm reaching uterus and are
available for men / women / men and women.
The morning-after pill contains high levels of synthetic oestrogen / progesterone
/ oestrogen and progesterone and kills sperm / suppresses ovulation / prevents
fertilisation.
IUDs are placed in the uterus / fallopian tubes / vagina and prevent ovulation / fertilisation
/ menstruation.
The rhythm method requires careful monitoring of temperature / cervical mucus /
temperature and cervical mucus and can be totally reliable / used after intercourse /
used without medical supervision.
The pill contains synthetic oestrogen / progesterone / oestrogen and progesterone and
is taken before intercourse / after intercourse / every day.
..........................................
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TOPIC 3. THE BIOLOGY OF CONTROLLING FERTILITY
3.5
Learning points
Summary
Fertile periods
• A fertile period is when a man is capable of fathering a child or when a
woman is capable of conceiving.
• Men are continuously fertile from puberty to death.
• Women are cyclically fertile for a few days every month.
• A woman can calculate her monthly fertile period and use this as a means
of contraception.
• Ovulation can be detected by a small rise in body temperature and change
in cervical mucus.
Treatments for infertility
• Causes of infertility
– Men and women can be made infertile by factors such as age,
genetics, disease and lifestyle.
• Artificial insemination (AI)
– AI involves the artificial injection of sperm into the vagina or uterus.
– Several samples of semen are collected over a period of days.
– AI is particularly used where a man has a low sperm count.
– If the male partner is infertile, another sperm donor may be used.
• Stimulating ovulation
– Ovulation is stimulated if a woman is not ovulating or is ovulating
irregularly.
– Superovulation may result in multiple births.
– Ovulation is also stimulated to cause superovulation so that several
eggs may be collected for IVF.
– Ovulation is enhanced by stimulating the secretion of FSH or by
blocking the oestrogen receptors in the pituitary.
– Ovulation may also be stimulated by increasing FSH and LH levels by
supplementing them with synthetic mimics.
• In vitro fertilisation (IVF)
– IVF involves the surgical removal of eggs from the ovaries after
superovulation.
– Ova are incubated in a culture medium along with a much larger
number of sperm.
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Summary Continued
– Embryos are cultured until they reach the 8-16 cell stage before
transfer to a uterus for implantation.
– Embryos at this stage may be tested using pre-implantation genetic
screening.
– Pre-implantation genetic screening identifies genetic disorders and
chromosome abnormalities.
• Intracytoplasmic sperm injection (ICSI)
– ICSI is a form of IVF in which a single sperm is injected into an ovum
with a needle to achieve fertilisation.
– ICSI is used where a man has a low sperm count or defective sperm.
Contraception
• Calendar-based methods (e.g. 'the rhythm method') are based on the
identification of the woman's fertile period.
• Intercourse must be avoided for a few days before and after ovulation.
• Barrier methods (e.g. condom, diaphragm) prevent sperm entering the
uterus.
• IUDs are placed in the uterus and reduce the motility of the sperm.
• Sterilisation involves cutting or closing the fallopian tubes of a woman.
• Sterilisation involves cutting and closing the vas deferens of each testis of a
man.
• Chemical contraceptives contain synthetic versions of oestrogen and
progesterone.
• The most common chemical contraceptive (the 'pill') contains oestrogen and
progesterone.
• The pill manipulates the negative feedback control of the menstrual cycle to
suppress ovulation by preventing the release of FSH and LH.
• Morning-after pills contain synthetic oestrogen and progesterone.
• Morning-after pills either suppress ovulation or prevent implantation.
• The mini-pill causes thickening of the cervical mucus, preventing sperm
from reaching the ovum.
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3.6
Extended response question
The activity which follows presents an extended response question similar to the style
that you will encounter in the examination.
You should have a good understanding of fertile periods for men and women before
attempting the question.
You should give your completed answer to your teacher or tutor for marking, or try to
mark it yourself using the suggested marking scheme.
Extended response question: Fertile periods
Give an account of fertile periods under the headings:
15 min
A) men; (4 marks)
B) women. (6 marks)
..........................................
3.7
End of topic test
End of Topic 3 test
Q11: Complete the sentences by matching the parts on the left with the parts on the
right. (9 marks)
A woman is capable of conceiving
progesterone.
Men are continuously fertile
during ovulation.
Cyclically fertile means that women are fertile
from puberty to
death.
A change in a woman's cervical mucus occurs
cervical mucous.
An example of a barrier method is
during the fertile
period.
A method of reducing the motility of sperm is
for a few days a
month.
The procedure that involves cutting the vas deferens is
the diaphragm.
The morning after pill contains synthetic oestrogen and
the intra-uterine
device.
The mini-pill causes thickening of
sterilisation.
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TOPIC 3. THE BIOLOGY OF CONTROLLING FERTILITY
..........................................
Q12: Complete the paragraph using the words from the list. (11 marks)
Men and women can both be made infertile by factors such as age, genetics, disease
and
.
is particularly used when a man has a low sperm count.
Artificial
.
Ovulation is stimulated when a woman is ovulating
births.
Super-ovulation may result in
as it provides several eggs that may be collected.
IVF uses
or supplementing
Ovulation is enhanced by stimulating the secretion of
mimics.
hormone levels with
after super-ovulation.
IVF involves removal of
are transferred when they reach the 8-16 cell stage.
identifies genetic disorders.
Pre-implantation genetic
is injected into an ovum.
ICSI differs from IVF in that a single
Word list: eggs, embryos, FSH, insemination, irregularly, lifestyle, multiple, screening,
sperm, super-ovulation, synthetic.
..........................................
The diagram below represents the negative feedback control of hormone production by
the testis in the male.
Q13: What is tissue A? (1 mark)
..........................................
Q14: Which stimulating hormone is involved? (1 mark)
..........................................
Q15: Which inhibiting hormone is involved? (1 mark)
..........................................
Q16: Explain how this negative feedback control leads to continuous fertility in males.
(3 marks)
..........................................
Q17: Why is the fertility of women described as cyclical? (1 mark)
..........................................
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Q18: State one form of infertility found in men and explain how it arises. (2 marks)
..........................................
Q19: State one form of infertility found in women and explain how it arises. (2 marks)
..........................................
Q20: What is superovulation? (1 mark)
..........................................
Q21: Describe one method of inducing superovulation. (2 marks)
..........................................
Q22: Under what circumstances would artificial insemination be an appropriate
treatment for infertility? (1 mark)
..........................................
Q23: State one similarity and one difference between intracytoplasmic sperm injection
and in vitro fertilisation. (2 marks)
..........................................
Q24: What is tested in pre-implantation genetic screening? (1 mark)
..........................................
Q25: When is the test carried out? (1 mark)
..........................................
Q26: What can the test identify? (1 mark)
..........................................
Q27: Under what circumstances is the test appropriate? (1 mark)
..........................................
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The following diagram shows one menstrual cycle.
Q28: On which day in the cycle would ovulation be expected to occur? (1 mark)
..........................................
Q29: State two changes to her body that a woman would notice at this time. (2 marks)
..........................................
Q30: Which days would be called the 'fertile period'? (1 mark)
..........................................
Q31: Give an account of the biological basis of the action of the different contraceptive
pills. (5 marks)
..........................................
..........................................
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Topic 4
Antenatal and postnatal screening
and care
Contents
4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2 Antenatal care . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
53
54
4.3 Antenatal screening . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
55
4.3.1 Pre-implantation genetic diagnosis (PGD) . . . . . . . . . . . . . . . . .
4.3.2 Ultrasound imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
55
56
4.3.3 Biochemical tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3.4 Diagnostic testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
56
57
4.3.5 Amniocentesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3.6 Chorionic villus sampling (CVS) . . . . . . . . . . . . . . . . . . . . . .
58
58
4.3.7 The moral dimension . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3.8 Rhesus antibody testing . . . . . . . . . . . . . . . . . . . . . . . . . . .
59
59
4.4 Postnatal screening . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.5 Learning points . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
61
62
4.6 Extended response question . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.7 End of topic test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
64
65
Learning Objectives
By the end of this topic, you should be able to:
• describe some of the techniques used to monitor the health of a pregnant woman
and the developing fetus;
• describe the background checks which can be made to determine the possibility
of an inherited condition in the fetus;
• describe the tests for inherited conditions which are carried out during IVF
treatment;
• describe routine tests which may be carried out on all pregnant mothers;
• describe scanning and diagnostic tests which can be carried out at different stages
in pregnancy;
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TOPIC 4. ANTENATAL AND POSTNATAL SCREENING AND CARE
• state the conditions which such tests can suggest or identify;
• describe the tests carried out after birth and the conditions which they may identify.
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4.1
Introduction
One of the most fascinating features of human development is not that things can
go wrong, but that it almost always goes perfectly well despite being an incredibly
complex process. With some 23,000 genes in the genome, the processes of mutation
and meiosis will inevitably generate some changes to the DNA which controls the
biochemistry of the body. Where these changes affect enzymes which form part of a
metabolic pathway, the outcome may be very serious; on the other hand, the effect may
be of little consequence, e.g. if only the colour of hair is altered.
Some problems arise not from genetic change, but from a phenotypic difference
between the embryo and the mother, e.g. in Rhesus blood type.
Advances in Screening
The last half-century has seen a revolution in our ability to detect and address these
problems. From the Guthrie blood test, developed in the 1960s (and still used today to
test for PKU), we have moved on to the screening of embryos in IVF, the various tests
which can be carried out on the fetus, and the battery of analyses which can be used
to check the newborn's blood and urine. Other techniques are applied without sampling
any tissue from the embryo or fetus, such as ultrasound imaging and blood testing of
the mother.
These developments have been possible in part because of the advances in technology,
e.g. ultrasound imaging and DNA analysis. Of equal importance have been the
advances in our knowledge of the underlying biochemistry of the observed medical
conditions so that we now understand the significance of particular chemicals appearing
in the urine of the mother or the newborn.
Advances in Treatment
Of course, the identification of a condition is of limited value unless there is some
treatment available to cure or ameliorate its effects. The type and success of our
responses vary considerably depending on the condition. Here are three examples.
1. The effects of Rhesus incompatibility can in most cases be completely prevented
by antenatal injections given to the mother which prevent the sensitisation of her
immune system. This condition arises when the mother is Rhesus negative and
the fetus Rhesus positive; red blood cells entering the mother's circulation as a
result of mixing of the blood during delivery trigger the release of antibodies by the
mother, which can cross back into the fetal blood circulation and cause the blood
to clot.
2. Phenylketonuria (PKU) is caused by a recessive allele found on one of the
autosomes. Thus, the condition occurs only when a child inherits this allele from
both parents. If left untreated, it results in severely impaired brain development.
Although it cannot be cured, by careful management of diet and suitable
medication the effects of the disease can be completely controlled.
3. Huntingdon's Disease results from the dominant mutation of a single autosomal
gene. It causes gradual degeneration of the brain, affecting first muscular coordination but leading to eventual dementia. In most cases, symptoms of the
disease only become obvious between the ages of 35 and 55 years. There is
no cure for the disease, but treatments are available to reduce the severity of the
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TOPIC 4. ANTENATAL AND POSTNATAL SCREENING AND CARE
symptoms and allow appropriate care to be given as the disease progresses. If
there is a family history of the disease, a couple may opt for IVF treatment so that
Pre-implantation Genetic Diagnosis may be carried out to ensure that the embryos
chosen for implantation do not carry the mutated allele.
4.2
Antenatal care
Learning Objective
By the end of this section, you should be able to:
• state the routine initial tests that are carried out on pregnant woman;
• state the general purpose of each test.
Once a woman has been confirmed as being pregnant, she will be routinely given
a physical examination by a doctor and her medical history will be checked for any
conditions which might complicate the pregnancy. The examination involves measuring
her blood pressure, assessing her general health, and taking blood and urine samples.
Height and weight measurements are used to calculate the Body Mass Index (BMI),
which will indicate whether a woman is significantly over- or underweight. BMI is
calculated as mass (kg) / height2 (m). A value of BMI = 30 or more is classified as
obese.
Blood pressure is checked as described in a later topic. Readings of up to 140/90 are
considered normal during pregnancy; higher readings indicate hypertension (one of the
risk factors for stroke and heart failure) and possible pre-eclampsia (symptoms include
protein in the urine) which can lead to a life-threatening conditions such as seizures.
Blood tests are used to determine blood group, and check levels of cholesterol (high
indicating increased risk of cardiovascular disease) and blood sugar (high indicating
diabetes and low indicating hypoglycaemia).
Urine tests comprise more than 100 tests which may be carried out on urine. Some are
simple, like colour, clarity, pH and smell; others more complex, such as the analysis of
ion or cell content. These tests may indicate a wide variety of potential problems, such
as infections or renal failure.
Medical history questions will be asked in case the woman has any conditions which
might complicate her pregnancy, e.g. type 1 diabetes, which can cause early labour or
birth defects, or tobacco smoking. Her family history will also be considered in case
there are any indications of inherited disorders (e.g. cystic fibrosis).
Antenatal care: Question
Q1: Complete the table by matching the conditions in the left column with the check or
test that may be used to detect them.
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Condition
55
Check/Test
Renal failure
Obesity
Cystic fibrosis
Pre-eclampsia
Diabetes
Check/test list: Blood pressure test; Blood sugar test; BMI; Medical history; Urine test.
..........................................
4.3
Antenatal screening
Learning Objective
By the end of this section, you should be able to:
• describe some of the tests which may be carried out on a pregnant woman or
her fetus;
• state the general purpose of each test;
• state the advantages and disadvantages of each test.
Once a woman has been confirmed as pregnant, a range of tests are available to assess
the health of both the mother and the fetus as the pregnancy progresses. Some, like
the ultrasound scans, are routine and available to all expectant mothers; others, such
as Chorionic Villus Sampling, are only carried out when there is good reason to expect
some abnormality may be present.
These tests may be mainly focussed on:
• the mother, such as the biochemical tests;
• both mother and fetus, e.g. ultrasound scans;
• the embryo/fetus, e.g. amniocentesis.
Procedures which do not involve removal of tissue from the fetus (e.g. ultrasound scans,
sampling mother's blood) are classed as non-invasive, whereas those that do are
invasive (e.g. amniocentesis, PGD).
4.3.1
Pre-implantation genetic diagnosis (PGD)
If a couple have a family history of a genetic disorder or they have had a previous child
who has been born with one, they may opt for in vitro fertilisation (IVF) which will allow
embryos to be screened for the presence of the disorder before they are implanted in
the mother's uterus.
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In the most common PGD procedure, two cells are removed from the 8-cell stage of
the embryo, their chromosomes are examined, and their DNA analysed. Transfer to
the uterus is delayed until the fifth day after fertilisation, thereby allowing time for the
analysis to be completed before the final choice of embryos is made.
As well as chromosomal abnormalities, the list of genetic disorders which can be
detected is considerable and includes: cystic fibrosis, sickle-cell disease, Huntingdon's
disease and Duchenne muscular dystrophy.
4.3.2
Ultrasound imaging
This procedure uses very high frequency sound to create an image of the fetus in the
uterus. A probe is placed over the area being investigated and pulses of sound are
directed into the abdomen. Different densities of tissue reflect different proportions of
the sound pulse and these reflections are interpreted electronically to form an image of
the uterus and the fetus. The frequencies used are typically between 2 and 18 MHz,
which is a thousand times greater than the highest frequencies we can hear.
Although this may appear to be quite a drastic treatment, there have been no negative
effects reported.
The first scan is carried out between 8 - 14 weeks of the pregnancy and is called a
dating scan as it is used to determine the age of the fetus, and hence the expected
delivery date (due date).
A second scan is also offered at 18 - 20 weeks, when the fetus is much larger. This is
known as an anomaly scan and is used to identify any aspects of physical development
of the limbs and vital organs which are unusual. An example of such an anomaly is a
high volume of fluid behind the neck, which in 60% of cases is an indicator of Down's
syndrome. If a further check reveals the absence of a particular bone in the nose, the
likelihood of the condition being present is increased to 95%.
4.3.3
Biochemical tests
The mother's blood is constantly transporting a whole range of chemicals around her
body, from the products of digestion, to her own hormones, and to substances which
have crossed the placenta from the fetus. Certain of these show significant changes if
there are problems with either the fetus or the mother, and therefore can be used as
chemical markers.
A blood test can be taken in conjunction with the anomaly scan, and the levels of such
marker chemicals in it compared with those expected at that stage in the pregnancy.
For example, low levels of certain fetal liver proteins (alpha-fetoprotein) and hormones
(estriol) associated with pregnancy are indicative of a fetus with Down's syndrome
(although there are other causes). In this case, the marker chemical is not itself part
of the disorder but an indicator that the disorder is likely to be present.
As the levels of these substances normally increase throughout pregnancy, it is
important that the recorded levels be matched to the age of the fetus to avoid incorrectly
diagnosing the condition. In the case of Down's syndrome, if the fetus was younger than
estimated, the presence of a normal level of estriol would result in it being incorrectly
diagnosed as having the condition. This would be a false positive result.
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It is also the case that these tests will yield a small proportion of false positives even if
correctly applied, i.e. some fetuses which are unaffected will be identified as having the
condition. Hence the need for carrying out a variety of tests and for the more precise
diagnostic tests which follow.
Biochemical tests: Question
Q2: State which results of the anomaly scan and the biochemical tests would indicate
that a further diagnostic test for Down's syndrome would be appropriate.
..........................................
4.3.4
Diagnostic testing
The tests mentioned in this topic are classified according to their precision: those that
indicate the possible presence of a disorder are screening procedures (e.g. ultrasound
scans); those that confirm the presence of a condition are diagnostic procedures (e.g.
amniocentesis). Screening procedures highlight symptoms, which may have more than
one cause, whereas diagnostic tests identify the causes of these symptoms.
All pregnant women are initially offered the screening procedures of ultrasound scans
and blood tests. If any of these tests suggest the potential presence of an abnormality,
then further diagnostic tests are indicated. At this point, a risk analysis must be carried
out because the diagnostic tests are invasive and may, in a few cases, cause damage
to the fetus or trigger a miscarriage. Refining the precision of ultrasound imaging, and
considering ultrasound together with the biochemical analysis of blood samples, can
give a much more certain indication of the presence of a disorder.
It is worth noting that all tests, even those based on karyotyping, have a small error
rate and that there are no prenatal tests which will detect all disorders or abnormalities.
Diagnostic testing: Question
Q3: Complete the paragraphs using the words from the list.
Soon after her pregnancy is confirmed, a woman is given a range of routine tests to
assess the possibility of complications to the pregnancy. These are
tests. Examples are
,
and
.
scan at
These are followed by a
. A second scan at
determines
scan.
physical problems and is called an
imaging.
weeks which
weeks seeks to identify
Both these scans use
All the tests mentioned so far identify the possibility of a disorder being present and are
tests. Tests which identify the presence of a disorder very
referred to as
tests. Examples of such tests are
precisely are called
and
.
Word list: amniocentesis; anomaly; blood pressure; blood tests; CVS; dating; diagnostic;
due date; screening; screening; ultrasound; urine tests; 8 - 14; 18 - 20.
..........................................
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4.3.5
Amniocentesis
This procedure involves the insertion (under anaesthetic) of a needle through the
abdomen wall, then the uterus wall, and finally (guided by ultrasound) into the amniotic
sac at a point away from the fetus. This sac, which is fluid-filled, supports and protects
the fetus. A sample of fluid (about 20ml) is extracted and the cells (usually white blood
cells) in it are removed and cultured. DNA can be extracted from these cells and
analysed.
These cells may also be treated to produce a karyotype. They are induced to divide
and the process is arrested at metaphase when the chromosomes are at their most
prominent. The cells are mounted on a slide, chemically fixed so that they will not alter,
and then stained to highlight the chromosome structure.
An image of the chromosomes of one cell arranged in homologous pairs, the
karyotype, is then analysed to identify any anomalies in terms of the numbers or
structure of the chromosomes.
This test is carried out after 15 weeks, usually at 18 weeks, with a risk of miscarriage of
less than 0.1 - 0.5%. In certain cases it may be carried out between 11 - 13 weeks, but
with increased risk.
4.3.6
Chorionic villus sampling (CVS)
During CVS, a sample of chorionic villi cells will be taken from the pregnant woman's
placenta. The villi are the structures in the placenta which contain the blood vessels that
are involved in the exchange of materials between the fetus and the mother.
Cells are obtaining using either:
• transabdominal CVS - where a needle is inserted through the abdomen (as in
amniocentesis);
• transcervical CVS - where a catheter is inserted through the cervix (the neck of
the womb).
In both cases, ultrasound guidance is used to ensure the safe placement of the
apparatus.
The cell sample is then prepared for karyotyping. The process is quicker than for
amniocentesis as the cells are actively dividing. Alternatively, DNA analysis may be
carried out.
CVS is normally carried out between 10 - 12 weeks, giving a much earlier indication of
any abnormalities, but has a higher risk of miscarriage of 0.5 - 1%.
Chorionic villus sampling: Question
Q4: Using suitable examples, explain the difference between a screening and a
diagnostic test.
..........................................
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4.3.7
The moral dimension
The screening and diagnostic tests described above provide information about the
health of the mother and the fetus. The results may range from indicating that the fetus
is perfectly healthy to suggesting that it will live for only a short period after birth and be
in constant distress. In the same way, the mother might be found to be in robust health
or suffering from some condition such as kidney disease (nephropathy) which would
threaten her life as the pregnancy proceeds.
Decisions have to be taken by the mother as to whether she will undergo any tests at all,
and then, if initial screening or family history point to it, whether she should expose her
fetus to diagnostic tests with their associated increased risks (small though they are).
If the diagnostic tests indicate that the fetus has a disorder which will seriously reduce
the quality of its life after birth or will mean that it will require round-the-clock care, the
mother may wish to consider the possibility of terminating the pregnancy.
Moral attitudes span the spectrum, from those who believe that the zygote is a human
being with an inalienable right to life to those who believe a woman has the right to
terminate any pregnancy. Others believe that there should be no medical intervention
of any sort during a pregnancy. It is not the purpose of this course to promote any
particular moral view, but you should be aware of the different views that people may
hold and of the issues that must be considered.
The moral dimension: Question
Q5: Type A Nieman-Pick Disease is a rare inherited disorder controlled by an
autosomal recessive allele which severely disrupts the function of many organs and
the nervous system. It is normally fatal by the age of four and there is no cure.
A woman has received a positive pregnancy test result from her home test kit. She
knows that her aunt had a child suffering from this condition, but her husband was
adopted and has no knowledge of his biological parents.
What tests would she and her fetus have the opportunity to undergo and what factors
might influence her decisions?
..........................................
4.3.8
Rhesus antibody testing
Any fetus is genetically different in many respects from its mother and so runs the risk
of the mother's immune system being alerted to this foreign invader and taking action
against it. One of the many functions of progesterone during pregnancy is to suppress
the mother's immune response. However, if cells from the fetus enter the mother's blood
circulation, white blood cells will rapidly find them and respond.
As you will discover in Unit 4 of the course, the human immune system reacts to the
presence of cells which have foreign, or 'non-self', protein markers (antigens) on their
cell membranes. The outcome of this reaction is the production of other proteins, called
antibodies, by white blood cells. These attach themselves to the antigens on the foreign
cells and help destroy them.
In the case of the Rhesus factor, problems arise if a mother who is Rhesus negative
(does not have the Rhesus antigen) is pregnant with a Rhesus positive fetus. If red blood
cells cross from the fetal blood circulation into the mother's blood during the pregnancy,
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the Rhesus antigens on the fetal cells will be registered as non-self.
During a first pregnancy, this is unlikely to cause a problem as the number of fetal cells
crossing is insufficient to trigger a response. However, during birth, many cells do cross
into the mother's blood and the immune system is sensitised, making white blood cell
which 'remember' the antigen. Sensitisation may also result from blood transfusions
or damage to the placenta during pregnancy, causing it to leak blood into the mother's
system.
Should the Rhesus antigen turn up again in the circulation, as it will do if the mother has
a second Rhesus positive child, these memory cells trigger a rapid and large production
of antibodies. This is not a problem for the mother as the few cells that pass from fetus
to mother are quickly removed.
However, as the antibody molecules are small enough to pass across the placenta into
the fetus, the damage occurs there in the form of the destruction of red blood cells.
The effect is to cause jaundice, which only becomes apparent after birth because the
breakdown products of the red blood cells are removed across the placenta before birth.
The condition can be fatal.
When it is known that a woman is Rhesus negative she is given injections of appropriate
anti-Rhesus antibodies (immunoglobulin proteins) twice; once late in the pregnancy and
once shortly after birth. These antibodies bind with any fetal red blood cells which have
entered the mother's blood, causing them to be destroyed before the mother's immune
system can be sensitised.
Rhesus antibody testing: Question
Q6: Under what circumstances will a mother's immune system react against a fetus
and how may this be avoided?
..........................................
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4.4
61
Postnatal screening
Learning Objective
By the end of this section, you should be able to:
• describe how PKU is detected;
• state how PKU is treated.
After the birth, both mother and baby are examined physically and, within three days of
birth, the baby is given a blood test. The blood test involves taking a small blood sample
by pricking the heel, and analysing the dried blood by a procedure first developed in
Scotland by Robert Guthrie in the 1960s. The test is commonly called the 'heel-prick' or
Guthrie test.
The blood sample is tested for several rare conditions, including Phenylketonuria (PKU).
The test is optional, but all of the conditions have very serious effects which can be
managed medically if treatment is started immediately. Also, such early warning enables
the parents to adapt to the necessary changes in their anticipated lifestyle.
Phenylketonuria is an autosomal recessive disorder that involves a mutation to the
gene coding for the enzyme which converts the amino acid phenylalanine to the
amino acid tyrosine. Without this enzyme, phenylalanine accumulates in the blood and
severely retards the development of the brain. Before birth, the excess phenylalanine is
transferred across the placenta into the mother's blood and removed by her liver. Thus,
the presence of the disorder only becomes apparent after birth.
Treatment started soon after birth can result in normal brain development. This can
be achieved through the strict control of diet, largely or totally eliminating foods which
are high in phenylalanine such as dairy products, meat, fish, nuts, peas and beans. In
addition, starchy food intake must also be carefully controlled. Various medications may
also be used and the action of some of these permits a more protein-rich diet.
Postnatal screening: Question
Q7: What is PKU, how is it detected, and how is it treated?
..........................................
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4.5
Learning points
Summary
Antenatal Care
• The initial assessments that are carried out once a woman is known to be
pregnant are: height and weight, blood pressure, blood tests, urine tests,
and medical history.
• An example of a potential complication that each assessment might reveal
is:
– height and weight - obesity;
– blood pressure - hypertension;
– blood tests - diabetes;
– urine tests - renal failure;
– medical history - cystic fibrosis.
Antenatal Screening
• Pre-implantation Genetic Diagnosis (PGD) may be carried out as part of the
IVF procedure.
• PGD is used to test the DNA of the embryo for the presence of specific
single gene disorders, and for chromosome abnormalities.
• Ultrasound imaging uses very high frequency sound to create images of the
fetus in the uterus.
• The first ultrasound scan (dating scan) is carried out between 10 - 14 weeks
to determine the age of the fetus and the date when it is due to be born (due
date).
• A second ultrasound scan (the anomaly scan) is carried out between 18 20 weeks to detect possible physical problems.
• A pregnant woman's blood carries a range of chemicals which can be used
to check that the pregnancy is progressing normally.
• The level of these marker chemicals normally varies throughout the
pregnancy as the mother's, and the fetus's, physiology change.
• The timing of these tests must be taken into account in order to avoid false
indications about the presence of a disorder.
• A false positive result would be obtained if the level of a marker chemical
indicated the presence of a disorder when in fact the fetus did not have it.
• Marker chemicals will be present in the mother's blood during a trouble-free
pregnancy, but the presence of a disorder may affect the level of a particular
marker.
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Summary Continued
• Screening tests, for example ultrasound scans and blood tests, indicate the
likelihood of a disorder being present.
• Diagnostic tests, for example amniocentesis and chorionic villus sampling,
confirm whether a condition is present or not.
• Diagnostic tests, for example amniocentesis and chorionic villus sampling,
are invasive and carry a small element of risk of inducing a miscarriage.
• Amniocentesis samples cells from the amniotic fluid.
• Cells from amniocentesis samples are multiplied up in culture and used to
produce a karyotype.
• A karyotype is an image of an individual's chromosomes, arranged in
homologous pairs.
• A karyotype is used to identify anomalies in terms of numbers or structure
of chromosomes.
• Chorionic villus sampling (CVS) removes cells from the placenta which can
be used immediately to produce a karyotype.
• CVS can be carried out earlier than amniocentesis but entails a
correspondingly higher risk of inducing a miscarriage.
Postnatal Screening
• A Guthrie heel-prick test is used to collect a blood sample from a newborn
child.
• Dried blood from this sample is screened for several inherited disorders.
• Phenylketonuria (PKU) is an autosomal recessive disorder causing an error
of metabolism.
• PKU results in high levels of phenylalanine which severely restrict brain
development.
• Individuals with PKU are given a restricted diet which lacks phenylalanine.
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4.6
Extended response question
The activity which follows presents an extended response question similar to the style
that you will encounter in the examination.
You should have a good understanding of tests which can be carried out during
pregnancy before attempting the question.
You should give your completed answer to your teacher or tutor for marking, or try to
mark it yourself using the suggested marking scheme.
Extended response question: Tests which can be carried out during
pregnancy
15 min
Give an account of the tests which can be carried out once a woman has been confirmed
as pregnant, under the headings:
A) screening tests; (6 marks)
B) diagnostic tests. (4 marks)
..........................................
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4.7
End of topic test
End of Topic 5 test
Q8: Complete the sentences by matching the parts on the left with the parts on the
right. (12 marks)
Tested as part of a general health check:
first ultrasound imaging.
A procedure that may be involved in IVF:
blood marker chemicals.
Usually taken 8-14 weeks after pregnancy:
chorionic villus sampling.
Used to detect the possibility of physical problems:
amniocentesis & chorionic
villus sampling.
Timing of tests taken into account when assessing:
amniocentesis.
Used to indicate the likelihood of a disorder:
pre-implantation genetic
diagnosis.
Used to confirm the presence of a disorder:
blood pressure.
Screening tests include:
screening tests.
Diagnostic tests include:
ultrasound and blood tests.
The name for an image of a foetus' chromosomes:
second ultrasound imaging.
Involves removing cells from the amniotic fluid:
diagnostic tests.
Involves removing cells from the placenta:
karyotype.
..........................................
Q9: Complete the paragraph using the words from the list. (8 marks)
heel-prick test is used to collect a
sample from a newborn
A
disorders. Phenyketonuria is an autosomal
child, which screens for several
disorder causing high levels of
which restricts
development. Individuals with
are given a restricted diet
phenylalanine.
Word list: blood, brain, Guthrie, inherited, lacking, phenylalanine, PKU, recessive.
..........................................
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Once her pregnancy is confirmed, a mother is given an initial examination consisting of
a number of screening tests.
Q10: What is tested to check for hypertension? (1 mark)
..........................................
Q11: What conditions can be indicated from taking blood tests? (1 mark)
..........................................
Q12: What is checked to test for any indication of cystic fibrosis? (1 mark)
..........................................
Q13: Explain the difference between a screening test and a diagnostic test. (2 marks)
..........................................
Q14: Name one ultrasound scan. (1 mark)
..........................................
Q15: State when the scan is carried out and its purpose. (2 marks)
..........................................
Q16: Describe how a marker chemical might indicate the presence of a fetal disorder.
(2 marks)
..........................................
Q17: Explain why a test for a marker chemical might give a false positive result. (2
marks)
..........................................
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The following illustration shows the process of obtaining an amniocentesis sample.
Q18: Suggest the purpose of the ultrasound probe. (2 marks)
..........................................
Q19: State two ways in which a diagram of chorionic villus sampling would be different
from the above diagram. (2 marks)
..........................................
Q20: State what a karyotype is and what it may reveal about an individual. (2 marks)
..........................................
Q21: What causes PKU? (1 mark)
..........................................
Q22: How does PKU affect development? (1 mark)
..........................................
Q23: How is PKU in infants treated? (1 mark)
..........................................
..........................................
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Topic 5
Patterns of inheritance
Contents
5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2 Genetic terms and their meanings . . . . . . . . . . . . . . . . . . . . . . . . .
70
70
5.3 Pattern of inheritance of a pair of alleles - one dominant, one recessive . . . .
5.3.1 Cystic fibrosis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
72
73
5.4 Dominant and incompletely dominant alleles . . . . . . . . . . . . . . . . . . .
74
5.4.1 Dominance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.4.2 Incomplete dominance . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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75
5.5 Sex-linked inheritance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.6 Learning points . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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82
5.7 End of topic test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
83
Learning Objectives
By the end of this topic, you should be able to:
• describe the pattern of inheritance of a pair of alleles where one is dominant and
one is recessive;
• describe the effects of alleles exhibiting dominance and incomplete dominance;
• describe the possible combinations of multiple alleles;
• describe sex-linked inheritance and the effects of the presence of genes on the
X-chromosome but not on the Y-chromosome.
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5.1
Introduction
Topics later in this unit are concerned with the various screening and diagnostic
procedures which are available to pregnant mothers. These seek to identify conditions
that cause aspects of metabolism to malfunction.
Many of these conditions are the result of a mutation which affects the action of a single
gene. This topic examines the patterns of inheritance of single genes (monohybrid
inheritance), considering in particular the behaviour of alleles which are dominant,
recessive or incompletely dominant, going on to study sex-linked inheritance and the
effects of genes which are present on the X but not on the Y chromosome.
This subject, even more than most, has its own specialist language and it is a very good
idea to keep a glossary of these words in your notes.
5.2
Genetic terms and their meanings
Learning Objective
By the end of this section, you should be able to:
• describe the meaning of various genetic terms.
There are many terms used in genetics which, if you understand their meanings, make
genetics more easily understood. You should know the meanings of the following terms:
• haploid - a cell containing one set of chromosomes (gametes);
• diploid - a cell containing two sets of chromosomes;
• gene - a discrete region of a chromosome whose DNA codes for the production of
a polypeptide or protein;
• allele - different forms of a gene;
• homozygous/a homozygote - an individual possessing two identical alleles of a
gene;
• heterozygous/a heterozygote - an individual possessing two different alleles of
a gene;
• genotype - the genetic makeup of an individual with respect to a particular
characteristic;
• phenotype - the expression of a gene in an individual in terms of appearance,
behaviour or biochemistry;
• dominant - an allele that is expressed in the phenotype of the heterozygote;
• recessive - allele that is not expressed in the phenotype of the heterozygote;
• incompletely dominant - an allele that is not completely masked by the dominant
allele and which therefore has some effect on an individual's phenotype;
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• pedigree chart - a diagram that shows the occurrence of the phenotypes of a
particular gene from one generation to the next;
• P generation - the parents at the start of any pedigree chart;
• F1 generation - the first generation produced by two parents in a cross;
• F2 generation - the generation produced by crossing two individuals from the F 1
generation;
• sex-chromosome - one of the pair of chromosomes which determine the sex of
the individual (XX in the female and XY in the male);
• autosome - one of the 22 pairs of chromosomes that control the general
functioning of the individual, but do not determine the sex;
• linked genes - genes carried on the same chromosome;
• sex-linked genes - genes carried on the X-chromosome;
• carrier - an individual who is heterozygous for a particular characteristic,
especially applied to genetic disorders which are caused by a recessive allele
carried on an autosome or an X-chromosome;
• siblings - offspring of the same parents, although the term can also be applied to
children who share a single parent (half- as opposed to full-siblings);
• twins - two individuals produced in the same pregnancy;
• identical twins - (also called monozygotic twins) two individuals produced by the
fertilisation of a single egg and the subsequent splitting of the ball of cells - they
are therefore genetically identical;
• non-identical twins - (also called dizygotic twins) two individuals produced by the
fertilisation of two eggs who are no more genetically similar than any other siblings.
Dominant alleles are represented by upper case (capital) letters while recessive alleles
are represented by lower case letters.
The symbol that represents male individuals
The symbol that represents female individuals
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5.3
Pattern of inheritance of a pair of alleles - one dominant,
one recessive
Learning Objective
By the end of this section, you should be able to:
• describe the pattern of inheritance of a pair of alleles where one is dominant
and one is recessive.
You should remember from a previous course the pattern of inheritance of a pair of
alleles that occurs where one is dominant and one is recessive. You should look at the
following two activities to revise the pattern of inheritance from the parental generation
through to the F 2 generation.
The monohybrid cross: F1 generation
10 min
The following illustrates the principle of the monohybrid cross and the inheritance of
genetic characteristics by the F1 generation.
The 'bobbit' is a small, roundish mammal. A a brown, male bobbit mates with a white,
female bobbit to produce the F 1 generation.
There are sixteen offspring, all of which are brown.
Q1: The colour of the bobbit is conferred by a single gene. A bobbit with the genotype
BB has a brown coat colour whereas a bobbit with the genotype bb is white. The B
allele is completely dominant to the b allele. Assuming that the parents in the cross
were homozygous, what were their genotypes?
..........................................
Q2:
The offspring in the F 1 generation are all brown. Why is this?
..........................................
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The monohybrid cross: F2 generation
The following illustrates the inheritance of genetic characteristics by the F2 generation
in a monohybrid cross. It is a continuation of the previous activity (The monohybrid
cross: F1 generation) which you should work through before looking at this one.
In the previous activity, a brown, male bobbit mated with a white, female bobbit to
produce the F1 generation. In this case, a male and female from the F 1 generation
mate to produce the F 2 generation.
In this case there are sixteen offspring, four of which are white and twelve of which are
brown.
Q3: What is the ratio of brown coloured bobbits to white coloured bobbits in the F 2
generation?
..........................................
Q4: Explain your answer to the previous question, with the aid of a Punnett square.
Remember that a bobbit with the genotype BB has a brown coat colour, whereas a
bobbit with the genotype bb is white. The B allele is completely dominant to the b
allele.)
..........................................
5.3.1
Cystic fibrosis
Cystic fibrosis is an inherited condition which causes the production of thick mucus
in the respiratory system. This increases an individual's susceptibility to respiratory
infections. Thick mucus is also produced in the digestive system and can cause the
blockage of ducts (for example in the pancreas) which prevents digestive enzymes
reaching the small intestine.
Cystic fibrosis is caused by a recessive allele. Individuals possessing only one such
allele are perfectly healthy. However, if someone inherits a recessive allele from each
parent, they will suffer from the condition.
The alleles involved in cystic fibrosis are inherited in the normal manner. The following
activity shows the pattern of inheritance of this condition when both parents are
heterozygous for this gene.
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TOPIC 5. PATTERNS OF INHERITANCE
Cystic fibrosis: Questions
10 min
Let N represent the normal allele and n represent the cystic fibrosis allele. Since both
parents are heterozygous for this gene, both of their genotypes will be Nn and their
phenotypes will be normal mucus production.
Both of these individuals will produce gametes, half of which will be N and half n.
Q5: Complete the Punnett square to show the possible genotypes of any offspring of
these two individuals.
n
N
N
n
..........................................
Q6:
What is the percentage chance that this couple's first child will have cystic fibrosis?
..........................................
Q7: The couple's first two children are normal and they are expecting their third. What
is the percentage chance that this child will have cystic fibrosis?
..........................................
Q8: In another couple, one individual is heterozygous while the other is homozygous
for the normal allele. What are the chances of any of their children having cystic fibrosis.
..........................................
..........................................
5.4
Dominant and incompletely dominant alleles
Learning Objective
By the end of this section, you should be able to:
• describe the effects of alleles which exhibit dominance and incomplete
dominance.
In this section we will look at the pattern of inheritance involving dominant, co-dominant
and incompletely dominant alleles.
5.4.1
Dominance
The disorder Huntington's chorea is a degenerative disorder characterised by slurred
speech, uncontrolled movements of the body and a progressive deterioration in mental
functions. The first symptoms generally appear between the ages of 35 and 55 and
become progressively worse for 10 to 25 years until the patient dies. The allele which
causes the condition is dominant to the normal form of the gene. An individual who is
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75
heterozygous for this gene will eventually develop the condition (as will a homozygous
dominant individual). The following shows the inheritance of this condition.
Dominance: Question
Let H represent the Huntington's chorea allele and h represent the recessive, normal
allele. In the following example, one individual is heterozygous for the gene, while the
other is homozygous recessive.
Q9: Complete the Punnett square to show the possible offspring genotypes, then
answer the questions which follow.
H
h
H
h
..........................................
Q10: What is the percentage chance that a child of this couple will develop Huntington's
chorea in later life?
..........................................
Q11: If a heterozygous child still had not developed symptoms by the time they were
40, what would be the chances of them being affected later?
..........................................
Q12: Normally such a lethal allele as Huntington's rapidly disappears from a population.
This does not happen with Huntington's. The frequency of the gene remains constant.
Why do you think this may be?
..........................................
..........................................
5.4.2
Incomplete dominance
Haemoglobin, a protein found in red blood cells and whose function is to carry oxygen,
is coded for by genes. Sometimes, a mutation can occur in one of these genes which
then codes for another type of haemoglobin called haemoglobin S. Haemoglobin S is
much less efficient at carrying oxygen than normal haemoglobin.
People who are homozygous for the abnormal allele suffer from the condition sickle
cell anaemia. All of their haemoglobin is type S which, apart from not carrying oxygen
efficiently, also causes the shape of their red blood cells to be a distorted, sickle
(crescent) shape rather than the normal disc shape. Such red blood cells are less
flexible than normal cells and tend to stick together. This can cause blockages in small
blood vessels. The result is extreme shortage of oxygen to the organs and normally
causes the death of the individual at a young age. The anaemia is caused by the
relatively short life span of the sickle cells.
People who are homozygous for the normal allele produce normal haemoglobin and
normal red blood cells.
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People who are heterozygous do not suffer from sickle cell anaemia, but from a milder
condition known as the sickle cell trait. These individuals can lead relatively normal
lives although they are slightly anaemic. Heterozygotes possess a phenotype that
is 'in-between' the two homozygous phenotypes because the normal allele does not
completely mask the effect of the abnormal one. In other words, the normal allele is
incompletely dominant to the sickle cell allele.
Since neither allele is completely dominant, each is represented by a different, upper
case letter. H is used to represent the normal allele, while S represents the allele for
haemoglobin S:
• HH individuals are normal;
• SS individuals have sickle cell anaemia - the red blood cells stick together, causing
problems in the circulatory system which can lead to severe organ damage and
usually death;
• HS individuals have sickle cell trait - the red blood cells are generally normal,
but individuals may show some signs of the disease when carrying out intense
physical activity; this occurs because the H allele is not completely dominant to
the S allele which means that S is partially expressed.
Normally, an allele causing a condition as serious as sickle cell anaemia would quickly
disappear from a population and, in fact, in most parts of the world, the frequency of the
sickle cell allele is very low. However, the frequency is much higher in certain parts of
Africa where malaria is endemic. This is because heterozygotes are more resistant to
malaria, as shown in the following table.
Genotype
Malarial region
Non-malarial region
HH
No protection from infection
with malaria - high infection rate
Will survive
HS
Protected from infection with
malaria - low infection rate
Sickle cell trait - will survive, but
are slightly anaemic
SS
Sickle cell anaemia, usually
fatal
Sickle cell anaemia, usually
fatal
Malarial protection conveyed by haemoglobin genotype
As a result, the HS genotype is favoured by natural selection in malarial regions.
The following graphic shows the correlation between the incidence of malaria and
the haemoglobin S allele. The sickle cell mutation provides a selective advantage
under certain conditions (where the incidence of malaria is high) but is otherwise a
disadvantage.
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Correlation between the distribution of the sickle cell mutation (S allele) and malaria in
Africa
Incomplete dominance: Questions
Q13: Why is there a high frequency of the S allele in regions with a high incidence of
malaria?
..........................................
Q14: Why do descendants of Central Africans living in other parts of the world have a
far lower incidence of sickle cell trait?
..........................................
5.5
Sex-linked inheritance
Learning Objective
By the end of this section, you should be able to:
• describe sex-linked inheritance, and the effects of the presence of genes on the
X-chromosome but not on the Y-chromosome.
Human diploid cells contain 46 chromosomes that are composed of 22 pairs of
autosomes (chromosomes that are not involved in sex determination) and one pair of sex
chromosomes. There are two types of sex chromosomes: X and Y. The Y-chromosome
is thought to have evolved from an X-chromosome which suffered an inversion mutation
and became unable to pair up with an X at meiosis. Thus, the X and Y-chromosomes
are no longer homologous.
As a result, the genes on the Y-chromosome have slowly mutated over time without
the exchange process of crossing over, and they now do not correspond to any genes
found on the X-chromosome. Strictly speaking, genes found on the X-chromosome are
X-linked, and those on the Y-chromosome are Y-linked, although the term sex-linked is
used generally to refer to genes carried on the X-chromosome.
In many animals, including human beings, females have two X-chromosomes whereas
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TOPIC 5. PATTERNS OF INHERITANCE
males have one X-chromosome and one Y-chromosome.
described genotypically as XX and males as XY.
Females are therefore
Males produce two types of gametes, each of which contains a full set of autosomes
but a different sex chromosome (X or Y). Females only produce one type of gamete,
consisting of the autosomes and a copy of the X-chromosome. If a male gamete
containing an X-chromosome fertilises a female gamete, then the offspring will be
female, but if the male gamete has a Y-chromosome, then the result will be a male.
The gender of the offspring is therefore determined by the sex chromosome carried by
the male gamete. There is an equal chance of the offspring being male or female.
Sex linkage refers to the inheritance of characteristics that are determined by genes
located on the sex chromosomes. The Y-chromosome contains almost no functional
genes and so most sex-linked characteristics are associated with the X-chromosome.
In a male, a sex-linked gene will always be expressed, even if the allele is recessive,
because there is no allele for the same gene on the Y-chromosome.
Females can be carriers of sex-linked disorders, but very rarely suffer from the conditions
themselves. This is because the alleles for these disorders are normally recessive and
will be masked by the normal allele on the other X-chromosome.
Haemophilia is a human sex-linked condition in which blood clotting factor VIII is
defective. Males who suffer from the disease (haemophiliacs) carry a mutated allele
of the factor VIII gene which means that their blood fails to clot properly. The smallest
wound can be fatal, especially if it is internal and goes undetected. Haemophilia is
described as a sex-linked recessive trait. Heterozygous females are carriers: they
possess one mutated allele, but do not suffer from haemophilia. There is a 50% chance
that the daughter of a female carrier will also be a carrier and a 50% chance that a son
will have the disorder (assuming that the father is not a haemophiliac). On the other
hand, all of the daughters of a male haemophiliac will be carriers and all of the sons will
be normal (because they inherit the X-chromosome from the mother).
The genotype of a male haemophiliac is often indicated by X h Y and that of a normal
male by XH Y. A female carrier would be genotypically X H Xh .
Red-green colour blindness in humans is also a sex-linked recessive disorder. Colourblind females are very rare, but carriers have a 50% chance of passing the allele to their
sons. Duchenne muscular dystrophy, a very serious inherited disease in human beings
which causes muscles to waste away, is another sex-linked recessive disorder.
Sex-linked inheritance: Questions
20 min
Q15: Complete each of the six boxes with the identity of the sex chromosomes of the
parents and the gametes
Woman
Man
Chromosome
Gametes
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..........................................
Q16: Enter the identity of the sex chromosomes of the offspring in the Punnett square.
Male gametes
X
Y
X
Female gametes
X
..........................................
Q17: Why is there an equal chance of human offspring being male or female?
..........................................
Q18: A couple have three children: two boys and a girl. They are about to have a fourth
child. What is the chance of that child being a girl?
a)
b)
c)
d)
25%
50%
75%
100%
..........................................
Let C represent the normal allele for colour vision and c represent the allele for red-green
colour blindness.
Q19: The following image shows a cross between a normal female and a colour-blind
male.
Match the phenotypes on the right with the correct genotypes on the left.
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..........................................
Q20:
The following image shows a cross between a carrier female and a normal male.
Complete the Punnett squares using the phenotypes on the right hand side.
..........................................
Q21: In the cross between a normal female and a colour-blind male, what are the
chances of the couple producing:
• a normal son;
• a carrier daughter;
• a colour-blind son.
..........................................
Q22: In the cross between a carrier female and a normal male, what are the chances
of the couple producing:
• a normal son;
• a carrier daughter;
• a colour-blind son;
• a colour-blind child.
..........................................
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A gene A is sex-linked. The recessive allele of this gene, a, causes a disorder in males.
Q23: Study the family tree and then complete it with the genotype of each individual.
..........................................
Q24: Recessive sex-linked disorders are often said to 'skip a generation'.
In
other words, an affected male rarely has male children with the disorder, but male
grandchildren born to the daughter of an affected male are more likely to be affected.
Why is this?
..........................................
..........................................
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5.6
Learning points
Summary
• You should know the meanings of the following terms: haploid, diploid,
gene, allele, homozygous, heterozygous, genotype, phenotype, dominant,
incompletely dominant, F 1 generation, F2 generation, autosome, sex
chromosome, recessive, carrier, pedigree chart, P generation, siblings,
twins, identical twins, non-identical twins, linked genes, sex-linked genes.
• An individual has two alleles for each gene (except for some sex-linked
genes).
• For a pair of alleles where one is dominant and one recessive, the dominant
allele is represented by an upper case letter and the recessive allele by
the same letter in lower case. Tongue rolling, for example, is controlled
by a single gene. An individual who is homozygous dominant (TT) or
heterozygous (Tt) will be able to roll their tongue. A homozygous recessive
individual (tt) will not be a tongue-roller.
• Huntington's chorea is a condition caused by the presence of the dominant
allele of a gene. HH or Hh individuals will eventually develop the disorder in
middle age. hh individuals will not develop it at all.
• An example of incomplete dominance is sickle cell anaemia. Since both
alleles have an effect on the phenotype, each one is represented by
a different upper case letter. H represents the normal allele, while S
represents the sickle cell allele. HH individuals have normal haemoglobin.
SS individuals have sickle shaped red blood cells and suffer from sickle cell
anaemia. HS individuals have a mixture of normal and sickle shaped red
blood cells and suffer from the milder sickle cell trait. The HS genotype
offers a selective advantage over HH in regions where malaria is prevalent.
• Humans have 46 chromosomes of which one pair, the sex chromosomes,
determine sex. Females have two X-chromosomes, which are homologous,
whereas males have one X and one Y, which are non-homologous.
• Homologous chromosomes are of the same size and shape, and carry the
same genes (but not necessarily the same alleles) at the same locations.
• Genes carried on the X-chromosome are said to be sex-linked.
• Some human sex-linked conditions are red-green colour blindness,
haemophilia and Duchenne muscular dystrophy.
• Males inherit sex-linked conditions from their mother because they receive
a Y-chromosome from their father. If their mother is heterozygous for the
gene (XC Xc for colour vision), then her son has a 1 in 2 chance of receiving
the abnormal allele. The mother will not be affected since she has a normal
allele in addition to the recessive, abnormal one.
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End of topic test
End of Topic 4 test
Q25: Complete the sentences by matching the parts on the left with the parts on the
right. (12 marks)
Diploid:
the expression of the alleles of a gene.
Gamete:
a chromosome which does not affect gender.
Alleles:
a genotype with two identical alleles.
Genotype:
a cell containing two of each chromosome.
Phenotype:
brothers and sisters.
Homozygous:
a chromosome which carries sex-linked genes.
Dominant:
a pair of alleles carried by a gene.
Recessive:
a cell containing one of each chromosome.
Incomplete
dominance:
alternative forms of a gene.
Autosome:
an allele which is always expressed in the phenotype if
present.
X:
allele which is only expressed by a homozygous person.
Siblings:
causes three different phenotypes for a gene.
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Q26: Select the correct words from the alternatives provided to complete the following
sentences.
Number of alleles a man has for an autosomal gene on his Y chromosome: 0 / 1 / 2
Genotype of a woman heterozygous for tongue-rolling: TT / X T Xt / Tt
Genotype of a man who will not develop Huntingdons' disease (caused by a dominant
autosomal allele): Xh Yh / Hh / hh
Genotype of a woman who shows sickle cell trait as a result of incomplete dominance:
HH / HS / XH XS
Number of alleles a man carries for a gene with multiple alleles: 1 / 2 / 3
Genotype of a woman with type O blood group: AO / BO / OO
Not true of homologous chromosomes: same alleles / same size / same shape
Percentage chance of a girl inheriting a sex-linked allele from her father: 25% / 50% /
100%
Gametes of the daughter of a father unaffected by recessive condition and a
heterozygous mother: XB and XB / B and B / XB and Xb
Proportion of heterozygotes among the children of parents who are heterozygous for a
condition which is lethal in early pregnancy: 1 in 2 / 2 in 3 / 3 in 4
..........................................
Q27: An individual who possesses two identical alleles controlling a characteristic is
for that characteristic. (1 mark)
said to be
..........................................
Q28: An individual's
characteristic. (1 mark)
is their genetic makeup with respect to a particular
..........................................
Q29: Characteristics which have three phenotypes are caused by alleles which show
. (1 mark)
..........................................
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TOPIC 5. PATTERNS OF INHERITANCE
A monohybrid cross is performed between an individual with the genotype AA and
another individual with the genotype aa.
Q30: What percentage of the F 1 offspring would be expected to be heterozygotes? (1
mark)
a)
b)
c)
d)
25%
50%
75%
100%
..........................................
Q31: What percentage of the F 2 offspring would be expected to be heterozygotes? (1
mark)
a)
b)
c)
d)
25%
50%
75%
100%
..........................................
Q32: What percentage of the F 2 offspring would be expected to be homozygotes? (1
mark)
a)
b)
c)
d)
25%
50%
75%
100%
..........................................
A monohybrid cross is performed between an individual with the genotype Aa and
another individual with the genotype Aa.
Q33: Which of the following phenotypic ratios in the offspring indicates incomplete
dominance? (1 mark)
a)
b)
c)
d)
3:1
1:2:1
2:1
1:1
..........................................
Q34: Which of the following phenotypic ratios in the offspring indicates that the
genotype aa is lethal to the fetus? (1 mark)
a)
b)
c)
d)
3:1
1:2:1
2:1
1:1
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Q35: Huntington's disease is an inheritable disorder that leads to the degeneration of
nerve cells. Symptoms do not usually appear until after the age of about 35. The
disorder is caused by a dominant mutation in a gene on chromosome 4.
A couple give birth to a child. The mother is heterozygous for the mutation, but the father
is unaffected. What is the percentage chance that the child will inherit the disorder? (1
mark)
a)
b)
c)
d)
25%
50%
75%
100%
..........................................
Cystic fibrosis is an inheritable disorder caused by a recessive mutation in a gene on
chromosome 7. Symptoms of the disease include very salty sweat and the accumulation
of thick mucus in the lungs.
A couple give birth to a child. Both parents are heterozygous for the mutation and are
described as carriers.
Q36: What is the percentage chance that the child will suffer from the disorder? (1
mark)
a)
b)
c)
d)
25%
50%
75%
100%
..........................................
Q37: What is the percentage chance that the child will be a carrier of cystic fibrosis? (1
mark)
a)
b)
c)
d)
25%
50%
75%
100%
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A man with a dominant sex-linked disorder and a woman who is unaffected have a child.
Q38: If the child is a boy, what is the percentage chance that he will suffer from the
disorder? (1 mark)
a)
b)
c)
d)
0%
25%
50%
100%
..........................................
Q39: If the child is a girl, what is the percentage chance that she will suffer from the
disorder? (1 mark)
a)
b)
c)
d)
0%
25%
50%
100%
..........................................
Duchenne muscular dystrophy is a recessive, sex-linked disorder in human beings that
causes severe muscle wastage. A couple have a child. The mother is heterozygous (a
carrier), and the father is unaffected.
Q40: If the child is a girl, what is the percentage chance that she will be a carrier? (1
mark)
a)
b)
c)
d)
0%
25%
50%
100%
..........................................
Q41: If the child is a girl, what is the percentage chance that she will suffer from the
disorder? (1 mark)
a)
b)
c)
d)
0%
25%
50%
100%
..........................................
Q42: If the child is a boy, what is the percentage chance that he will suffer from the
disorder? (1 mark)
a)
b)
c)
d)
0%
25%
50%
100%
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Q43: Red-green colour blindness in humans is a sex-linked trait.
A man and a woman, neither of whom is colour-blind but who both have colour-blind
fathers, decide to have a child.
What is the percentage chance of the child being colour-blind? (1 mark)
a)
b)
c)
d)
50%
25%
12.5%
0%
..........................................
Q44: Explain why the daughter of a man affected by a recessive sex-linked trait will not
be affected, although any son that she herself may have can be affected. Assume the
daughter's mother is not a carrier for the condition. (3 marks)
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Topic 6
Blood vessels
Contents
6.1 Why have a cardiovascular system? . . . . . . . . . . . . . . . . . . . . . . . .
6.2 Blood vessels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
90
91
6.2.1 Arteries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.2.2 Capillaries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
92
93
6.2.3 Vasoconstriction and vasodilation . . . . . . . . . . . . . . . . . . . . . .
94
6.2.4 Veins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.3 Exchange of materials between the blood and the cells . . . . . . . . . . . . .
94
95
6.3.1 The mechanism that causes flow in and out of capillaries . . . . . . . .
6.3.2 Lymph vessels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
96
97
6.4 Learning points . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.5 Extended response question . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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99
6.6 End of topic test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
100
Learning Objectives
By the end of this topic, you should be able to:
• describe the structure of the three main types of blood vessel in the blood
circulation system;
• explain how this structure is related to their different functions;
• describe how the purpose of the circulatory system is achieved by the movement
of materials in and out of capillaries.
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TOPIC 6. BLOOD VESSELS
6.1
Why have a cardiovascular system?
Life on earth consists of cells, which either live singly (unicellular), or in communities
(multicellular) with varying levels of complexity. All cells are bounded by an outer
membrane and contain a liquid through which runs a system of membranes where many
of the processes of life take place. To carry out these processes, the cell must obtain
raw materials to build new cellular material and substrates from which it can release
energy. These must come from the surrounding fluid. In addition, the cell must be able
to release waste into its surroundings.
The human organism consists of some 10 13 cells (or 1014 if gut and other bacteria are
included). They may live freely in a liquid, e.g. the cells in the blood, or in tight-knit
tissues, e.g cardiac muscle cells. Wherever they are in the body, they must be supplied
with oxygen and glucose (or another respiratory substrate) to generate the ATP that
drives cell processes, and all of the substances that are necessary for their metabolism,
e.g. amino acids. Carbon dioxide and any other products (e.g. hormones) must be
taken away.
In humans, and all other vertebrate animals, the supply role is carried out by the
cardiovascular, or blood circulatory, system, with the heart pumping a liquid tissue, the
blood, around the body through vessels. On its route, the blood reaches all parts of
the body, allowing not only the transport of the substances that are vital to the cells
themselves, but also carrying the hormones which help the body function as a coordinated whole, and the white blood cells which enable the immune system to function.
Fluids flow from areas of high pressure to areas of low pressure. By its contraction, the
heart creates high pressure which forces the blood to move away from it, while valves
prevent any backflow into its chambers. The repeated contractions of the heart cause
the blood to flow through the arteries, which are vessels that carry the blood round the
body. Close to the heart, these are thick-walled and muscular to absorb the pressure
waves, but where they divide to supply the tissues they have much thinner walls (and
are known as arterioles).
Although the major arteries contract as each wave of blood passes, pressure is gradually
lost as the blood moves towards the arterioles. However, there is still a higher pressure
in the finest branches of the arterial system than in the tissues. At this point are found
the smallest vessels, the capillaries, where fluid is forced out into the tissues and bathes
the cells.
The tissue fluid then returns to the capillaries, which join to form small veins (venules)
and then the veins which return the blood to the heart. About 1% of the tissue fluid goes
into the lymphatic system before returning to the blood.
The sections which follow in this topic consider in more detail the structure of these
blood vessels and the process of exchange of materials with the cells.
It should be noted that there are other ways to achieve some of these tasks. Insects, for
example, which in terms of diversity of form are the most successful type of animal ever
to evolve, supply air almost directly to their cells through a system of air capillaries. The
blood of an insect is left to carry out the other less urgent transport functions.
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Why have a cardiovascular system: Questions
Q1: Complete the paragraph about the requirements of living cells by using the words
from the list.
to provide
Living cells need to take in a respiratory
power metabolism. They also require raw materials such as
. Waste materials such as
and products like
must be removed.
to
acids and
Word list: ATP, amino, carbon dioxide, fats, hormones, substrate.
..........................................
Q2: Re-arrange the following statements to put them into the correct order to describe
the circulation of the blood.
• Venules join to form veins to carry blood back to heart
• Cells exchange substances with tissue fluid
• Arteries divide to form smaller arterioles
• The heart contracts, increasing the pressure of the blood
• Blood in arterioles is at higher pressure that fluid in tissues
• Tissue fluid returns to capillaries
• Fluid is forced from capillaries into tissues
• Blood flows through major arteries
• Capillaries join to form venules
• Muscular walls contract to maintain blood pressure
..........................................
6.2
Blood vessels
Learning Objective
By the end of this section, you should be able to:
• describe the structure of the main types of blood vessel;
• state the function of each type of blood vessel.
In the simplest terms, arteries carry blood under high pressure from the heart to the
tissues, capillaries exchange materials with the tissues, and veins carry blood back to
the heart under low pressure. The key features of the structure of each type of vessel
relate directly to these functions.
Transfer into and out of the tissues only takes place in the beds of capillaries; arteries
and veins are the pipe-work through which the blood is transported.
All of the vessels are tubes with walls composed of different tissues dependent of the
type of vessel. The central space is called the lumen and it is bounded by a layer of
cells called the endothelium.
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6.2.1
Arteries
Arteries distribute blood from the heart to tissues of the body. To achieve this flow, the
heart creates high hydrostatic pressure by contracting and forcing blood into the major
arteries which leave each side of the heart. The structure of these arteries must absorb
this high pressure, maintain the flow of the blood, and minimise any obstruction to flow.
The structure of an artery
The elastic fibres in the outer and middle layers allow the artery wall to stretch as each
wave of blood arrives, and to contract as the wave passes, so helping to maintain flow.
The layer of smooth muscle allows the diameter of the artery to be reduced to control
flow through the vessel.
The endothelium is not elastic, so when the lumen is reduced by the contraction of the
elastic fibres, it becomes folded.
As the blood gets further from the heart, the pressure falls because energy has been
dissipated during the process of stretching the vessels walls and as a result of friction
against the walls. Accordingly, the thickness of the different layers in the walls is
reduced. The smallest arteries, the arterioles, supply blood to the capillaries where
exchange with the tissues takes place.
Arteries: Question
Q3: Complete the sentences about artery structure by matching the phrases on the
left with the phrases on the right.
The centre of all vessels is the
connective tissue with elastic fibres.
All vessels are lined by
elastic walls of the arteries.
The outer layer of the artery wall is
endothelium.
Smooth muscle and elastic fibres make
up the
lumen.
The surge of blood from the heart is
middle layer of the artery wall.
accommodated by the
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6.2.2
93
Capillaries
All tissues of the body contain capillaries which radiate out from the arterioles to form
a bed, such that no cell is generally more than about 20 microns from a capillary. This
is the point at which materials enter or leave the circulation, and so, unlike arteries or
veins, capillaries have walls which allow the movement of fluid between the blood and
the surrounding tissue.
Accordingly, the wall of the capillary consists of only one layer of endothelium cells and
a surrounding layer of connective tissue cells. The diameter of the lumen is only wide
enough, at 8 microns, to allow the passage of a single red blood cell at a time. The
functional length of each capillary, i.e. where transfer can occur, is roughly 20 times its
diameter (this is very variable).
The structure of a capillary
A single arteriole may supply many capillary beds, and the entry of blood to each bed is
controlled by smooth muscles in the arteriole walls. Bypass arterioles (vascular shunts)
are also present so that when vasoconstriction reduces the flow into the capillary bed,
blood passes directly from arteriole to venule.
Capillaries: Question
Q4: Complete the sentences about capillary structure by matching the phrases on the
left with the phrases on the right.
A capillary
allows blood to flow into the capillary bed.
A capillary wall
exchanges materials with the tissues.
Vasoconstriction
consists of two layers of cells: endothelium and
connective tissue.
Vasodilation
involves the contraction of the smooth muscle in artery
walls.
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6.2.3
Vasoconstriction and vasodilation
The smooth muscle in the walls of the medium and small arteries is also used to control
the flow of blood. We probably all remember our fingers and toes being numb with cold
when we went sledging or snowballing. That effect is the result of the blood supply being
shut down to the extremities to reduce heat loss. In the same way, our faces go white as
blood is diverted away from our skin and into our muscles when we are very frightened
or angry.
These effects are caused by the sustained contraction of the smooth muscle in the walls
of the arteries and arterioles involved in the supply of blood to the skin. This is known as
vasoconstriction and it is used to close down the flow of blood into the capillary beds
supplied by these arteries.
When it is required to resupply the tissues with blood, the muscles relax and blood flows
again into the capillaries. This is known as vasodilation and can be accompanied by
pain, e.g. when it is numb fingers that are involved!
If you think about this, you will notice that it means that there must be more space in
the blood vessels than there is blood to fill it, otherwise it would be impossible to reduce
flow to particular areas. The balancing of this process is primarily carried out by the
autonomic nervous system, and so it is not under our voluntary control. Many other
factors may be involved, including hormones circulating in the blood (e.g. adrenaline) or
changes within the blood vessels themselves (e.g. damage to the vessel walls).
6.2.4
Veins
The function of the veins is to return blood to the heart from the tissues. However, this is
not achieved directly by the contraction of the heart. By the time the blood returns to the
venules and veins, it is under very low pressure. The result is that the force of gravity
tends to keep the blood in the legs, arms and abdomen in a standing person.
Pressure is exerted on the veins by the surrounding muscles, e.g. during walking or by
the action of breathing, and the presence of valves in the veins ensures that the resulting
flow is back towards the heart. When muscles contract, they get shorter and fatter which
causes them to press against the veins; when we breathe out, we increase the pressure
in the thorax and abdomen to force the air out of the lungs.
The structure of veins
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Veins have a larger lumen in comparison to arteries at an equivalent point in the
circulation. The middle and outer layers of the wall are also thinner and, most obviously,
contain valves at regular intervals. The valves work like a one-way door: they allow the
blood to flow back towards the heart, shutting to prevent any flow away from the heart.
The difference in blood pressure in the arteries and veins is mirrored in the rate of flow,
and in the distribution of blood between the different parts of the circulatory system. At
any one time, the heart, arteries and capillaries contain about a third of the blood, the
other two thirds being in the veins.
Veins: Question
Q5: Complete the sentences about vein structure by matching the phrases on the left
with the phrases on the right.
Veins carry the blood
to flow through veins in only one direction.
Valves cause the blood
connective tissue with elastic fibres.
Veins have a much thinner
back to the heart under low pressure.
Veins have an outer layer of
muscular wall than arteries.
..........................................
6.3
Exchange of materials between the blood and the cells
Learning Objective
By the end of this section, you should be able to:
• describe the movement of materials in and out of the blood circulation;
• state the substances which move into and out of the blood and tissues;
• state how lymph is formed and what happens to it.
The capillaries are the only vessels in which movement of materials between the blood
and the tissues occurs. The thin wall acts as a filter which allows the liquid fraction of the
blood, the plasma, to pass into the surrounding tissue where it forms the tissue fluid
(interstitial fluid) that bathes the cells.
In most parts of the circulation, the wall acts like a fine sieve, allowing the water from
the plasma, carrying glucose, oxygen, and other dissolved substances, to pass out of
the capillary. Note that, although the vast majority of the oxygen in the blood is carried
by haemoglobin in the red blood cells, to pass into the tissues oxygen must dissociate
from the haemoglobin and pass into solution. Large plasma protein molecules and cells
are too big and are kept in the blood.
A moment's reflection will suggest that this cannot be the whole story for, after all, the
blood is full of red blood cells which must be both added to the blood and removed from it.
Thus, in the red bone marrow, the capillaries contain much larger pores which allow the
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TOPIC 6. BLOOD VESSELS
passage of cells. In other areas, passage out of the capillaries is relatively much more
restricted, e.g. in the brain. Here, the capillaries are only permeable to oxygen and ions;
larger molecules, e.g. glucose, only pass out of the lumen by active transport. This is
the 'blood-brain barrier' which allows very strict regulation of the conditions in the central
nervous system.
The blood contains water, cells of various types (red blood cells, white blood cells,
platelets), and a wide variety of other substances including carbon dioxide, glucose,
amino acids, fatty acids, lactic acid, urea, various ions, hormones, and proteins.
The proteins carry out a wide variety of functions, e.g. as antibodies (immune system),
fibrinogens (blood clotting), enzymes, and hormones. However, the largest proportion
(60%) of the blood proteins are the albumins which act as carrier molecules (for some
hormones and fatty acids) and, most importantly, maintain the osmotic pressure of the
blood.
6.3.1
The mechanism that causes flow in and out of capillaries
The diagram shows the pressure differences which cause the flow of fluid out of and into
the capillaries. These are measured in mmHg, the standard unit used in medicine for
blood pressure.
The flow in and out of a capillary
The movement of the fluid is caused by the difference between the hydrostatic pressure
in the blood, resulting from the contractions of the heart, and the osmotic pressure,
caused by dissolved substances (principally albumins in the blood). On the arterial side
of the capillary bed, the hydrostatic pressure exceeds the osmotic pressure in the blood
which means that fluid is forced out into the tissues.
As the blood passes through the capillary, the loss of fluid causes the hydrostatic
pressure to drop until, at the venous end of the bed, the fluid is flowing back into the
vessel as the osmotic pressure has remained largely unchanged. This is because the
albumin responsible has stayed in the blood.
The returning tissue fluid carries with it the carbon dioxide released by the respiration
of the cells, any wastes from their metabolism (e.g. lactic acid, urea), and any products
(e.g. hormones). The waste products are then carried around the body until they reach
the organs which excrete them (e.g. carbon dioxide in the lungs, urea in the kidneys).
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TOPIC 6. BLOOD VESSELS
6.3.2
97
Lymph vessels
Most tissue fluid returns to the capillaries, but up to 10% drains into the lymphatic
system through a system of lymph capillaries. Although this is sometimes described
as excess tissue fluid, this flow is an essential part of the circulatory system because
of its importance in the functioning of the immune system. The fluid in the lymphatic
vessels is called lymph, but it is just tissue fluid by another name!
There are a few differences: lymph contains more white blood cells, especially when
it leaves lymph nodes, and the lymph formed in the small intestine is white with fats
(triglycerides).
The lymph capillaries join to form lymph vessels which lead to lymph nodes. These are
dispersed throughout the body, but are especially frequent in the chest, neck, pelvis,
armpit, groin and intestinal regions. These are bean-shaped structures which contain
a matrix of connective tissue, enmeshing a variety of white blood cells (especially
lymphocytes). The principal locations give a clue to the function of the lymph nodes;
they are found where lymph will have been collected from parts of the body which could
have been invaded by bacteria.
Other lymph vessels carry the processed lymph away from the nodes; it is eventually
returned to the blood circulation in veins that are close to the heart (the subclavian
veins).
The pressure of the lymph is even lower than that of the blood in the veins. Like veins,
lymph vessels contain valves and the pressure of muscles contracting in the surrounding
tissues forces the lymph to flow. In addition, there is also peristaltic action by the muscle
layers in the vessel walls.
Lymph vessels: Question
Q6: Complete the sentences about capillay bed exchange by matching the phrases on
the left with the phrases on the right.
Fluids leave capillaries because
large protein molecules.
Fluids re-enter the capillaries because
capillaries in the tissue fluid.
Lymph is formed from
oxygen, glucose and other materials.
Lymph vessels return lymph to
the pressure is higher than the tissue
fluid.
The waste and other products of cells
return to the
tissue fluid, which enters the lymphatic
capillaries.
Tissue fluid is similar to blood plasma
but lacks
veins near the heart.
the osmotic pressure exceeds the
hydrostatic pressure.
..........................................
Tissue fluid supplies cells with
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6.4
Learning points
Summary
General
• The blood circulation carries blood from the heart, through arteries to the
tissues, and back through veins to the heart.
• Blood pressure decreases with distance from the heart.
• All blood vessels have a central lumen lined by the endothelium.
Arteries
• Compared to veins, arteries have thicker muscular walls.
• The outer layer of artery walls is made of connective tissue with elastic
fibres.
• The middle layer of artery walls contains smooth muscle and elastic fibres.
• The contractions of the heart create surges of blood under high pressure.
• The elastic fibres in the artery walls allow them to stretch and recoil as these
surges of blood pass.
• Contraction of the smooth muscle in the middle layer of the artery
wall reduces the diameter of the lumen of the artery; this is called
vasoconstriction.
• Vasoconstriction reduces the flow of blood into capillary beds.
• Relaxation of the smooth muscle of the middle layer of the artery wall
causes the opposite effect; this is called vasodilation.
• Vasodilation increases the diameter of the lumen and allows blood to flow
into a capillary bed.
Capillaries
• Capillaries allow exchange of substances with the tissues.
• The wall of the capillary is very thin, consisting of a layer of endothelium
cells and a surrounding layer of connective tissue.
Veins
• Compared to arteries, veins have thinner walls with much less smooth
muscle.
• Veins contain valves which ensure blood only flows in one direction, back to
the heart.
• Blood flow in veins is caused by the contraction of muscles in the vicinity of
the vein, so squeezing the vein and causing the blood to move through the
valves.
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Summary Continued
Exchange of materials
• The pressure difference between the blood in the capillary and the fluid in
the surrounding tissue causes lymph to flow out of the capillary.
• In most capillaries, the escaping fluid consists of water with a variety of
dissolved substances including glucose, oxygen, hormones, amino acids
and mineral ions.
• This escaping fluid becomes tissue fluid.
• Tissue fluid is similar to blood plasma but does not contain plasma proteins.
• Tissue fluid returning to the capillaries carries with it carbon dioxide and
metabolic wastes for excretion.
Lymph vessels
• Ninety percent of the tissue fluid returns to the capillaries.
• Ten percent of the tissue fluid enters lymph vessels.
• Lymph is returned to the veins near the heart.
6.5
Extended response question
The activity which follows presents an extended response question similar to the style
that you will encounter in the examination.
You should have a good understanding of the exchange of materials between the blood
and the body tissues before attempting the question.
You should give your completed answer to your teacher or tutor for marking, or try to
mark it yourself using the suggested marking scheme.
Extended response question: The exchange of materials between the
blood and the body tissues
Give an account of the exchange of materials between the blood and the body tissues
under the headings:
A) movement of fluid in and out of the blood; (6 marks)
B) substances transferred. (4 marks)
..........................................
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TOPIC 6. BLOOD VESSELS
6.6
End of topic test
End of Topic 6 test
Q7:
Complete the paragraph using the words from the list. (15 marks)
to the tissues and
The blood circulation carries blood from the heart through
to the heart. Blood pressure
with distance from
back through
lined by the
.
the heart. All blood vessels have a central
Capillaries allow exchange of substances with the tissues. The wall of the capillary
is very thin, consisting of a layer of endothelium cells and a surrounding layer of
tissue. The
difference between the blood in the
and
causes
to flow out of the capillary.
the fluid in the surrounding
In most capillaries, the escaping fluid consists of water with a variety of dissolved
, oxygen,
, amino acids and mineral ions.
substances including
which is similar to blood plasma but
This escaping fluid becomes tissue
.
does not contain plasma
Tissue fluid returning to the capillaries carries with it carbon dioxide and
wastes for excretion.
Word list: arteries, capillary, connective, decreases, endothelium, fluid, glucose,
hormones, lumen, metabolic, plasma, pressure, proteins, tissue, veins.
..........................................
Q8: Complete the sentences by matching the parts on the left with the parts on the
right. (10 marks)
Thick muscular walls are a feature of
vasodilation.
Arteries are made from an outer layer of connective tissue with
capillaries.
The middle layer of artery walls is made from
vasoconstriction.
Contraction of the smooth muscle of the artery wall is known as
arteries.
The process that increases the diameter of the artery lumen is
elastic fibres.
Thin walls with little smooth muscle are a feature of
lymph.
Blood flow in only one direction is ensured by
veins.
The blood flow in veins is caused by the contraction of
smooth muscle.
90% of tissue fluid returns to the
valves.
A substance that is returned to veins near the heart is
surrounding
muscle.
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Q9: Select the correct words from the alternatives provided to complete the following
sentences. (4 marks)
Blood travels from the heart to the tissues in arteries / capillaries / veins.
As blood gets further from the heart, the pressure in the vessels increases / decreases
/ stays the same.
The centre of all blood vessels is lined by the connective tissue / endothelium / lumen.
Compared to veins, arteries have similar / thicker / thinner muscular walls.
..........................................
Q10: What is the function of the elastic fibres in the outer layer of the artery walls? (2
marks)
..........................................
Q11: Name the effect caused by the contraction of the muscle cells in the middle layer
of the artery wall. (1 mark)
..........................................
Q12: What is the purpose of the contraction of this tissue? (2 marks)
..........................................
Q13: Under what circumstances might this occur? (2 marks)
..........................................
Q14: In which blood vessels does exchange with the tissues take place? (1 mark)
..........................................
Q15: What feature of these vessels allows them to carry out this function? (1 mark)
..........................................
Q16: Explain how blood moves through veins. (3 marks)
..........................................
Q17: What causes fluid to leave the blood and bathe the cells? (2 marks)
..........................................
Q18: What name is given to the fluid that bathes the cells? (1 mark)
..........................................
Q19: How does the composition of this fluid differ from blood plasma? (1 mark)
..........................................
Q20: Name two substances that cells might release into this fluid. (1 mark)
..........................................
Q21: What percentage of tissue fluid is returned directly to the blood? (1 mark)
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Q22: Into which vessels does the excess fluid drain? (1 mark)
..........................................
Q23: Where does this excess fluid return to the blood circulation? (1 mark)
..........................................
Q24: What name is given to this excess fluid? (1 mark)
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Topic 7
Structure and function of the heart
Contents
7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.2 The structure of the heart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
104
104
7.3 The human circulatory system . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.4 The control of heart rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
106
108
7.4.1 Effect of exercise on the body . . . . . . . . . . . . . . . . . . . . . . . .
109
7.4.2 Effect of exercise on cardiac output . . . . . . . . . . . . . . . . . . . . .
7.5 The cardiac cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
109
111
7.6 The cardiac conducting system . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.6.1 Control of the heartbeat . . . . . . . . . . . . . . . . . . . . . . . . . . .
114
114
7.6.2 Electrocardiograms (ECGs) . . . . . . . . . . . . . . . . . . . . . . . . .
7.7 Blood pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
117
119
7.8 Learning points . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.9 Extended response question . . . . . . . . . . . . . . . . . . . . . . . . . . . .
124
127
7.10 End of topic test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
127
Learning Objectives
By the end of this topic, you should be able to:
• describe the structure of the heart and the blood circulatory system;
• explain how blood is pumped through the heart - the cardiac cycle;
• describe how the function of the heart is controlled;
• explain how blood pressure is measured.
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7.1
Introduction
The pressure which causes the blood to flow round the body's arteries is provided by
the contractions of the heart. This structure is largely composed of a type of muscle,
called cardiac muscle, which is unique in the body:
• it comprises individual cells like smooth muscle, but unlike it the cells are striped
in the same way as skeletal muscle;
• it contracts constantly, at a resting rate of between 60 and 90 beats per minute
(bpm);
• it does not tire, and in the average human lifetime will contract roughly 3 × 10 9
times;
• the cardiac muscle cells are myogenic, contracting spontaneously to their own
internally generated rhythm.
7.2
The structure of the heart
Learning Objective
By the end of this section, you should be able to:
• identify the main features of the structure of the heart;
• name the blood vessels entering and leaving the heart;
• explain the function of the valves in the heart;
• explain the difference between the left and right ventricles;
• state that equal volumes of blood flow through each side of the heart.
The heart is a muscular pump which keeps blood flowing continuously in one direction
round the body. It starts pumping before we are born and can even continue pumping
for a short time after death.
The heart is a muscular bag which consists of four chambers through which blood flows.
The right and left sides of the heart are completely separated from each other by the
septum. The two upper chambers are called atria, while the lower chambers are known
as ventricles.
The left atrium receives oxygenated blood (bright red in colour) from the lungs via two
pulmonary veins. The blood flows into the left ventricle through the bicuspid valve. When
the ventricle contracts, the blood is forced out through the semilunar valve into the
aorta, the largest artery in the body. The blood is passed around the body, materials
are exchanged and deoxygenated blood (blue in colour) flows back to the right atrium
via two main veins called the superior and inferior venae cavae (superior means 'above'
while inferior means 'below'). The blood flows through the tricuspid valve into the right
ventricle, and from there into the pulmonary artery which carries it to the lungs.
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The bicuspid and tricuspid valves are known as the atrio-ventricular valves. They prevent
the backflow of blood into the atria when the ventricles contract. These valves are
attached to the ventricle walls by tendons, called chordae tendinae, which prevent the
valves inverting (turning inside-out) when the ventricles contract. The semilunar valves
(found at the entrance to the aorta and pulmonary artery) prevent blood flowing back
into the ventricles when they relax.
The walls of the atria are thinner than those of the ventricles because they only have
to force blood a very small distance into the ventricles. The walls of the ventricles are
thicker because they have to provide enough force to pump the blood further. The wall
of the left ventricle is thicker than that of the right since the right ventricle only needs
to pump blood to the lungs (which are next to the heart) while the left ventricle has to
pump blood all round the body. The volume of blood passing through the two sides of
the heart is, of course, the same!
The structure of the heart: Questions
Q1: Complete the diagram which shows the internal structure of the heart.
30 min
..........................................
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Hint: when labelling the sides of the heart imagine this is your heart being viewed from
the front.
Q2:
Why are the bicuspid and tricuspid valves also known as atrio-ventricular valves?
..........................................
Q3:
What is the function of valves in the heart?
..........................................
Q4:
What is the function of the chordae tendinae?
..........................................
..........................................
7.3
The human circulatory system
Learning Objective
By the end of this section, you should be able to:
• identify the vessels carrying blood to and from the heart, head, kidneys, liver
and small intestine.
Although a detailed knowledge of the blood supply to the various parts of the body
is not specified at this point in the syllabus, this section is included as it provides the
background which is essential to the understanding of other parts of the course.
Oxygenated blood is pumped out of the left ventricle into the aorta. Almost immediately,
the aorta begins to divide into various other arteries including:
• the coronary arteries suppling the heart muscle itself;
• the carotid artery carrying blood to the head and brain;
• the hepatic artery supplying the liver;
• the renal arteries supplying the kidneys.
At the same time, deoxygenated blood is pumped from the right ventricle into the
pulmonary artery which carries it to the lungs to be reoxygenated. From here, the
oxygenated blood is carried back to the left atrium via the pulmonary veins.
As blood passes through the body organs other than the lungs, it loses oxygen and
becomes deoxygenated. Various veins, for example the jugular vein from the head, the
hepatic vein from the liver, and the renal veins from the kidneys, carry this deoxygenated
blood back to the right atrium of the heart.
An additional vein is found between the intestines and the liver. Known as the hepatic
portal vein, it carries blood containing the products of digestion from the intestine to the
liver.
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The human circulatory system: Questions
Q5: Complete the diagram which shows the circulatory system.
20 min
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..........................................
Q6: Complete the table to show the arteries and veins associated with various parts
of the body.
Organ/area of body
Associated artery
Associated vein
heart
head
kidney
liver
small intestine
Artery/vein list: carotid artery, coronary artery, coronary vein, gut arteries, hepatic artery,
hepatic portal vein, hepatic vein, jugular vein, renal artery, renal vein.
..........................................
7.4
The control of heart rate
Learning Objective
By the end of this section, you should be able to:
• explain the role of the autonomic nervous system in the control of heart rate;
• describe the effect of hormones and exercise on heart rate;
• state what is meant by the terms cardiac output, heart rate and stroke volume;
• explain the calculation of cardiac output.
The heart rate is controlled by nerves from the autonomic nervous system which
is controlled by centres in the medulla of the brain and consists of two parts: the
sympathetic and the parasympathetic which act antagonistically to each other; that is
they produce opposite effects to each other.
Nerves from the sympathetic part of the autonomic nervous system increase the rate at
which the heart beats by the secretion of noradrenaline (norepinephrine). Nerves from
the parasympathetic part of the autonomic nervous system decrease the heart rate by
the secretion of acetylcholine.
The hormone adrenaline produced by the adrenal glands also increases the heart rate.
The following illustration shows the effect that the sympathetic and parasympathetic
systems have on the heart rate.
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TOPIC 7. STRUCTURE AND FUNCTION OF THE HEART
The effect of sympathetic and parasympathetic nervous systems on heart rate
It can be seen that one heartbeat is longer under the influence of the parasympathetic
nervous system. This means that there will be fewer beats per minute, therefore
the heart rate is slower when the heart is stimulated by nerve impulses from the
parasympathetic system.
7.4.1
Effect of exercise on the body
During exercise, the metabolic rate of skeletal muscles increases sharply, therefore cells
need much more glucose and oxygen than they would at rest. Cells also produce more
carbon dioxide than they would at rest. These changes interrupt the 'steady state' of
the body which brings about changes to the respiratory system and the cardiovascular
system. As a result, the body is returned to its 'steady state'.
7.4.2
Effect of exercise on cardiac output
The stroke volume is the volume of blood pumped out of the heart during one heartbeat.
The heart rate is the number of heartbeats produced per minute. Cardiac output is the
volume of blood pumped out of the heart per minute.
Cardiac output = stroke volume × heart rate
During exercise, the increased levels of carbon dioxide in the blood are detected by the
medulla, aorta and carotid arteries. The medulla increases the number of sympathetic
nerve impulses being sent to the pacemaker (sino-atrial node) in the heart. These
increased sympathetic nerve impulses increase both the heart rate and stroke volume
of the heart. This increases the cardiac output so that glucose and oxygen can be
transported faster to the skeletal muscles. The sympathetic nerves also stimulate the
adrenal glands to secrete adrenaline which further stimulates the sino-atrial node in the
heart.
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How exercise affects the body
The following illustrates the immediate effects of exercise on the body.
10 min
Muscles are working at a higher intensity than normal and require more energy.
To provide the extra energy to pump blood around the body more quickly, the breathing
becomes deeper and faster to get more air into the lungs.
The rate of metabolism also increases in order to metabolise energy stores
(carbohydrates and fats) to produce ATP.
Sweat is produced to cool the body down.
Lactate is produced during anaerobic exercise.
..........................................
Effect of exercise on cardiac output: Question
Complete the table by working out the cardiac output for each of the body states.
Remember that:
Cardiac output (l/min) = (stroke volume (ml) × heart rate (bpm)) / 1000
Make sure that you look at the units at the top of each column.
Q7:
State of body
Stroke volume
(ml)
Heart rate
(bpm)
at rest
60
60
during gentle
exercise
70
120
during strenuous
exercise
80
180
Cardiac output
(l/min)
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7.5
The cardiac cycle
Learning Objective
By the end of this section, you should be able to:
• state the sequence of contraction and relaxation of the muscular walls of the
heart during systole and diastole;
• similarly, state the sequence and cause of the opening and closing of the heart
valves;
• explain the heart sounds heard through a stethoscope.
The cardiac cycle refers to the pattern of contraction and relaxation of the heart during
one complete heartbeat. Contraction of the heart muscle is known as systole while
relaxation is known as diastole; the complete cycle of systole and diastole is one heart
beat. If an individual has a heart rate of 75 beats per minute, then the average length of
time of one heartbeat is 0.8 seconds.
The atria (upper chambers) contract a fraction of a second before the ventricles contract.
During atrial systole, the two atria contract simultaneously, the atrio-ventricular valves
(bicuspid and tricuspid valves) are open, and blood is forced through into the ventricles.
At this point, the ventricles are relaxed (in diastole) and the semilunar valves are closed.
Atrial systole is followed about 0.1 seconds later by ventricular systole. The atrioventricular valves are closed and blood is forced out through the semilunar valves into
the arteries.
It is the opening and closing of the atrioventricular (AV) and semilunar (SL) valves that
create the heart sounds which are heard with a stethoscope. This apparatus collects
sound waves in a diaphragm-covered bell and carries them through a plastic tube to a
pair of earpieces. It has been modified since its invention in the 1850s to enhance the
detection of particular frequencies, yet it still remains one of the simplest, but most
symbolic, items of medical equipment. With experience, it can be used to provide
information about the functioning of not only the heart, but also the lungs, intestines,
and the blood flow in the arteries and veins.
During atrial and ventricular diastole, blood from the pulmonary veins and the venae
cavae fill up the atria. Then the cycle repeats.
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The contraction of the chambers of the heart generates pressure differences in the blood
which cause the opening and closing of the various heart valves. During diastole, when
the muscle of all the chambers is relaxed, the pressure of blood flowing in from the vena
cava and the pulmonary vein opens the AV valves and blood flows into the atria and
through into the ventricles.
During atrial systole, the muscle of the atrial walls contracts. This closes the veins
entering the chambers and increases the pressure in the atria relative to that in the
ventricles. As a result, the ventricular walls are stretched as more blood is forced into the
ventricles. When ventricular systole begins, the atrial muscle relaxes and the pressure
difference is reversed. This forces shut the AV valves, preventing back flow into the atria,
and, at the same time, pushes open the semilunar valves, allowing blood to flow into the
aorta and the pulmonary artery.
When the ventricles relax and the heart enters diastole, the blood pressure in the
arteries leaving the heart forces shut the semilunar valves, ensuring that blood flows
off to the body and lungs rather than back into the ventricles.
The cardiac cycle: Questions
Q8: What is meant by the terms systole and diastole?
..........................................
Q9: During atrial systole, which heart valves are open and which are closed?
..........................................
Q10: During which process, ventricular systole or diastole, is the pressure greater in
the ventricles? Give a reason for your answer.
..........................................
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7.6
The cardiac conducting system
Learning Objective
By the end of this section, you should be able to:
• state that heart muscle is myogenic;
• explain the role of the pacemaker (SAN) in co-ordinating the contractions of the
heart;
• state that the autonomic nervous system and hormones regulate heart rate;
• state the role of the atrioventricular node (AVN);
• describe an electrocardiogram and explain what it shows.
The mechanism of controlling the beating of the heat is described first, followed by a
description of electrocardiograms and their diagnostic value.
7.6.1
Control of the heartbeat
Cardiac muscle is different from other types of muscle in that it is myogenic. This
means that it does not need an electrical impulse in order to contract. However the
synchronisation of the heartbeat and the rate at which it beats is controlled by the
nervous system. This synchronisation is controlled by an inbuilt pacemaker and the
conducting system of nerves in the heart itself.
The conducting system of the heart
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The pacemaker is properly known as the sino-atrial (SA) node and is located in the
wall of the right atrium, close to the entrance of the superior vena cava. This SA node
generates rhythmical waves of electrical impulses which spread throughout the walls
of the atria, making them contract. The cells of the SAN, although modified muscle
cells, do not themselves contract. They are autorhythmic, sending out between 60 and
100 impulses per minute without any external stimulation. The effect of the autonomic
nervous system is to vary this rate.
Between the atria and ventricles there is another node called the
atrio-ventricular (AV) node, which passes the electrical impulses on to a network
of conducting fibres which spread throughout the walls of the ventricles. The wave
of electrical impulses is transmitted very quickly down to the base of the septum (in
the 'bundle of His'), then upwards and outwards throughout the ventricle walls (by the
Purkinje fibres). This causes the ventricles to contract from the bottom up (the most
efficient way of emptying them).
This coordination ensures that the atria contract a fraction of a second before the
ventricles and enables both the atria and ventricles to fill up with blood before they
contract. In this way blood enters and leaves the heart efficiently.
Control of the heartbeat: Steps
The following shows the steps involved in the process of controlling the heat beat.
10 min
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..........................................
The cardiac conducting system: Questions
Q11: Where is the sino-atrial node found in the heart?
..........................................
Q12: Where do the conducting fibres initiate contraction of the ventricles? Why is this
important?
..........................................
7.6.2
Electrocardiograms (ECGs)
Learning Objective
By the end of this section, you should be able to:
• state that an electrocardiogram shows the electrical activity in the heart muscle
as the wave of excitation spreads first across the atrial walls and then through
the walls of the ventricles;
• state that, in a healthy heart, a single beat is represented by a series of peaks
and troughs reflecting first atrial, then ventricular systole, and finally a region of
low electrical activity corresponding to diastole.
An electrocardiogram (ECG) is a graph showing the electrical activity in the heart
muscle. This activity can be detected by electrodes placed on the skin of the chest
and shown on a machine called an electrocardiograph.
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Diagram (a) shows an electrocardiogram of a healthy heart. In the diagram, the P wave
corresponds to a wave of excitation spreading over the atrial walls, the QRS and T waves
correspond to a wave of excitation spreading over the walls of the ventricles. Notice that
the pattern repeats at regular intervals.
Diagram (b) shows an electrocardiogram of a damaged heart. Notice that there is no
obvious regular pattern. This heart is said to be in ventricular fibrillation (VF). What has
happened here is that the coordination of the heartbeat has been interrupted. Instead of
the electrical impulses from the atrio-ventricular node passing down the Purkinje fibres
in an ordered way, they have become chaotic. Rather than there being one coordinated
wave of contraction from the bottom up, followed by a coordinated wave of relaxation,
some parts of the ventricle walls are contracting while others are relaxing.
The effect is called fibrillation and results in little blood being pumped out of the heart.
Fibrillation can be fatal unless it is treated immediately by passing a strong electric
current through the chest wall to the heart. This usually stops the heart completely for a
few seconds, after which it often begins to beat again in a coordinated way.
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119
Blood pressure
Learning Objective
By the end of this section, you should be able to:
• state that blood pressure changes in the arteries as a result of the cardiac cycle;
• state that blood pressure is measured by a sphygmomanometer;
• explain the use of the sphygmomanometer;
• state a typical blood pressure reading and explain it;
• state that hypertension is a major risk factor for many diseases including
coronary heart disease.
Blood flows through the vessels of the circulatory system as a result of the
pressure generated when the heart contracts. Blood pressure is measured using a
sphygmomanometer. This instrument measures both the systolic pressure (when the
heart contracts) and the diastolic pressure (when the heart relaxes). Because the
heart pumps blood into arteries, the blood pressure is greatest in arteries. As blood
is forced along the arteries there is little decrease in pressure because the arteries
have elastic walls which stretch during ventricular systole and recoil during ventricular
diastole, continuing to force the blood along them.
Blood pressure is measured most accurately just above the elbow on the upper arm,
although measurements can also be taken at the wrist or even the finger. The unit is
the millimetre of mercury (mmHg), which sounds like a rather archaic unit compared
to the pascal or the torr. It stems from the original method of determining pressure by
measuring the height of the column of mercury that was supported in the U-tube of a
manometer.
Measurements are made of the systolic and diastolic pressure, and a person's blood
pressure is expressed as systolic/diastolic, e.g. a typical reading for a young adult would
be 120/70mmHg. A reading in excess of 140/90mmHg is classified as hypertension,
which is a major risk factor for many diseases, including coronary heart disease.
However once blood reaches the smaller arterioles the blood pressure decreases as
a result of the general increase in surface area provided by the greater number of
arterioles. Imagine water in a large pipe being passed into a network of smaller but
more numerous pipes; the pressure in each of the smaller pipes would be less than
the pressure in the single large pipe as the water is spread out. Also, as a result of
the smaller arterioles' inner diameter, the rate at which the blood flows decreases - this
further decreases the blood pressure.
As the arterioles divide further into capillaries the rate of blood flow and blood pressure
continue to decrease. Remember it is here that exchange of materials between the
blood and tissues occur, so a slow rate of flow is desirable here.
Blood pressure continues to decrease as blood is passed into venules and veins. This
is why veins need to have valves present to prevent the backflow of blood.
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Measuring blood pressure: Steps
The following steps show the process of measuring blood pressure.
10 min
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Blood pressure: Questions
Q13: When is the blood pressure lowest?
10 min
a) During systole
b) During diastole
..........................................
Q14: Margaret has a systolic blood pressure of 130 and a diastolic blood pressure of
85. How should her blood pressure be recorded?
a) 85/130
b) 130/85
..........................................
Q15: Tom's blood pressure is 140/90. What is his systolic blood pressure in mm Hg?
..........................................
Q16: A woman's blood pressure is measured as 165/95. How would you describe her
blood pressure?
a)
b)
c)
d)
Low
Normal
High
Hypertensive
..........................................
Q17: Why do you think the systolic value is higher than the diastolic value?
..........................................
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TOPIC 7. STRUCTURE AND FUNCTION OF THE HEART
Q18: Complete the graph, which shows how blood pressure varies in arteries,
capillaries and veins, using the labels provided.
..........................................
Q19: Can you explain why the blood pressure oscillates (goes up and down) in the
arteries? (Hint - think about the heartbeat.)
..........................................
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7.8
Learning points
Summary
The structure of the heart
• The heart consists of four chambers, two (upper) atria and two ventricles.
• Blood enters the heart from the body into the right atrium via the (superior
and inferior) vena cava.
• Blood leaves the heart for the lungs from the right ventricle via the
pulmonary artery.
• Blood enters the heart from the lungs into the left atrium via the pulmonary
vein.
• Blood leaves the heart for the body from the left ventricle via the aorta.
• The atrioventricular valves prevent backflow from the ventricles into the
atria.
• The semilunar valves prevent backflow from the aorta and pulmonary artery
into the ventricles.
• The same volume of blood is pumped through each ventricle.
The human circulatory system
• The heart muscle is served by the coronary artery and vein.
The control of heart rate
• Heart rate is externally regulated by the autonomic nervous system and by
hormones.
• The medulla of the brain raises the heart rate through the sympathetic
nervous system.
• The medulla of the brain lowers the heart rate through the parasympathetic
nervous system.
• The sympathetic
antagonistically.
and
parasympathetic
nervous
systems
act
• Nerve cells of the sympathetic nervous system accelerate heart rate by
secreting noradrenaline (norepinephrine).
• Nerve cells of the parasympathetic nervous system slow heart rate by
secreting acetylcholine.
• The hormone adrenaline increases heart rate.
• Exercise increases heart rate.
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TOPIC 7. STRUCTURE AND FUNCTION OF THE HEART
Summary Continued
• Cardiac output is the volume of blood leaving each ventricle in one minute
(ml/min).
• Heart rate is the number of times the heart beats in one minute (bpm).
• Stroke volume is the volume of blood pumped from each ventricle during
one heartbeat (ml).
• Cardiac output (l/min) = (stroke volume (ml) × heart rate (bpm)) / 1000
The cardiac cycle
• The cardiac cycle is the pattern of contraction and relaxation of the heart
during one complete heartbeat.
• Systole refers to contraction of the muscle and diastole to its relaxation.
• Atrial systole starts slightly before ventricular systole, forcing blood through
the AV valves into the ventricles.
• Ventricular systole follows, closing the AV valves and forcing blood through
the SL valves into the pulmonary artery and the aorta.
• During diastole, relaxation of the muscles of the atria and ventricles causes
blood to flow into the atria and through into the ventricles.
• Both atria and ventricles are in diastole for about half of a heartbeat.
• During ventricular diastole, backflow of blood under high pressure from the
arteries is prevented by closure of the SL valves.
• The opening and closing of the AV and SL valves cause the heart sounds
detected with stethoscope.
The cardiac conducting system
• Heart muscle cells are myogenic, i.e. they contract and relax to their own
internal rhythm without external stimulation.
• Heart beating is regulated by the autonomic nervous system and by
hormones.
• The sinoatrial node (SAN), or pacemaker, sets the rate of contraction of the
heart muscle.
• The cells of the SAN are autorhythmic, not contracting themselves but
sending out impulses which co-ordinate the contraction of the heart muscle.
• Impulses from the SAN travel across the atria, causing them to contract
simultaneously, and to the atrioventricular node (AVN).
• The AVN passes the impulses through the ventricles, causing them to
contract simultaneously.
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TOPIC 7. STRUCTURE AND FUNCTION OF THE HEART
Summary Continued
• The impulses from the SAN and the AVN generate currents which are
detected by electrocardiography and recorded on an electrocardiogram.
Electrocardiograms (ECGs)
• state that an electrocardiogram shows the electrical activity in the heart
muscle as the wave of excitation spreads first across the atrial walls and
then through the walls of the ventricles;
• state that, in a healthy heart, a single beat is represented by a series of
peaks and troughs reflecting first atrial, then ventricular systole, and finally
a region of low electrical activity corresponding to diastole.
Blood pressure
• The contraction and relaxation of the heart muscle during the cardiac cycle
cause the variations in blood pressure in the arteries.
• These variations in pressure are detectable in arteries as the pulse.
• Blood pressure is measured by a sphygmomanometer.
• The steps in the use of the sphygmomanometer are:
– an inflatable cuff stops blood flow and deflates gradually;
– blood flow restarts as systolic pressure is reached, and the pulse is
felt;
– once blood flows freely, and the pulse is no longer felt, diastolic
pressure has been reached.
• Blood pressure is expressed as systolic pressure/diastolic pressure.
• A typical reading for a young adult is 120/70mmHg.
• Hypertension is defined as blood pressure above 140/90mmHg.
• Hypertension is a major risk factor for many diseases including coronary
heart disease.
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TOPIC 7. STRUCTURE AND FUNCTION OF THE HEART
7.9
127
Extended response question
The activity which follows presents an extended response question similar to the style
that you will encounter in the examination.
You should have a good understanding of the cardiac cycle before attempting the
question.
You should give your completed answer to your teacher or tutor for marking, or try to
mark it yourself using the suggested marking scheme.
Extended response question: The cardiac cycle
Give an account of the cardiac cycle under the following headings:
15 min
1. atrial systole; (3 marks)
2. ventricular systole; (5 marks)
3. diastole. (2 marks)
..........................................
7.10
End of topic test
End of Topic 7 test
Q20: Complete the sentences by matching the parts on the left with the parts on the
right. (9 marks)
Atria:
vessel which carries blood from the heart to the lungs.
Ventricles:
chamber which pumps blood round the body.
Right ventricle:
artery which carries blood from the heart to the body.
Pulmonary artery:
structures between upper and lower chambers of the heart.
Left ventricle:
vessel which supplies blood to the heart muscle.
Aorta:
lower chambers of the heart.
Atrioventricular
valves:
chamber which pumps blood to the lungs.
Semilunar valves:
upper chambers of the heart.
Coronary artery:
structures which prevent backflow of blood to the heart.
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Q21: Complete the sentences by matching the parts on the left with the parts on the
right. (11 marks)
Autonomic:
secreted by nerves to decrease heart rate.
Medulla:
volume from each ventricle during one heartbeat.
Sympathetic:
secreted by nerves to increase heart rate.
Parasympathetic:
part of the nervous system that increases heart rate.
Antagonistic:
part of the nervous system that controls heart rate.
Noradrenaline:
hormone that increases heart rate.
Acetylcholine:
number of heartbeats per minute.
Adrenaline:
area of the brain that controls heart rate.
Stroke volume:
action of the nervous system that controls heart rate.
Heart rate:
calculated by multiplying stroke volume by heart rate.
Cardiac output:
part of the nervous system that decreases heart rate.
..........................................
Q22: Complete the sentences by matching the parts on the left with the parts on the
right. (10 marks)
Cardiac cycle:
contraction of ventricle muscle.
Atrial systole:
valves which open when the atria contract.
Ventricular systole:
muscle which can contract without external stimulation.
Diastole:
contraction and relaxation of the heart during one complete
beat.
Atrioventricular:
record of impulses generated by the SAN and AVN.
Semilunar:
contraction of the muscle of the atrium.
Myogenic:
co-ordination of the heart muscle contraction.
Sinoatrial node:
valves which open when the ventricles contract.
Ventricular:
relaxation of heart muscle.
Electrocardiogram:
contraction controlled by the atrioventricular node.
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129
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Q23: Complete the paragraph using the words from the list. (12 marks)
The contraction and
of the heart muscle during the
cycle
which are detectable as
cause large variations in blood pressure in the
.
the
Blood pressure is measured using a
. An inflatable
stops blood
and
flow and gradually deflates. As systolic pressure is reached, blood flow
the pulse is felt. Once blood is flowing freely, no pulse is felt and diastolic pressure is
measured.
Blood pressure is expressed as
mm Hg.
reading being
pressure /
pressure, a typical
Hypertension, which is indicated by blood pressure reading of over
heart disease.
is a major risk factor for
mm Hg,
Word list: 120/70, 140/90, arteries, cardiac, coronary, cuff, diastolic, pulse, relaxation,
restarts, sphygmomanometer, systolic.
..........................................
. (1
Q24: The volume of blood pumped by the left and right ventricles is
mark)
..........................................
Q25: The cardiac output of a person with a heart rate of 80 bpm and stroke volume of
. (1 mark)
75 ml is
..........................................
Q26: During atrial systole, the muscle of the right
is
. (2 marks)
..........................................
into the right atrium.
Q27: During diastole, blood flows from the
valves are open and the
valves are closed.
At this stage, the
valves are open and the
valves
During ventricular systole, the
are closed.
and the right ventricle
Blood therefore flows from the left ventricle into the
. (7 marks)
into the
..........................................
Q28: Select the correct words from the alternatives provided to complete the following
sentences. (5 marks)
Cardiac muscle cells are AV node / autorhythmic / electrocardiogram / myogenic /
sinoatrial node.
The cells of the pacemaker are AV node / autorhythmic / electrocardiogram / myogenic
/ sinoatrial node.
The rate at which cardiac muscles contract is set by the AV node / autorhythmic /
electrocardiogram / myogenic / sinoatrial node.
The contraction of the ventricles is triggered by the AV node / autorhythmic /
electrocardiogram / myogenic / sinoatrial node.
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Impulses in the heart can be recorded on an AV node / autorhythmic / electrocardiogram
/ myogenic / sinoatrial node.
..........................................
Q29: The two branches of the autonomic nervous system are said to be
.
of the brain.
They are controlled by the
the rate of the SAN by the secretion of
The sympathetic nervous system
.
the rate of the SAN by the secretion
The parasympathetic nervous system
. (6 marks)
of
..........................................
Q30: What causes the changes in blood pressure in the aorta? (1 mark)
..........................................
Q31: What instrument is used to measure blood pressure? (1 mark)
..........................................
Q32: Which blood pressure is this instrument measuring when a pulse ceases to be
detected? (1 mark)
..........................................
Q33: What is the correct term for high blood pressure? (1 mark)
..........................................
Q34: What unit is used to measure blood pressure? (1 mark)
..........................................
Q35: A blood pressure reading for a middle-aged man was 145/100. Explain what this
means. (1 mark)
..........................................
Q36: Suggest a cardiovascular disease that this man would be at increased risk of
contracting. (1 mark)
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Topic 8
Cholesterol and cardiovascular
disease
Contents
8.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.2 Cholesterol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
132
132
8.2.1 What is cholesterol? . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
132
8.2.2 Functions of cholesterol in the body . . . . . . . . . . . . . . . . . . . .
8.2.3 Transport of cholesterol in the body . . . . . . . . . . . . . . . . . . . .
134
135
8.2.4 Controlling cholesterol levels . . . . . . . . . . . . . . . . . . . . . . . .
8.2.5 Familial hypercholesterolaemia . . . . . . . . . . . . . . . . . . . . . . .
137
138
8.3 Atherosclerosis and associated diseases . . . . . . . . . . . . . . . . . . . . .
8.3.1 Atherosclerosis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
139
139
8.3.2 Thrombosis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.3.3 Peripheral vascular disorders . . . . . . . . . . . . . . . . . . . . . . . .
140
140
8.3.4 ASSIGN score . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.4 Learning points . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
141
143
8.5 Extended response question . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.6 End of topic test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
145
146
Learning Objectives
By the end of this topic, you should be able to:
• describe the chemical nature of cholesterol, its sources and its roles in the body;
• explain the factors which affect levels of cholesterol in the body;
• describe the development and effects of atherosclerosis, thrombosis and
peripheral vascular disease.
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TOPIC 8. CHOLESTEROL AND CARDIOVASCULAR DISEASE
8.1
Introduction
The diseases covered in this topic (atherosclerosis, thrombosis, peripheral vascular
disorders) are all related to the role of cholesterol in the body and its disruption. Despite
its being essential to all animal life, cholesterol is often portrayed in the media as a
chemical responsible for ill-health. This it certainly can also be when, in association with
certain lipoproteins, its concentration in the blood becomes too high, in which case it
contributes to the formation of plaques on the artery walls.
It is this plaque formation which gives rise to atherosclerosis, leading to high blood
pressure, peripheral vascular disorders, angina, cardiac infarction (heart attack) and
stroke.
8.2
Cholesterol
Learning Objective
By the end of this section, you should be able to:
• describe the chemical nature of cholesterol;
• state how cholesterol is formed in, and eliminated from, the body;
• name dietary sources of cholesterol;
• describe some of the roles of cholesterol in the body;
• describe the transport of cholesterol in the body;
• explain how the level of cholesterol may be increased and decreased in the
body;
• explain the cause, effects, detection and treatment of familial hypercholesterolaemia.
This section describes the structure, functions, transport and blood concentration of
cholesterol.
8.2.1
What is cholesterol?
Cholesterol is a waxy solid which is virtually insoluble in water, but easily dissolved
in organic solvents, e.g. acetone and methanol. It is a steroid, a fat, and, like all
fats, cholesterol is composed of carbon, hydrogen and oxygen atoms, with the formula
C27 H46 O. All steroids have a core of twenty carbon atoms, bonded together in the form
of four fused rings.
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TOPIC 8. CHOLESTEROL AND CARDIOVASCULAR DISEASE
The structure of cholesterol
Although up to 75% of the body's supply of cholesterol comes from the diet, cholesterol
is also synthesised within the body. All animal cells make cholesterol, starting with one
molecule each of acetyl CoA and the related acetoacetyl CoA. There follows a sequence
of enzyme-assisted reactions, one of which is sensitive to cholesterol levels and acts as
part of their homeostatic control, which is the point at which statin drugs act.
One type of prescription statin
The body's daily production of cholesterol is roughly 1g for a 70kg person, up to a
quarter of which is produced in the liver. The liver is also responsible for the elimination
of cholesterol as a component of bile. Occasionally, this cholesterol develops into
crystalline accretions in the gall bladder, which are known as gallstones. These oval
structures can be 3cm in length and are the most common type of gallstone.
In the diet, the principal source of cholesterol is the animal fat found in cheese, eggs,
beef, pork, poultry, fish and shellfish. It is not found in plant foods to any significant
extent. Generally, less cholesterol is absorbed from food than is synthesised in the
body. Although consumption of saturated fat has been linked to cholesterol levels,
there is considerable debate about the nature of the connection between the two.
Alternative explanations suggest that lifestyle and consumption of other foods, such as
carbohydrates, as causative factors. It should be remembered that excess consumption
of any energy foods will lead to the build-up of fat stores in the body.
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TOPIC 8. CHOLESTEROL AND CARDIOVASCULAR DISEASE
Cholesterol: Questions
10 min
Q1: Complete the paragraph using the words from the list. (some words may be used
more than once)
compound (a fat). It is synthesised in all cells, but
Cholesterol is a waxy
. Dietary sources of cholesterol are
the largest producer is the
foods, such as
and
products.
fats may lead to increased cholesterol levels.
A diet high in
in the form of
.
Cholesterol is eliminated from the body by the
Word list: animal, bile, dairy, liver, meat, saturated, steroid.
..........................................
Q2: Why is cholesterol only found attached to carrier molecules in the blood and not
free in solution?
..........................................
8.2.2
Functions of cholesterol in the body
Far from being some alien chemical which causes disease, cholesterol is fundamental
to the proper functioning of the body of all animals.
Cholesterol is used to build and to maintain the membranes of the cell. As part of
membrane structure, it controls the permeability of the plasma membrane to hydrogen
and sodium ions as well as other substances.
Within the membrane, cholesterol is also involved in the transport of substances into the
cell (endocytosis), the processing of cell signals (such as antigens or hormones), and
accelerating the conduction of nerve impulses (as part of the myelin sheath).
As an important constituent of bile, cholesterol aids the absorption of vitamins A, D, and
other fat-soluble vitamins. It is also a precursor for the synthesis of vitamin D (in the
skin) and the steroid hormones, e.g. testosterone, oestrogen and progesterone.
Functions of cholesterol: Question
Q3: Complete the table, which contains information about the functions of cholesterol
in the body, using the words from the list. (some words may be used more than once)
Function
Example
Location
Control permeability
Endocytosis
Vitamin D
Interstitial cells
Word list: hydrogen/sodium ions, plasma membrane, precursor, skin, testosterone,
transport into cell.
..........................................
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TOPIC 8. CHOLESTEROL AND CARDIOVASCULAR DISEASE
8.2.3
Transport of cholesterol in the body
Given its near insolubility in water, cholesterol does not circulate freely round the body
but is transported attached to carrier molecules. These are lipoproteins which surround
and contain insoluble lipid molecules, and enable them to be carried in the blood or
across membranes.
The most important lipoproteins involved in cholesterol transport are high- and lowdensity lipoproteins (HDLs and LDLs). The difference in density reflects the relative
proportion of fat in the molecule, fat having a lower density than protein.
HDLs are the smallest of the lipoprotein molecules and have the highest proportion
of protein. They are made in the liver and are released into the blood stream, where
they pick up excess cholesterol from body cells and transport it back to the liver for
elimination. Most significantly, they remove cholesterol from the atheromas, which are
part of the plaques formed in atherosclerosis. This cholesterol is sometimes referred to
as 'good', but is just the same as that found attached to LDLs or, indeed, in atheromas.
As a result, high levels of HDLs can prevent or reduce the accumulation of cholesterol
in the artery walls.
LDLs are also synthesised in the liver, but have a much larger fat component. They
are created in response the presence of the level of fatty acids carried in the blood.
Their function is to carry cholesterol to the body cells. These cells, when they require
cholesterol, make LDL receptors which are located on the plasma membrane. When
attached to a molecule of LDL, they are absorbed into the cell and the cholesterol it
carries is released.
The concentration of cholesterol in the cell acts in a negative feedback loop to control
the transcription of the LDL-receptor gene; a high level of cholesterol will reduce the
production of the receptor molecules, and vice versa. Where there is an excess of LDL
in the blood beyond that required by the body cells, the LDLs may deposit cholesterol in
the endothelium of the artery walls, forming atheromas. LDLs circulate in the blood for
a few days and, if not absorbed into other body cells, attach to LDL-receptors on liver
cells and are digested.
The ratio of HDL/LDL in the blood is important in determining the chances of
atherosclerosis. A high ratio will cause the removal of cholesterol from the atheromas
in the arterial walls, reducing atherosclerosis. The link between saturated fats and
cholesterol levels depends on the observation that, whereas increased intake of all fats
increases the concentration of HDLs, only saturated fats increase the level of LDLs (and
hence the transport of cholesterol to the body cells).
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TOPIC 8. CHOLESTEROL AND CARDIOVASCULAR DISEASE
High density and low density lipoproteins: Question
Q4:
Select the correct option to complete the sentences about cholesterol transport.
1. Cholesterol is transported by fats / proteins / lipoproteins.
2. Excess cholesterol is carried from body cells to the liver by LDLs / HDLS / lipids.
3. Cholesterol is transported from the liver to the body cells by LDLs / HDLs / lipids.
4. LDL-receptors are found are found on only liver cells / most cells / all cells.
5. When a cell is making cell membranes, LDL-receptor production is unchanged /
increased / decreased.
6. Excess LDLs may deposit cholesterol in body cells / atheromas / lipoproteins.
7. Atherosclerosis is reduced by a HDL/LDL ratio which is low / medium / high.
8. LDL levels in the blood are increased by a diet high in fats which are saturated /
unsaturated / both.
..........................................
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TOPIC 8. CHOLESTEROL AND CARDIOVASCULAR DISEASE
8.2.4
Controlling cholesterol levels
The concentration of cholesterol in the blood is dependent on the ratio of HDLs to
LDLs, which in turn is determined by the balance between the intake of energy and its
consumption by the cells of the body. Such changes can be brought about by exercise,
which increases energy use, and dietary changes, which alter the intake of energy,
especially in the form of fats.
Regular physical activity tends to raise HDL levels. However, it does depend on the type
and duration of the activity. If the exercise is too gentle, e.g. ambling along with the dog,
then respiration can be fuelled by glucose from the blood. Moderate exercise, such as
a brisk walk that puts you slightly out of breath, does mobilise the fat reserves and so
helps weight loss. Repeated daily for about half an hour, it also raises HDL levels, as
does intense exercise, although the latter does not draw on the fat reserves to the same
extent as it uses up stored glycogen as a more immediate energy source.
Dietary changes can alter both the intake of cholesterol and fats. Reducing the intake
of food from animal sources, especially dairy products, meat, poultry, fish and shellfish,
directly reduces the quantity of cholesterol taken in, making the body compensate with
its own production to supply the needs of the cells. Thus, there will be no excess
cholesterol circulating in the blood.
In the same way, reducing the intake of fat (saturated fat in particular) will reduce the
production of LDLs and hence the cholesterol level in the blood.
Drugs may also be used to reduce blood cholesterol levels. Statins act at a critical
point in the reaction sequence of the metabolic pathway synthesising cholesterol in the
liver. That stage is controlled by negative feedback; statins inhibit the enzyme involved,
mimicking the effect of high cholesterol levels and cutting back production.
Controlling cholesterol levels: Question
Q5: Complete the sentences which relate to controlling cholesterol levels in the blood
by matching the phrases on the left with the phrases on the right.
Regular physical activity raises
the level of LDLs.
The concentration of cholesterol is reduced by
lowering
cholesterol intake.
Reducing the unsaturated fat in the diet reduces
cholesterol production by
liver cells.
Reducing the content of animal food in the diet
reduces
HDL levels.
Statins are drugs which inhibit
the fat content of the diet.
Statins reduce
an enzyme in the liver.
..........................................
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TOPIC 8. CHOLESTEROL AND CARDIOVASCULAR DISEASE
8.2.5
Familial hypercholesterolaemia
Familial hypercholesterolaemia (FH) is an inherited condition that causes high
cholesterol levels in the blood, and in particular very high concentrations of LDLs, which
bring about the early onset of cardiovascular disease. It is one of the most common
genetic disorders, occurring in 1 in 500 people.
Most commonly, people that suffer from the disorder have a mutation in the autosomal
gene which encodes the protein of the LDL-receptor. The effect of this is to reduce the
number of LDL-receptors on the liver cells (other mutations alter the receptor structure).
LDLs circulate for twice as long the blood. This increases the cholesterol concentration
in the blood and hence deposition in atheromas in the arterial walls. In the most serious
cases, no LDL-receptors are formed.
The mutation shows incomplete dominance: heterozygotes develop cardiovascular
disease between 30 and 40, whereas homozygotes show the symptoms in childhood.
In families with a history of FH, especially if it has developed early in adult life, it is
important that children are genetically tested for the presence of the condition. If it is
present, a diet should be followed which is low in total and saturated fats. In addition, the
condition can be treated with drugs (usually statins), which cause the liver to produce
more LDL-receptors, reducing LDL-cholesterol levels in the blood.
Familial hypercholesterolaemia: Question
Q6:
Select the correct option to complete the sentences about FH.
i
FH is usually caused by a mutation of which gene? LDL-receptor / HDL-receptor
ii
What inheritance pattern does this mutation show?
codominance
incomplete dominance /
iii What type of chromosome is this gene found on? allosome / autosome
iv In FH patients, what is reduced on liver cell membranes? HDL-receptors / LDLreceptors
v
FH causes the early onset of what disease? cardiovascular / Raynaud's
vi This results from increased levels of what in the blood? HDLs / LDLs
vii In consequence, what is deposited in artery walls? cholesterol / muscle cells
viii What should be reduced in the diet of FH patients? far / sugar
ix What drugs may be used to control FH? coumadins / statins
..........................................
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TOPIC 8. CHOLESTEROL AND CARDIOVASCULAR DISEASE
8.3
Atherosclerosis and associated diseases
139
Learning Objective
By the end of this section, you should be able to:
• describe the development of atherosclerosis;
• name some of the diseases which atherosclerosis causes;
• describe the development of thrombosis;
• state the conditions which can be caused by thrombosis;
• describe the development of peripheral vascular disease;
• describe the development of deep vein thrombosis.
This section considers some of the diseases that are associated with cholesterol.
8.3.1
Atherosclerosis
Atherosclerosis is associated with a thickening of the artery walls that is caused by a
build-up of fatty material, especially cholesterol. Although typically associated with older
people, this is a progressive, but largely symptomless, condition which begins much
earlier in life (20-30 years old). Also known as hardening, or furring, of the arteries, it is
not clear what causes the onset of the condition, although a response to the deposition
of LDL-cholesterol would seem to be involved. Contributory factors include diabetes
mellitus, low HDL/LDL ratio, hypertension, a family history of CVD, and smoking.
The thickening of the artery wall is caused by a structure called a plaque, which is
composed of plaques of fibrous material, calcified tissue (hardened with calcium salts),
and atheromas containing cholesterol. The following diagram shows the development
of the plaque beneath the endothelium; the narrowing effect on the lumen of the artery
can be clearly seen.
The development of plaque beneath the endothelium
At the same time as narrowing the lumen of the artery, the plaques also reduce the
elasticity of the artery wall, both of which restrict blood flow and increase blood pressure.
Atherosclerosis is the root cause of many of the cardiovascular diseases, including
angina, heart attack (myocardial infarction), stroke and peripheral vascular disease.
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8.3.2
Thrombosis
The plaques which develop in atherosclerosis can be divided into two broad groups:
stable and unstable plaques. It is the latter group that can rupture and lead to serious
complications.
The outer cap of an unstable plaque is weak and quite likely to break open, rupturing
the endothelium and exposing the fibrous material and cholesterol to the blood plasma.
These act as clotting factors, which cause the platelets and blood proteins to trigger
the cascade of reactions, resulting in the formation of a blood clot or thrombus. One
of the later steps in this extended sequence of reactions involves the conversion of the
enzyme precursor prothrombin into its active form thrombin. This enzyme then converts
the soluble plasma protein fibrinogen into the insoluble fibres of fibrin, which link together
to form a clot.
In normal circumstances, the clotting process is initiated when tissue is injured, e.g. by
a cut. The clot of fibrin threads forms a meshwork which traps red blood cells and seals
the wound. This allows the tissue in the damaged area to be cleaned up by white blood
cells. New cells are then produced, creating scar tissue.
The formation of a thrombus in an artery is known as thrombosis. If the thrombus
occurs in a small artery, it has the potential to block it and cause the death of the cells
it serves. A clot which breaks free (called an embolus) travels along in the blood with
the potential to block an artery or arteriole. If this happens, the cells served by the
blocked vessel die within minutes because of oxygen deprivation, with potentially fatal
consequences.
If a thrombus forms in a coronary artery, an ensuing embolus causes a heart attack
(myocardial infarction). A thrombus in a carotid artery taking blood to the head, or in an
artery in the brain, may lead to a stroke.
8.3.3
Peripheral vascular disorders
Peripheral Vascular Disease (PVD)
When atherosclerosis or thrombus formation causes the narrowing of arteries other than
those serving the heart and brain, it is known as peripheral vascular disease. Most
commonly, the arteries serving the legs are affected (although others can be involved)
and the effects range from mild pain when walking to extreme difficulty walking, tissue
loss because of the onset of gangrene, and amputation. All of these effects stem from
the reduced supply of oxygen which is caused by the narrowing of the arterial lumens.
Deep Vein Thrombosis (DVT)
Whereas all of the preceding conditions have involved atherosclerosis, and by
definition arteries, deep vein thrombosis occurs in veins and does not involve plaque
development. Although most commonly developing in the legs, these clots may form in
many other areas of the body. Unlike emboli in arteries, those arising from DVT pass
back first to the heart and then through the pulmonary artery to the lungs where they
may lodge, causing a pulmonary embolism. This leads to a variety of possible effects,
ranging from breathing difficulties, to collapse or sudden death.
Although most people would know of DVT as a risk involved in long-haul air travel,
immobilisation in the sitting position for long periods in any situation can cause it. There
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are many other causal factors, some being the same as those of atherosclerosis, like
advancing age, obesity and heavy smoking, but there are several others, such as major
surgery, cancer, pregnancy, trauma (e.g. a blow to the thigh) and a variety of inherited
conditions.
8.3.4
ASSIGN score
Although the ASSIGN Score is not in the syllabus, this section is included to show how
a very complex topic like CVD can be approached to allow people who are at risk to
be identified. The information has been largely taken from the NHS Scotland website
(http://assign-score.com).
Scotland has an exceptionally high level of cardiovascular disease. The preceding
section makes clear that CVD has many contributory causes, and so it is possible for a
person to be below the critical level on any one factor, but still at high risk because of
the combined effect. The ASSIGN Score provides a tool which combines the various
factors to calculate a single risk assessment.
From a system of predicting a person's risk of CVD, which was produced in the
United States (the Framingham Score), the ASSIGN score was developed by Dundee
University in conjunction with the Scottish Intercollegiate Guidelines Network. ASSIGN
is tailored to the Scottish population in which much cardiovascular disease is associated
with social deprivation and family history.
The ASSIGN score number is the estimated percent risk of getting cardiovascular
disease over ten years. It is based on what happened to 13,000 Scots men and women
in the 1980s and 1990s (when risk factors were not being systematically treated).
ASSIGN 20 therefore means a 20% risk, but the actual risk is almost certainly now
less than that (because overall health has improved). What matters more is what
somebody's score is in relation to other people's scores, and therefore how badly they
need prevention now to lower their risk.
The ASSIGN score is produced by combining the factors which identify people who are
at increased risk of cardiovascular disease. These risk factors are:
• age (being older);
• sex (being a man);
• where you live (higher risk in poorer areas);
• family history (relatives who have or had coronary disease or stroke);
• diabetes mellitus (a person has sugar diabetes);
• cigarette smoking;
• the blood pressure reading (high);
• the blood total cholesterol reading (high) - mainly 'bad cholesterol';
• the blood HDL ('good cholesterol') to cholesterol reading - (low HDL/total reading).
The ASSIGN score will lie in the range 1 to 99. The higher the score, the higher the risk
of cardiovascular disease.
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A score of 20 or more is considered to be high, and is used to identify those people in
greatest need of advice and treatment to reduce their risk.
In Scotland you would be offered advice, support and treatment. That support might
include being invited to join groups to help you with your diet, to stop smoking, or to take
exercise suitable for your age and health. Treatment would probably include medication
to help reduce the chances of developing cardiovascular disease. These medicines
could be:
• low-dose aspirin to reduce the risk of blood clots (thrombosis);
• statin tablets to reduce cholesterol levels.
If your ASSIGN score was not as high as 20, you might still be given advice about things
you could do to improve your own health and lower your risk such as:
• taking regular exercise;
• controlling your waistline and your weight;
• eating healthily;
• avoiding tobacco.
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8.4
Learning points
Summary
Cholesterol
• Cholesterol is a waxy solid, a steroid and a fat.
• It is synthesised in all body cells, but the liver produces most in the body.
• Cholesterol is present in animal foods, especially dairy products, meat and
poultry.
• A diet high in saturated fats may cause increased cholesterol levels.
• It is eliminated from the body in the bile produced by the liver.
Functions of cholesterol
• It is a component of cell membranes, where it controls permeability.
• It is a precursor of vitamin D and steroid hormones.
Transport of cholesterol
• Cholesterol is transported by lipoproteins synthesised in the liver.
• High density lipoproteins (HDLs) carry excess cholesterol from the body
cells to the liver.
• High levels of HDLs in the blood can reduce the presence of cholesterol in
plaques in the artery walls.
• Low density lipoproteins (LDLs) transport cholesterol from the liver to body
cells.
• LDLs become attached to LDL-receptors, found on the cell membrane of
most cells.
• The LDL-receptors pass into the cell and release the cholesterol.
• The production of LDL-receptors is controlled by negative feedback.
• Once a cell has sufficient cholesterol, LDL-receptor production is
suppressed.
• Excess LDL circulates in the blood and can become absorbed into
atheromas in plaques in artery walls.
• A high ratio of HDLs to LDLs lowers the level of cholesterol in the blood,
reducing the development of atherosclerosis.
Controlling cholesterol levels
• Regular physical activity raises HDL levels, reducing cholesterol levels.
• Reducing the content of unsaturated fat in the diet reduces the level of LDLs.
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Summary Continued
• Reducing the content of animal foods rich in cholesterol reduces cholesterol
intake.
• Statins are drugs which reduce cholesterol levels.
• Statins inhibit an enzyme in the pathway that produces cholesterol in the
liver.
Familial hypercholesterolaemia
• FH is caused by an incompletely dominant autosomal mutation of the LDLreceptor gene.
• The mutant allele results in fewer (or altered) LDL-receptors on liver cells.
• This causes high levels of LDL-cholesterol in the blood and the early onset
of cardiovascular disease.
• The presence of the FH mutation can be detected by genetic testing.
• FH can be treated by changes to lifestyle such as following a diet low in total
fats and saturated fats.
• FH can be treated using drugs such a statins which reduce LDL-cholesterol
levels in the blood.
Atherosclerosis
• Atherosclerosis is the thickening of artery walls by a build-up of fatty
material.
• The thickening is in the form of plaques beneath the endothelium, consisting
of fibrous material, an atheroma of fatty material (mainly cholesterol), and
calcified tissue.
• The growth of the plaque reduces the lumen of the artery.
• This reduces the blood flow and increases blood pressure.
• The growth of the plaque also reduces the elasticity of the artery wall which
increases blood pressure.
• Atherosclerosis is the root cause of cardiovascular diseases such as angina,
heart attack (myocardial infarction), stroke and peripheral vascular disease.
Thrombosis
• Unstable plaques may rupture, damaging the endothelium, and exposing
the fibrous tissue and atheroma cholesterol to the blood plasma.
• These act as clotting factors, which activate the cascade of reactions of the
clotting process.
• As part of this process, the enzyme precursor prothrombin is converted to
the active form thrombin.
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Summary Continued
• Thrombin catalyses the conversion of the soluble plasma protein fibrinogen
to insoluble fibres of fibrin.
• The fibrin threads form a meshwork which traps red blood cells and forms a
clot, sealing the wound and allowing the growth of scar tissue.
• When such a clot forms in an artery it is called a thrombus.
• If such a clot breaks loose into the blood stream, it is called an embolus
which will travel through the blood circulation and may block a blood vessel.
• In a coronary artery, this leads to a heart attack (myocardial infarction).
• In an artery in the brain, it causes a stroke.
• In these cases, the cells served by the blocked arteries/arterioles die from
lack of oxygen.
Peripheral vascular disorders
• Peripheral vascular disease (PVD) is atherosclerosis of arteries other than
those serving the heart or the brain.
• PVD is most common in the arteries of the legs.
• As elsewhere in the body, the atherosclerosis of PVD deprives cells of
oxygen, causing symptoms for mild pain to gangrene leading to amputation.
• Deep vein thrombosis (DVT) is the formation of a clot in a deep vein, most
commonly in the leg.
• If the clot becomes detached, forming an embolus, it may pass through the
heart and lodge in a branch of the pulmonary artery.
• If the embolus lodges in the pulmonary artery, this is called a pulmonary
embolism.
8.5
Extended response question
The activity which follows presents an extended response question similar to the style
that you will encounter in the examination.
You should have a good understanding of cholesterol in the body before attempting the
question.
You should give your completed answer to your teacher or tutor for marking, or try to
mark it yourself using the suggested marking scheme.
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Extended response question: Cholesterol in the body
Give an account of cholesterol in the body under the headings:
15 min
1. sources and removal of cholesterol; (3 marks)
2. high density lipoproteins and low density lipoproteins. (7 marks)
..........................................
8.6
End of topic test
End of Topic 8 test
Q7:
30 min
Complete the paragraph using the words from the list. (10 marks)
which is synthesised in all body cells, although the
Cholesterol is a waxy
produces most in the body. It is a component of cell
, where
. It is also a precursor of
and steroid
.
it controls
produced by the liver.
Cholesterol is eliminated from the body in the
Cholesterol is present in
. A diet high in
foods, especially dairy products, meat and
fats may cause increased cholesterol levels.
Word list: animal, bile, hormones, liver, membranes, permeability, poultry, saturated,
steroid, vitamin D.
..........................................
Q8: Complete the sentences by matching the parts on the left with the parts on the
right. (10 marks)
Lipoproteins:
drugs used to reduce cholesterol levels.
High density
lipoproteins:
present on the cell membrane of most cells.
Low density
lipoproteins:
reduces the development of atherosclerosis.
LDL-receptors:
become incorporated into atheromas.
Negative feedback:
synthesised in the liver and used to transport cholesterol.
Excess LDLs:
raises HDL levels.
High HDL : low LDL:
control of LDL-receptor production.
Regular physical
activity:
reduced by less animal foods in diet.
Cholesterol intake:
carriers of cholesterol from the liver to the body cells.
Statins:
carriers of cholesterol from the body cells to the liver.
..........................................
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Q9: Complete the sentences by matching the parts on the left with the parts on the
right. (12 marks)
Atheroma:
result of narrower lumen and reduced elasticity of the artery
wall.
Plaque:
root cause of angina, myocardial infarction, stroke and PVD.
Increased blood
pressure:
caused by an embolus lodged in the pulmonary artery.
Atherosclerosis:
a clot which breaks loose in the blood stream.
Fibrin:
caused by a loose clot in an artery in the brain.
Thrombus:
build-up of fatty material beneath the endothelium.
Embolus:
atherosclerosis in arteries other than in brain or heart.
Myocardial
infarction:
formed by a mesh of fibrin threads and red blood cells.
Stroke:
thickening consisting of fibrous material, calcified tissue and
fat.
Peripheral vascular
disease:
caused by a loose clot in the coronary artery.
Deep vein
thrombosis:
formed by the effect of thrombin on fibrinogen.
Pulmonary
embolism:
formation of a clot in a deep vein.
..........................................
Q10: Complete the sentences by matching the parts on the left with the parts on the
right. (6 marks)
Incompletely dominant
autosomal:
a method of detecting FH.
Fewer LDL-receptors on liver
cells:
the drugs used to treat FH.
High levels of LDL-cholesterol
in blood:
a type of mutation causing familial FH.
Genetic screening:
the effect of a mutation on the LDL-gene.
A diet low in fat and saturated
fat:
the result of a mutation to the LDL-gene.
Statins:
the natural method of treating FH.
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..........................................
Q11: Complete the sentences by matching the parts on the left with the parts on the
right. (4 marks)
Liver:
transport cholesterol from body cells to the liver.
Animal foods:
the cell structure that contains the most cholesterol.
High density
lipoproteins/HDLs:
a type of food which is high in cholesterol.
Cell membrane:
the organ which makes and eliminates cholesterol.
..........................................
Q12: Explain how cholesterol is transported from the organ in which it is produced to
body cells. (3 marks)
..........................................
Q13: State one natural and one artificial method of reducing cholesterol levels. (2
marks)
..........................................
Q14: How is the presence of familial hypercholesterolaemia detected? (1 mark)
..........................................
Q15: How may the condition be treated naturally? (1 mark)
..........................................
Q16: Explain how atherosclerosis leads to an increase in blood pressure. (3 marks)
..........................................
Q17: List two cardiovascular diseases resulting from atherosclerosis. (1 mark)
..........................................
Q18: Describe the steps by which an unstable plaque may lead to an embolism. (4
marks)
..........................................
Q19: State the reason that peripheral vascular disease may lead to gangrene. (1 mark)
..........................................
Q20: Explain the link between deep vein thrombosis (DVT) and pulmonary embolism
(PE). (2 marks)
..........................................
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Topic 9
Pathology of cardiovascular
disease
Contents
9.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9.2 Regulation of blood glucose levels . . . . . . . . . . . . . . . . . . . . . . . . .
150
150
9.2.1 Blood glucose levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
151
9.2.2 Homeostatic control of blood glucose levels . . . . . . . . . . . . . . . .
9.2.3 The effect of exercise on blood glucose levels . . . . . . . . . . . . . . .
152
154
9.3 Blood glucose levels and diabetes . . . . . . . . . . . . . . . . . . . . . . . . .
9.4 Blood glucose levels and vascular disease . . . . . . . . . . . . . . . . . . . . .
155
158
9.5 Obesity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9.5.1 Measuring obesity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
160
162
9.5.2 Obesity and diet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9.5.3 Obesity and exercise . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
162
163
9.6 Learning points . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9.7 Extended response question . . . . . . . . . . . . . . . . . . . . . . . . . . . .
165
167
9.8 End of topic test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
167
Learning Objectives
By the end of this topic, you should be able to:
• explain how the concentration of glucose in the blood is maintained within a narrow
range;
• explain the effect of exercise on blood sugar levels;
• describe the contribution of chronically raised blood sugar levels on cardiovascular
disease;
• describe the different forms of diabetes and their effects;
• describe the causes and effects of obesity, how it is measured and how it may be
reduced.
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9.1
Introduction
The diseases covered in this topic are all related to the regulation of the level of glucose
sugar in the blood, the failure of that regulation which results in diabetes, and the
imbalance of diet and exercise which causes obesity.
At a number of points, rather more information is included than is required by the
syllabus. This has been done to provide a deeper understanding of the biological
mechanisms involved, making it easier to comprehend the syllabus topics. The material
that will be examined is specified at the start of each section and again in the Learning
Points section at the end of the topic.
9.2
Regulation of blood glucose levels
Learning Objective
By the end of this section, you should be able to:
• state that blood glucose levels are homeostatically regulated by negative
feedback;
• explain that receptors in the pancreas detect the concentration of glucose in the
blood;
• state that high blood glucose levels stimulate the release of insulin by the
pancreas;
• state that insulin stimulates the liver (and other tissues) to convert glucose to
glycogen;
• state that low blood glucose levels stimulate the release of glucagon by the
pancreas;
• state that glucagon stimulates the liver to convert glycogen to glucose;
• state that exercise and fight or flight situations stimulate the release of
adrenaline from the adrenal glands;
• state that adrenaline stimulates the liver (and other tissues) to convert glycogen
to glucose;
• state that adrenaline also stimulates the release of glucagon and inhibits the
release of insulin by the pancreas.
This section considers the role of glucose in the body and the control of its blood
concentration.
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9.2.1
Blood glucose levels
Although fatty acids, and to a lesser extent amino acids, may act as
respiratory substrates, glucose is the principal one. Therefore, there is a constant
demand for glucose to keep all cells alive. This base level of demand is added to
whenever energy must be expended for other functions, such as movement or keeping
warm.
Glucose is stored in the body as glycogen, a polysaccharide that is very similar to
starch, to which it is readily converted (and vice versa). In total, about 125g of glycogen
are stored in the average adult, of which about 75% is in the skeletal muscles. However,
the intramuscular glycogen cannot be released as glucose into the circulation so only
the glycogen held in the liver is able to be mobilised to raise blood glucose levels.
Humans cannot convert fat directly to glucose which means that glycogen stores
must be replenished either with glucose from food intake (glycogenesis: genesis =
making), or derived from other substrates, e.g. lactate, glycerol and some amino acids
(gluconeogenesis: neo = new). During fasting or starvation, the latter is the only source
of blood glucose. Although much more energy is stored in the form of fat than in the form
of glycogen, it is less readily available to the body cells as it must first be split into fatty
acids and glycerol in the fat depots (lipolysis), then circulated in the blood to the body
cells and the liver respectively.
For the average adult at rest, the body uses about 10 grams of glucose per hour, of
which more than half goes to supply the brain. It is worth noting that because all of
the glucose taken up by the brain is respired and none is stored, the brain is extremely
sensitive to reduced blood glucose levels.
The structure of glucose
In an average adult, about 5g of glucose is circulating in the blood at any one time.
Within cells, much more is held in combination with phosphate and other groups, which
aids the transport of glucose into the cells by lowering the internal concentration of
glucose itself. As there is only sufficient available glucose in the body to fuel
metabolism for about 20 minutes, there must be a constant replenishment of the blood
glucose to compensate for its absorption into the body cells.
The blood glucose level is generally maintained at between 4.5 and 6 mmol/L (81 and
110mg/100mL) and the consequences of allowing the concentration to stray outside
this range can be very serious. If it falls below 4mmol/L, hypoglycaemia develops,
potentially leading to coma and death. Concentrations above the range do not cause
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such immediately serious consequences, but if the condition (hyperglycaemia) persists
and becomes chronic, then there are many deleterious effects on health.
Blood sugar levels are lowest in the morning. The basic measure of blood glucose level
is taken after an 8 hour fast. After a meal, a maximum of 10mmol/L might be reached
after 90 minutes, rapidly dropping back to 7mmol/L or less. Given that a few minutes of
exercise or the eating of a chocolate bar is going to remove or add enough glucose to
the blood to push the glucose concentration outside its normal range, it is clear that the
body must have a homeostatic mechanism for keeping it within these boundaries.
Blood glucose levels: Question
Q1:
10 min
Complete the paragraph using the words from the list.
in the body. It circulates in solution in the blood
Glucose is the principal
. These stores
plasma and is stored as an insoluble polysaccharide called
, but only the store in the
can be
are found in the liver and the
mobilised to raise blood sugar levels.
Blood glucose levels are kept in a very narrow range, by a process of
. If
sets in with
the glucose concentration falls below this range, the condition of
potentially fatal results. If the glucose concentration exceeds the normal range, the
.
condition is known as
Word list: glycogen, homeostasis, hyperglycemia, hypoglycaemia, liver, muscles,
respiratory substrate.
..........................................
9.2.2
Homeostatic control of blood glucose levels
Blood glucose levels are controlled by two types of cells located in groups within the
pancreas (called the islets of Langerhans). In response to changing blood glucose
levels detected by their receptors, these cells release different hormones which stimulate
or inhibit the inter-conversion of glucose and glycogen.
When low levels of glucose (less than 4mmol/L) are detected by one type of cell (α-cells),
they respond by increasing their production of glucagon. Glucagon is carried in the
blood to the liver, where it attaches to glucagon receptors on the cells and stimulates the
conversion of glycogen to glucose. This glucose then passes into the blood, increasing
the concentration. Muscle cells lack glucagon receptors, which means that they are not
influenced by glucagon.
Relatively high levels of glucose (more than 5mmol/L) are detected by receptors on
the other type of cell (β-cells), which release insulin in response. Insulin attaches to
receptors on the liver and muscle cells, stimulating glucose uptake and causing these
cells to convert glucose to glycogen. In addition, insulin inhibits the release of glucagon
(although glucagon does not inhibit insulin production, for complex reasons).
Glucose, insulin and glucagon
The following diagram shows how the level of glucose in the blood changes in relation
to the consumption of food and the release of insulin and glucagon. The dashed
line indicates the average blood glucose concentration, the set point around which the
homeostatic process operates.
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The action of the pancreatic hormones glucagon and insulin on the liver is a form
of negative feedback system, which acts to maintain blood glucose levels within a
very narrow range. If blood glucose levels fall below a critical point, glucagon is
released, stimulating conversion of glycogen to glucose and a rise in blood glucose
levels. Conversely, a rise in blood glucose levels above the normal range triggers the
release of insulin, which causes glucose to be removed from the blood and stored as
glycogen.
This can be summarised as follows:
Insulin and glucagon also influence the cells of the fat depots in the same way. Thus,
glucagon stimulates the release of fatty acids and glycerol, whereas insulin stimulates
the conversion of glucose (via acetyl-CoA) to fatty acids and lipids. In this way, excess
carbohydrate is converted and stored as fat.
Insulin can be seen as the most influential factor of all in homeostasis, i.e. in the
maintenance of steady-state conditions within the body. Not only does it play a principal
part in the control of carbohydrate, lipid and protein metabolism, it even acts within the
brain to improve learning and memory. Together with glucagon, it balances metabolism,
with insulin promoting broadly anabolic reactions that store energy and build up protein,
and glucagon stimulating catabolic activities such as the release of stored energy.
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Homeostatic control of blood glucose levels: Question
Q2: Complete the sentences concerning the homeostatic control of blood sugar levels
by matching the phrases on the left with the phrases on the right.
Blood glucose levels are controlled by hormones released from
cells in
receptors.
Blood glucose levels are detected on the surface of these cells
by
glucagon.
The hormone stimulating conversion of glycogen to glucose is
the pancreas.
The hormone released in response to low blood glucose levels
is
insulin.
The hormone stimulating conversion of glucose to glycogen is
glucagon.
The hormone released in response to high blood glucose levels
is
the liver.
The organ which both releases and stores glucose as glycogen
is
..........................................
9.2.3
insulin.
The effect of exercise on blood glucose levels
Exercise increases the body's energy demand which lowers the blood glucose level.
This triggers the release of glucagon, but vigorous exercise also causes the release of
growth hormone from the pituitary gland, which promotes glucose formation in the liver,
and thyroxine from the thyroid gland, which increases metabolic rate. It should be noted
that both of these hormones have a wide variety of other effects.
In response to the excitement and stress of exercise, the sympathetic nervous system
triggers the release of another hormone, adrenaline, from the adrenal glands which
are attached to the top of the kidneys. Adrenaline has a wide variety of effects, all of
which prepare the body for physical action; the 'fight or flight' response. These include
the inhibition of the release of insulin, stimulation of the release of glucagon, and the
promotion of the conversion of glycogen to glucose in the liver and the muscles.
The effect of exercise on blood glucose levels: Question
Q3: Select the correct option to complete the sentences about blood glucose levels
and exercise.
1. Lowering the blood sugar level causes the release of insulin / glucagon / adrenaline.
2. Vigorous exercise stimulates the release from the thyroid gland of growth hormone
/ thyroxine / adrenaline.
3. Excitement and stress cause the release of glucagon / insulin / adrenaline.
4. Adrenaline inhibits the release of insulin / glucagon / thyroxine.
5. Adrenaline has the same effect on the liver as insulin / glucagon / growth hormone.
..........................................
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9.3
155
Blood glucose levels and diabetes
Learning Objective
By the end of this section, you should be able to:
• explain the commonest causes of hypo- and hyperglycemia;
• state that diabetes is a common cause of hyperglycemia;
• state that diabetics are unable to control a rise in blood glucose levels;
• state that chronic vascular disease is a complication of diabetes;
• state that there are two forms of diabetes called Type 1 and Type 2;
• explain that Type 1 diabetes typically appears in childhood, resulting from a
failure of insulin production, and is treated by regular insulin injections;
• explain that Type 2 diabetes typically develops in adulthood, resulting from
a reduced cellular sensitivity to insulin caused by a decreased number of
receptors on the liver cells;
• state that Type 2 diabetes is frequently associated with obesity and is mainly
treated by adjustments to diet and exercise regimes;
• state that both forms of diabetes cause abnormally high blood glucose levels
after meals;
• explain that, at these high blood glucose concentrations, the kidneys are unable
to reabsorb all of the glucose from the glomerular filtrate, and so glucose is
excreted in the urine;
• state that a positive result for glucose in a urine sample test is a strong indicator
of diabetes;
• state that a glucose tolerance test assesses a blood sample taken after an 8
hour fast;
• state that a glucose tolerance test uses two blood samples, one taken before
drinking a solution containing 75g of glucose and a second taken 2 hours later.
Blood glucose levels
The homeostatic control of blood glucose levels was described in the previous section.
If the blood glucose level falls below the normal range, then hypoglycaemia develops
(hypo = under). This is most usually a result of excessive insulin levels caused by overproduction or as a complication in the treatment of Type 1 diabetes.
If blood glucose levels are consistently (chronically) above the normal range, the
condition is known as hyperglycaemia (hyper = over) and various forms of chronic
vascular disease result. The most common cause of hyperglycemia is diabetes, which
occurs in two main forms, both of which result in the body being unable to control the
rise in blood glucose levels after a meal.
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Type 1 diabetes
Type 1 diabetes (also known as 'insulin-dependent diabetes' or 'juvenile diabetes')
results from the body's failure to produce insulin, requiring treatment by regular insulin
injection. It is an autoimmune disorder, which destroys the insulin-producing β-cells
in the pancreas. The condition typically develops in childhood and usually occurs in
people who have a family history of the disease, i.e. there is a genetic component. It
also appears that there is a need for an environmental trigger, e.g. a viral infection or
cold weather. The lack of insulin means that much higher than normal concentrations of
glucose are found in the blood for long periods after meals.
Type 2 diabetes
Type 2 diabetes (also known as 'adult-onset diabetes' or 'non-insulin dependent
diabetes') typically develops in adulthood and, although it may be treated with insulin
injections, it is usually controlled by adjusting diet, weight and exercise regimes. This is
by far the most common form of diabetes, accounting for some 90% of cases, and, like
Type 1 diabetes, it has both genetic and environmental causes. Obesity is thought to
be the main trigger for the development of the condition in those who are genetically at
risk, but it is possible that more complex lifestyle factors are also involved.
Usually, this condition results from a decreased sensitivity of the muscle and liver cells to
insulin, meaning that they have a reduced response to normal levels of insulin released
by the pancreas. Fat cells tend not to lose sensitivity to insulin, continuing to convert
glucose into fat, thus contributing to the development of obesity.
Type 2 diabetes is typically associated with a 'Western' diet, high in saturated fats and
simple carbohydrates, containing more energy than is required to maintain the body's
metabolism. Other aspects of the urban lifestyle which may contribute to increased
obesity are reduced physical activity, more sedentary occupations, reduced sleep and
increased stress.
The modern tendency to 'graze' on snacks (often with a high fat and sugar content)
will lead to a more continuously elevated blood glucose level, and hence production of
insulin, than would result from the traditional two or three meals a day.
In such a situation, the pancreas is continuously producing high levels of insulin to
combat the intake of glucose from the small intestine. These high levels of insulin
eventually inhibit the production of insulin receptors on the key liver and muscle cells,
reducing the uptake of insulin, and consequently glucose, into these cells. In other
cases, the production of insulin is significantly reduced. The net effect of these changes
is to reduce the conversion of glucose to glycogen, leaving a high concentration of
glucose circulating in the blood. This excess glucose is then:
• absorbed into the fat depots, which still retain their insulin receptors and are
stimulated by the high level of insulin to store the excess glucose as fat;
• taken into blood vessel walls, causing damage to them and contributing to vascular
disease;
• excreted by the kidneys in the urine because the cells lining the proximal tubule
are no longer able to reabsorb all of the glucose passed out of the blood in the
glomerular filtrate by active transport.
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Testing for diabetes
Both Type 1 and Type 2 diabetes make the blood glucose levels after a meal so high that
glucose is lost in the urine. This happens when blood glucose concentrations exceed
11 mmol/L.
Urine test: a glucose test-strip is dipped into a urine sample and a positive result is an
indication of diabetes. However, the urine test result must be confirmed by blood tests
which are much more reliable.
Measuring urine glucose levels
Blood tests: blood samples can be tested in the following ways:
• random glucose test: glucose levels are taken at a random time on two occasions
- any concentration above 11.1mmol/L indicates diabetes;
• fasting glucose test: the glucose level is measured after an overnight (8 hour) fast
on two different days - concentrations above 7.0mmol/l indicate diabetes.
Measuring blood glucose concentration
A further glucose tolerance test can be carried out if these tests prove to be
inconclusive. It involves taking a standard glucose drink, containing 75g of glucose,
after an overnight (8 hour) fast. Blood samples are taken before the drink is given and
two hours later; a glucose level above 11.1mmol/L is a diagnosis of diabetes.
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Diabetes: Question
Q4: Complete the paragraph about diabetes using the words from the list. (some
words may be used more than once)
The condition of blood glucose levels consistently below the normal range is called
. Diabetes is the most common cause of
. A common
disease. Type 1 diabetes is also known
complication of diabetes is chronic
diabetes. Type 1 diabetes typically develops in
as known as
and is treated with
injections. Type 2 diabetes typically develops in
and results from
resistance caused by the reduced number
. Typically, Type 2 diabetes is associated with
and is treated
of
and increasing
. Diabetics excrete glucose in their
by altering
. The glucose tolerance test is taken before and
hours after
g of glucose.
taking a drink containing
Word list: 75, adulthood, childhood, diet, exercise, glucose, hyperglycaemia,
hypoglycaemia, insulin, insulin dependent, obesity, receptors, two, urine, vascular.
..........................................
9.4
Blood glucose levels and vascular disease
Learning Objective
By the end of this section, you should be able to:
• state that chronic high blood glucose levels lead to endothelial cells taking in
abnormally large quantities of glucose, damaging the lining of the blood vessels;
• explain that, in larger blood vessels, this may lead to atherosclerosis,
cardiovascular disease, stroke or peripheral vascular disease;
• explain that, in smaller blood vessels, this may lead to haemorrhaging in the
retina, renal failure or peripheral nerve dysfunction.
The cells of the endothelium, which line the various types of artery, lack insulin receptors
and so their uptake of glucose is dependent on the concentration in the blood which
they carry. High blood glucose levels will result in high glucose intake. If high blood
glucose concentration (above 7 mmol/L) is a chronic condition, as it is in diabetes Types
1 and 2, then the vessel walls become thicker and weaker, slowing the flow of blood
and causing hypertension. Eventually, the endothelium begins to break up. This has
different outcomes dependent on the size of the vessels involved.
In larger arteries, the damage to the endothelium causes the development of
atherosclerosis and peripheral vascular disease. The leakage of proteins into the blood
can trigger clot formation, with the danger of stroke and myocardial infarction.
In smaller arteries and arterioles, certain tissues are particularly at risk. In the eye, a
lack of oxygen caused by the reduced blood flow to the retina stimulates the proliferation
of more arterioles, which are equally fragile and so also leak blood. As a result, vision
becomes blurred and may ultimately be largely lost.
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In the kidneys, the result of the endothelial damage is progressive destruction of the
glomeruli, ultimately ending in kidney failure.
In the peripheral nervous system, the lower blood flow and the associated reduced
oxygen supply slow, and ultimately stop, the conduction of nerve impulses in the
sensory-motor and autonomic nervous systems. In other words, virtually all body
systems and organs may be affected, and the range of effects is exceptionally wide.
Blood glucose levels and vascular disease: Question
Q5: Complete the paragraph about glucose levels and vascular disease using the
words from the list.
cells which line the blood vessels are damaged by
high blood
The
glucose concentrations. As a result, the flow of blood is slowed, causing
. In small arteries the effects are particularly bad in certain tissues. In the eye, the
is affected, causing blurring and eventual loss of vision. In the
nervous system, conduction of nerve
is slowed or stopped. In the kidney,
eventually leads to kidney failure. The effects in large
the damage to the
and peripheral
disease, which may ultimately lead to
arteries are
and myocardial
.
Word list: atherosclerosis, chronic, endothelial, glomeruli, hypertension, impulses,
infarction, peripheral, retina, stroke, vascula.
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9.5
Obesity
Learning Objective
By the end of this section, you should be able to:
• describe the origins of obesity;
• state that obesity is a major risk factor for cardiovascular disease and Type 2
diabetes;
• explain that BMI (body mass index) is calculated as mass / (height 2 ), where
mass is measured in kg and height in metres;
• state that a BMI in excess of 30 indicates obesity;
• state that obesity results from an excess of body fat in relation to lean body
tissue (muscle);
• state that, in order to accurately measure body fat, a measurement of body
density is needed;
• state that obesity is linked to high fat diets and reduced physical activity;
• state that, to counter obesity, the content of fats and free sugars in the diet
should be limited;
• state that dietary fat should be limited because fats contain twice as much
energy per gram as proteins or carbohydrates;
• state that dietary free sugars should be limited because their digestion requires
no metabolic energy expenditure;
• state that exercise increases energy expenditure and preserves lean tissue;
• state that exercise can reduce the risk of developing cardiovascular disease
by helping control weight, reducing stress and hypertension, and raising HDL
levels.
Why obesity?
Obesity is a condition in which fat has accumulated in the body to the extent that
it begins to have an adverse effect on health. It is largely a product of our modern
'Western' way of life and is rarely found in the developing world, only appearing when
some of the population achieve affluence. Following the increasing incidence of obesity
in the population is a whole host of associated diseases, yet storing fat is neither an
unnatural nor an intrinsically bad thing.
It has been said that although we live in the 21 st century, our bodies remain in the Stone
Age. In other words, our social development has taken place at a far faster rate than
our physiological evolution. If we assume five generations to the century, and that the
Stone Age only finished about 4000 years ago in Britain, then there have only been
some 200 generations since its end. More importantly, it is only in the last 200 years
(at most) that the majority of the British population have ceased to be dependent on the
seasonal availability of food. Not much evolutionary change can occur in 10 generations
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except in the face of extreme selection pressure, and selection pressure on the human
population in Western society has decreased to zero for all practical purposes over this
period.
Fat stores represent a means of storing the maximum energy in the minimum mass.
Historically, when we had an excess of energy in our food, we stored it as fat against
those inevitable times when food would be short. In order to survive the hungry months
of winter, a wild mammal like a red deer can lose 30% of its body mass as it draws on
the fat stores laid down in the preceding summer.
While we have physiological mechanisms to stop us eating when we are satiated, and
to prompt us to eat when our bodies require it, we have no such means to stop us laying
down fat. Until the very recent past, any excess of food would have naturally come to
an end in autumn with the end of the plant growing season. By spring, most of our fat
stores would have been gone.
In today's Western/developed world, for most people there are no seasonal food
shortages. The deepfreeze, supermarket and global air-transport systems mean that
most foods are available all year round. If you can afford it, you can eat as much as you
can every day.
Of course, there is much more to the 'Western' lifestyle than food supply. With increasing
affluence comes a dietary shift towards foods of animal origin, an increase in the fat
content of the diet, and the increased intake of alcohol and refined foodstuffs (e.g.
sugar). In addition, people's occupations tend to be more sedentary and more stressful,
while hours of sleep and exercise levels have been reduced.
Breakfast!
In this complex and biologically unnatural situation, unless we consciously balance our
energy intake and energy use, we can easily tend to obesity with its increased risk of
cardiovascular disease and Type 2 diabetes.
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9.5.1
Measuring obesity
A person is generally classified as obese if they have an excess of body fat compared
to their lean body tissues, such as muscle. The degree of obesity can be estimated by
calculating the Body Mass Index (BMI) as:
BMI =
mass (kg)
height (m) 2
BMI values can be used to assign people to body condition categories as shown in the
following table.
BMI range
Category
< 18.5
underweight
18.5 - 24.9
normal
25 - 29.9
overweight
30+
obese
These categories are further sub-divided into 'severe', 'moderate' and 'mild', but it would
be unwise to go too far with this. For example, the square of a person's height is used,
but mass is dependent on volume (and so on the cube power); thus, if two people have
the same build, but are of very different heights, the taller one will have a higher BMI.
Also, as muscle is much denser than fat, a very muscular person with hardly any fat will
be classed as obese, and vice versa. Most modern professional rugby players would
be classified as obese, but you may be ill-advised to tell one that! The BMI index was
originally developed as a means of comparing groups of people and was not intended
for use with specific individuals.
In order to accurately measure body fat content, it is necessary to measure body
density. This is done in much the same way as Archimedes would have done it over
2,000 years ago. When a person is immersed in water, the volume of water displaced
is the same as the volume of the person. After making an adjustment for the air in the
lungs, the volume of body tissue is obtained. An accurate measure of mass then allows
density to be calculated.
9.5.2
Obesity and diet
Obesity develops insidiously, and countering it requires a change of lifestyle - there are
no permanent quick fixes! Within the general mix of features which characterise the
'Western' way of life, two are particularly linked to obesity: a diet high in fats and simple
sugars, and a lack of physical activity.
After we eat a meal, our blood stream is flooded with nutrients which means that our
homeostatic systems have to work very hard to keep the levels of these chemicals within
their 'safe' ranges. What isn't immediately used is stored to be slowly drawn upon until
the next meal replenishes the stock. If storage exceeds use, then fat will build up. This
applies whatever the diet, but some foods create bigger problems than others.
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After digestion in the small intestine, fats pass into the lymphatic system and are then
circulated in the blood to the cells, which means that little energy is used in their
breakdown. In the same way, simple carbohydrates, e.g. sucrose, require little energy
to make them available to cells. Proteins require much more 'work' before their energy
becomes available to cells. Although sugars and proteins contain much the same energy
per gram, sugars yield much more of it to the cells. Fats also contain more than twice
as much energy per gram as carbohydrates (hence their use for energy storage by
animals).
The sensation of hunger is linked to two main factors: blood glucose levels and stomach
fullness. Foods which are easily digested, e.g. those rich in sucrose, quickly pass
into the blood, emptying the stomach. Equally, they also lead to a sudden rise in blood
glucose, which triggers a surge in insulin secretion and a subsequent severe drop in
glucose levels. These combined effects will induce the hypothalamus to generate
the sensation of hunger. Frequent consumption of sugary or starchy food, or indeed
high fat food, will lead to persistently high levels of insulin, leading eventually to insulin
resistance.
9.5.3
Obesity and exercise
Taking regular exercise will increase the amount of energy that is used up in metabolism
simply because of the increased work that is being done. However, the effects are much
more wide-ranging than that. Regular exercise will build up muscle tissue at the same
time as reducing fat. As muscle has a higher metabolic rate that fat tissue, the effect is
to raise the basal metabolic rate, increasing the body's overall energy use, even when
inactive. Also, the increased demand on the heart increases its size, reducing heart
rate, increasing stroke volume, and making it more able to cope with sudden physical
stress.
A girl riding a bicycle
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Exercise increases the HDL/LDL ratio, helping to counter atherosclerosis and reducing
hypertension as the high density lipoproteins transport cholesterol back to the liver. The
risk of cardiovascular disease is also reduced by the fact that exercise reduces stress
levels. That, in turn, cuts the production of the steroid hormone cortisol, which causes
an increase in blood glucose levels and the development of fat tissue.
It is important for us all to realise that we have to work at keeping ourselves at a healthy
weight, and to be aware of the effects of changes in lifestyle such as diet and exercise
levels.
Obesity: Question
Q6: Complete the paragraph about obesity using the words from the list. (some words
may be used more than once)
Obesity is defined as an excess of
disease and
a major risk factor in
compared to
.
tissue. It is
. A BMI in excess of 30 indicates obesity. Accurate
BMI stands for
estimation of body fat content requires a measurement of body
.
and free
To counter obesity, a person should reduce their dietary intake of
, and increase their
activity. Compared with
,
per gram. Exercise increases
fats contain twice as much
expenditure and preserves
tissue. Exercise can reduce the risk of
disease by helping control
, reducing
developing
and
, and raising
levels.
Word list: Body Mass Index, carbohydrates, cardiovascular, density, energy, fat, lean,
physical, sugar, Type 2 diabetes.
..........................................
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9.6
Learning points
Summary
Regulation of blood glucose levels
• Blood glucose levels are homeostatically regulated by negative feedback.
• Receptors in the pancreas detect the concentration of glucose in the blood.
• High blood glucose levels stimulate the release of insulin by the pancreas.
• Insulin stimulates the liver (and other tissues) to convert glucose to
glycogen.
• Low blood glucose levels stimulate the release of glucagon by the pancreas.
• Glucagon stimulates the liver to convert glycogen to glucose.
• Exercise and fight/flight situations stimulate the release of adrenaline from
the adrenal glands.
• Adrenaline stimulates the liver (and other tissues) to convert glycogen to
glucose.
• Adrenaline also stimulates the release of glucagon and inhibits the release
of insulin by the pancreas.
Blood glucose levels and diabetes
• Diabetes is a common cause of hyperglycemia.
• Diabetics are unable to control a rise in blood glucose levels.
• Chronic vascular disease is a complication of diabetes.
• There are two forms of diabetes called Type 1 and Type 2.
• Type 1 diabetes typically appears in childhood, resulting from a failure of
insulin production, and is treated by regular insulin injections.
• Type 2 diabetes typically develops in adulthood, resulting from a reduced
cellular sensitivity to insulin caused by a decreased number of receptors on
the liver cells.
• Type 2 diabetes is frequently associated with obesity and is mainly treated
by adjustments to diet and exercise regimes.
• Both forms of diabetes cause abnormally high blood glucose levels after
meals.
• At these high blood glucose concentrations, the kidneys are unable to
reabsorb all the glucose from the glomerular filtrate and so glucose is
excreted in the urine.
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TOPIC 9. PATHOLOGY OF CARDIOVASCULAR DISEASE
Summary Continued
• A positive result for a glucose in urine sample test is a strong indicator of
diabetes.
• A glucose tolerance test assesses a blood sample taken after an 8 hour
fast.
• A glucose tolerance test uses two blood samples: one taken before drinking
a solution containing 75g of glucose, and a second taken 2 hours later.
Blood glucose levels and vascular disease
• Chronic high blood glucose levels lead to endothelial cells taking in
abnormally large quantities of glucose, damaging the lining of the blood
vessels.
• In larger blood vessels, this may lead to atherosclerosis, cardiovascular
disease, stroke or peripheral vascular disease.
• In smaller blood vessels, this may cause haemorrhaging in the retina, renal
failure or peripheral nerve dysfunction.
Obesity
• Obesity results from an excess of body fat in relation to lean body tissue
(muscle).
• Obesity is a major risk factor for cardiovascular disease and Type 2
diabetes.
• BMI (body mass index) is calculated as mass / (height2 ), where mass is
measured in kg and height in metres.
• A BMI in excess of 30 indicates obesity.
• In order to accurately measure body fat, a measurement of body density is
needed.
• Obesity is linked to high fat diets and reduced physical activity.
• To counter obesity, the content of fats and free sugars in the diet should be
limited.
• Dietary fat should be limited because fats contain twice as much energy per
gram as proteins or carbohydrates.
• Free sugars should be limited because their digestion requires no metabolic
energy expenditure.
• Exercise increases energy expenditure and preserves lean tissue.
• Exercise can reduce the risk of developing cardiovascular disease by
helping control weight, reducing stress and hypertension, and raising HDL
levels.
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TOPIC 9. PATHOLOGY OF CARDIOVASCULAR DISEASE
9.7
167
Extended response question
The activity which follows presents an extended response question similar to the style
that you will encounter in the examination.
You should have a good understanding of Type 2 diabetes before attempting the
question.
You should give your completed answer to your teacher or tutor for marking, or try to
mark it yourself using the suggested marking scheme.
Extended response question: Type 2 diabetes
Give an account of Type 2 diabetes under the headings:
15 min
1. causes; (2 marks)
2. effects. (6 marks)
3. diagnosis and treatment (2 marks)
..........................................
9.8
End of topic test
End of Topic 9 test
Q7: Complete the paragraph using the words from the list. (10 marks)
regulated by
feedback. Receptors
Blood glucose levels are
detect the concentration of glucose in the blood.
blood
in the
glucose levels stimulate the release of insulin by the pancreas. Insulin stimulates the
(and other tissues) to convert glucose to glycogen. Low blood glucose
by the pancreas. Glucagon stimulates the
levels stimulate the release of
to glucose.
liver to convert
Exercise and fight/flight situations stimulate the release of
glands.
from the adrenal
.
Adrenaline stimulates the liver (and other tissues) to convert glycogen to
the release of insulin
Adrenaline also stimulates the release of glucagon and
by the pancreas.
Word list: adrenaline, glucagon, glucose, glycogen, high, homeostatically, inhibits, liver,
negative, pancreas.
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TOPIC 9. PATHOLOGY OF CARDIOVASCULAR DISEASE
..........................................
Q8: Complete the sentences by matching the parts on the left with the parts on the
right. (8 marks)
Hyperglycemia:
develops during adulthood.
Chronic vascular
disease:
two blood samples tested after drinking glucose
solution.
Insulin production:
a strong indicator of diabetes.
Type 2 diabetes:
a complication of diabetes.
Insulin receptors on liver
cells:
the inability to control a rise in blood sugar levels.
Obesity:
Type 1 diabetes is caused by its failure.
Positive urine test for
glucose:
Type 2 diabetes is caused by their reduction in number.
frequently associated with Type 2 diabetes.
Glucose tolerance test:
..........................................
Q9: Complete the sentences by matching the parts on the left with the parts on the
right. (8 marks)
Endothelial cells:
contains twice as much energy per gram as protein.
Atherosclerosis:
their digestion requires no metabolic energy.
Peripheral nerve
dysfunction:
an indicator of obesity.
Body mass index:
damaged by chronic high blood glucose levels.
BMI > 30:
reduces stress and hypertension, and raises HDL levels.
Fat:
result of chronic high glucose levels in smaller blood
vessels.
Free sugars in diet:
calculated by dividing mass (kg) by height (m) squared.
Exercise:
result of chronic high glucose levels in larger blood vessels.
..........................................
Q10: What detects the glucose concentration of the blood? (1 mark)
..........................................
Q11: What is released in response to low levels of blood glucose? (1 mark)
..........................................
Q12: Which organ does this stimulate? (1 mark)
..........................................
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Q13: What is its effect? (1 mark)
..........................................
Q14: Adrenaline is released under certain circumstances. Where is ite produced? (1
mark)
..........................................
Q15: When is it produced? (1 mark)
..........................................
Q16: What is its effect? (1 mark)
..........................................
Q17: What are diabetics unable to control? (1 mark)
..........................................
Q18: Name a common complication of diabetes. (1 mark)
..........................................
Q19: Complete the following paragraph by selecting the correct words or phrases from
the options provided. (4 marks)
Type 1 diabetes typically appears in childhood / adulthood
It is treated with insulin injections / change of diet.
Type 2 diabetes is usually developed in childhood / adulthood.
It is more common in developing / developed countries and in people who are underweight / over-weight.
..........................................
Q20: What indicator substance is present in the urine of diabetics? (1 mark)
..........................................
Q21: What test is used for this substance (if urine and blood tests are inconclusive)? (1
mark)
..........................................
Q22: How is this test applied? (2 marks)
..........................................
Q23: Explain how diabetes leads to vascular disease. (2 marks
..........................................
Q24: State one condition which results from high blood sugar levels in larger blood
vessels. (1 mark)
..........................................
Q25: State one condition which results from high blood sugar levels in smaller blood
vessels. (1 mark)
..........................................
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Obesity is major cause of disease in Western societies.
Q26: Obesity results from an excess of body fat in relation to
. (1 mark)
..........................................
Q27: What two features of a Western lifestyle are particularly linked to obesity? (2
marks)
..........................................
Q28: BMI is used to give an estimate of obesity. What does it stand for? (1 mark)
..........................................
Q29: What is the formula for calculating BMI? (1 mark)
..........................................
Q30: What value of BMI is critical is determining obesity? (1 mark)
..........................................
Q31: State two ways in which exercise helps counter obesity. (2 marks)
..........................................
..........................................
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Topic 10
End of unit test
Contents
172
TOPIC 10. END OF UNIT TEST
End of Unit 2 test
Q1: Complete the sentences by matching the parts on the left with the parts on the
right. (12 marks)
Sperm are produced in the seminiferous
follicles.
The ova are contained within
vesicles.
The zygote divides repeatedly to become the
superovulation.
FSH and LH production form part of a negative feedback
tubules.
Testosterone activates the seminal
amniocentesis.
The phase of the menstrual cycle after ovulation is called
blastocyst.
IVF involves the surgical removal of eggs after
disorder.
IUDs are placed in the uterus and reduce the sperm
cycle.
XC Xc is the genotype of a colour vision
phenylalanine.
Screening tests indicate the likely presence of a
luteal.
CVS carries a higher risk of inducing a miscarriage than
carrier.
Individuals with PKU are given a diet which lacks
motility.
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..........................................
Q2: Complete the sentences by matching the parts on the left with the parts on the
right. (12 marks)
Reduces the flow of blood into capillary beds:
diabetes.
Action of the sympathetic and parasympathetic nervous
systems:
sphygmomanometer.
Volume of blood leaving each ventricle in one minute:
statins.
Measures blood pressure:
cardiac output.
Major risk factor for coronary heart disease:
antagonistic.
Component of cell membranes which controls permeability:
obesity.
Transport cholesterol from the liver to body cells:
hypertension.
Drugs which reduce cholesterol levels:
vasoconstriction.
Embolus lodged in the pulmonary artery:
low density
lipoproteins.
Release stimulated by low blood glucose levels:
cholesterol.
Common cause of hyperglycemia:
glucagon.
A BMI (mass/(height squared)) in excess of 30 indicates:
embolism.
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TOPIC 10. END OF UNIT TEST
The following diagram represents the homeostatic control of testosterone production.
Q3:
Identify hormone X. (1 mark)
..........................................
Q4:
Identify tissue Y. (1 mark)
..........................................
Q5:
State two processes which are stimulated by testosterone. (2 marks)
..........................................
Q6: With reference to the diagram, explain what is meant by negative feedback control.
(2 marks)
..........................................
Q7:
Name the part of the brain which initiates puberty. (1 mark)
..........................................
© H ERIOT-WATT U NIVERSITY
TOPIC 10. END OF UNIT TEST
The following diagram shows the average sperm count of samples of 30 men taken at
intervals over a period of 60 years. In order to make the trend clear, a line of best fit has
been drawn.
Q8: Using the line of best fit, calculate the percentage decline in sperm count between
1960 and 2000. (1 mark)
..........................................
Q9: In which year was the maximum average sperm count recorded? (1 mark)
..........................................
Q10: In which year was the minimum average sperm count recorded? (1 mark)
..........................................
Q11: In some years, more than one sample group was assessed. In the years when
this happened, what was the biggest difference recorded? (1 mark)
..........................................
Q12: Suggest two lifestyle factors which may contribute to the decline in sperm count
observed over the period. (2 marks)
..........................................
Q13: In the selection of fertile men to be included in the sample groups, what key factor
should be taken into account? (1 mark)
..........................................
Q14: Describe one way in which ovulation may be stimulated during the treatment of
infertility in women. (1 mark)
..........................................
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TOPIC 10. END OF UNIT TEST
Q15: Under what circumstances is Intracytoplasmic Sperm Injection (ICSI) used? (1
mark)
..........................................
Q16: What are the main features of the ICSI procedure? (2 marks)
..........................................
Q17: Synthetic hormones are used as chemical contraceptives. Explain two ways in
which they achieve their effect. (2 marks)
..........................................
The following diagram shows the heart and the nerves which control its activity.
Q18: To which nervous system do the sympathetic and parasympathetic nerves
belong? (1 mark)
..........................................
Q19: What effect does stimulation by the parasympathetic nerve have on the heart? (1
mark)
..........................................
Q20: Name the chemical released by the sympathetic nerve cells which affects heart
muscle. (1 mark)
..........................................
Q21: Name structure B. (1 mark)
..........................................
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TOPIC 10. END OF UNIT TEST
177
Q22: What is the function of structure B? (1 mark)
..........................................
Q23: Cardiac output, heart rate and stroke volume are linked. If a man has a stroke
volume of 125ml, and his cardiac output 6L/min, what is his heart rate? (1 mark)
..........................................
The following table shows the results of an investigation into the effect of a short period
of exercise on blood pressure. The blood pressure of five students was measured before
and after running on the spot for a short period.
Student
Initial blood pressure (mm
Hg)
Final blood pressure (mm
Hg)
1
120/75
134/82
2
127/79
145/86
3
118/70
124/72
4
134/81
144/83
5
121/75
133/77
Average
124/76
?
Q24: Calculate the average final blood pressure. (1 mark)
..........................................
Q25: Each blood pressure result includes two numbers. What do these represent? (1
mark)
..........................................
Q26: What conclusion can be drawn from these results? (1 mark)
..........................................
Q27: Suggest a suitable control for this investigation. (1 mark)
..........................................
Q28: List two factors which should be kept constant during the investigation. (2 marks)
..........................................
Q29: Type 1 diabetes typically develops in
. (1 mark)
..........................................
Q30: Type 2 diabetes is usually caused by an increase in insulin
mark)
..........................................
Q31: Diabetes is diagnosed by
tolerance test. (1 mark)
..........................................
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Q32: Name a drug used to control high cholesterol levels in the blood. (1 mark)
..........................................
Q33: Explain how it works. (1 mark)
..........................................
The following table contains data describing the effect of changing blood insulin levels
on the rate of glucose uptake by liver cells.
Concentration of insulin in the
blood (units/cm3 )
Rate of glucose uptake by
liver cells (μg/g of liver/hour)
0
5
10
15
20
25
1.8
3.4
4.0
4.4
4.6
4.8
Q34: Construct a line graph to show the data in the table. (3 marks)
..........................................
Q35: Predict the rate of uptake when the concentration of insulin is 30 units/cm 3 . (1
mark)
..........................................
Q36: Using a simple whole number ratio, compare the rate of glucose uptake at 0
units/cm3 of blood insulin with the rate of uptake at 25 units/cm 3 of blood insulin. (1
mark)
..........................................
..........................................
© H ERIOT-WATT U NIVERSITY
GLOSSARY
Glossary
Active transport
movement of substances across the cell membrane against a concentration
gradient, which requires an energy supply
Adrenaline
(epinephrine) peptide hormone (composed of amino acids) released by adrenal
glands next to the kidneys, causing a wide range of effects including stimulating
the liver and muscles to convert glycogen to glucose
Allele
one of the different forms of a gene
Anomaly scan
ultrasound scan taken between 18-20 weeks of pregnancy, used to identify any
aspects of physical development of the limbs and vital organs which are unusual
Archimedes
a Greek mathematician, physicist, engineer, inventor and astronomer who died
about 212BC; find out more about him online if you think the 21 st century has a
monopoly of genius - some of his inventions are still in use today
Artificial insemination
the placing of sperm into the reproductive tract of a female for the purpose of
impregnating the female by using means other than sexual intercourse
Atheroma
a swelling in the artery wall which consists mainly of cholesterol, calcium salts and
fibrous material; also referred to as plaques
Atherosclerosis
a condition associated with a thickening of the artery walls that is caused by a
build-up of fatty material, especially cholesterol
Atria
upper chambers of the heart
Atrio-ventricular (AV) node
A group of modified muscle cells which initiates the contraction of the ventricles
Atrio-ventricular valves
valves located between the atria and ventricles which prevent backflow into the
atria during ventricular systole
Autonomic nervous system
ANS - involuntary system which controls basic body functions, e.g. heart rate,
digestion, and breathing
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GLOSSARY
Autosome
one of the 22 pairs of chromosomes that control the general functioning of the
individual, but which do not determine the sex
Basal metabolic rate
the daily energy use when at rest
Blastocyst
mass of cells, formed by the process of cleavage, which embeds into the
endometrium
Body Mass Index (BMI)
BMI = Mass (kg) / (Height (m) 2 ); a crude measure of body fat content - a BMI
greater than 30 indicating obesity
Carbohydrates
chemicals containing only carbon, hydrogen and oxygen, with the latter pair in the
same proportion as they are found in water (general formula C x (H2 O)y )
Carrier
an individual who is heterozygous for particular characteristic, especially applied to
genetic disorders which are caused by a recessive allele carried on an autosome
or an X-chromosome
Cervix
lower, narrow portion of the uterus where it joins with the top of the vagina
Cholesterol
a waxy steroid of fat, which is both synthesised in the body and taken in from the
diet
Conception
the fusion of sperm and egg
Contraception
the prevention of fertilisation which may be achieved by natural or artificial means
Corpus luteum
develops from the follicle after ovulation and secretes high levels of progesterone
and lower levels of oestrogen
Cortisol
a steroid hormone produced in the adrenal gland in response to stress
Cystic fibrosis
an inherited condition which causes the production of thick mucus in the
respiratory system
Dating scan
ultrasound scan taken between 8-14 weeks of pregnancy, used to determine the
age of the foetus and hence the expected date of delivery
© H ERIOT-WATT U NIVERSITY
GLOSSARY
Deep Vein Thrombosis (DVT)
clots forming in veins, most usually in the legs, which can form an embolus which
may lodge in the lung causing a pulmonary embolism
Density
the mass of one cm3
Diabetes
a group of metabolic diseases in which a person has chronically high blood sugar
levels
Diastole
relaxation of the heart muscle
Diploid
a cell containing two sets of chromosomes
Dominant
an allele that is expressed in the phenotype of the heterozygote
Embolus
a blood clot which has broken free in an artery and may cause a stroke or heart
attack
Endometrium
inner lining of the uterus into which the blastocyst implants
Endothelium
layer of cells forming the inside of all blood vessel walls
Epididymis
narrow, coiled tube attached to the back of each testis in which sperm mature and
are stored
Evolution
the process of change in a species that is driven by the selection of the best
adapted genotypes as breeding stock
F1 generation
the first generation produced by two parents in a cross
F2 generation
the generation produced by crossing two individuals from the F 1 generation
Follicle
group of cells in the ovary containing the ovum
Follicle stimulating hormone
hormone produced by the pituitary which stimulates gamete production in the
testis and ovary
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GLOSSARY
FSH
follicle stimulating hormone
Gamete
sex cell: male - sperm; female - ovum/ova
Gene
a discrete region of a chromosome whose DNA codes for the production of a
polypeptide or protein
Genotype
the genetic makeup of an individual with respect to a particular characteristic
Germline
cells in the body (at any stage) which could pass their genes on to the next
generation, i.e. gametes or their precursors, including undifferentiated stem cells
(c.f. somatic cells)
Glomerular filtrate
the liquid from the blood which passes by pressure filtration from the glomerulus
into the Bowman's capsule in the nephrons of the kidney
Glomeruli
(glomerulus) a capillary bed surrounded by the Bowman's capsule of the kidney
nephron where blood filtration occurs
Glucagon
a peptide hormone (composed of amino acids) released by the α-cells of the islets
of Langerhans in the pancreas, which stimulates the liver to convert glycogen to
glucose
Gluconeogenesis
metabolic pathway located in the liver (and part of the kidney) which makes
glucose from non-carbohydrate organic substrates, e.g. glycerol, lactate and
certain amino acids
Glucose tolerance test
a biochemical test to establish how well a person responds to a large oral dose of
glucose; it is used as a diagnostic test for diabetes
Glycogenesis
synthesis of glucose into glycogen, in the liver and the muscles
Haploid
a cell containing one set of chromosomes (gametes)
HDL/LDL ratio
the ratio of high density lipoproteins to low density lipoproteins in the blood
© H ERIOT-WATT U NIVERSITY
GLOSSARY
HDLs
high-density lipoproteins that carry cholesterol from tissues to the liver
Heterozygous/a heterozygote
an individual possessing two different alleles of a gene/individual who is
heterozygous
Homologous pairs
pairs of chromosomes which carry the same genes at the same place, are the
same length and have the centromere in the same place
Homozygous/a homozygote
an individual possessing two identical alleles of a gene/an individual who is
homozygous
Hormone
a chemical released by a cell or gland in one part of the body which becomes
attached to receptors on particular cells, triggering activity in these target cells
Hyperglycaemia
blood glucose levels above 11mmol/L, causing a wide variety of symptoms
including, at worst, coma and death
Hypoglycaemia
blood glucose levels below 3mmol/L, causing a wide variety of symptoms
including, at worst, coma and death
Hypothalamus
located centrally at the base of the brain, it links the nervous system to the
endocrine system through the pituitary gland, produces hormones, and controls
body temperature, sleep, hunger, and daily body rhythm
ICSH
interstitial cell stimulating hormone, produced by the pituitary gland, which
stimulates testosterone in the interstitial cells of the testis; also known as LH
(luteinising hormone) in the female
ICSI
intracytoplasmic sperm injection
Identical twins
(also called monozygotic twins) two individuals produced by the fertilisation of a
single egg and the subsequent splitting of the ball of cells - they are therefore
genetically identical
Incompletely dominant
an allele that is not completely masked by the dominant allele and which therefore
has some effect on an individual's phenotype
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GLOSSARY
Insulin
a peptide hormone (composed of amino acids) released by the β-cells of the islets
of Langerhans in the pancreas, which stimulates the liver and muscle cells to
absorb glucose and convert it to glycogen
Interstitial cells
also known as Leydig cells, located between the seminiferous tubules of the testis,
which release testosterone in response to ICSH
Invasive
test procedures requiring the removal of tissue, e.g. amniocentesis
IVF
in vitro fertilisation
Karyotype
an image of the chromosomes of one cell arranged in homologous pairs, the
karyotype, which is then analysed to identify any anomalies in terms of the
numbers or structure of the chromosomes
LDLs
low-density lipoproteins that carry cholesterol from the liver to body cells
LH
luteinising hormone
Linked genes
genes carried on the same chromosome
Lipolysis
the break down of fat to fatty acids and glycerol in fat cells
Lumen
central space in a blood vessel
Luteal phase
portion of the menstrual cycle from ovulation to the end of menstruation, during
which the corpus luteum develops and releases progesterone
Luteinising hormone
hormone produced by the pituitary which triggers ovulation and also stimulates the
development of the corpus luteum
Lymphocytes
a type of white blood cell, which includes natural killer (NK) cells, T cells and B
cells
Medulla
part of the central core of the brain which regulates the basic processes of
breathing, heart rate, arousal and sleep
© H ERIOT-WATT U NIVERSITY
GLOSSARY
Meiosis
process of cell division taking place in the testis or ovary which produces gametes
with one of each pair of chromosomes, so half the number in other body cells
Menopause
the stage at the end of a woman's reproductive life (typically between the ages
of 45 and 55) when the ovaries cease to release ova and menstruation no longer
occurs
Menstrual cycle
physiological changes which take place in women of reproductive age, consisting
of three phases: follicular, ovulation, and luteal
Menstruation
shedding of the endometrium over 3-5 days
Metabolism
a term referring to all of the chemical reactions that go on in cells to keep an
organism alive
Mitosis
process of cell division which gives rise to all cells in the body apart from the sex
cells, which exactly copies all of the chromosomes into the daughter cells
mmol/L
millimoles per litre, i.e. the molecular weight in grams, divided by 1000; for glucose
1mmol = 0.18g
Motile
ability to move on its own, using energy to do it
Myocardial infarction
a heart attack that is caused by the interruption of the blood supply to part of the
heart muscle, leading to its death
Non-identical twins
(also called dizygotic twins) two individuals produced by the fertilisation of two
eggs who are no more genetically similar than any other siblings
Non-invasive
test procedures not requiring the removal of tissue, e.g. ultrasound scans
Obesity
a condition in which fat has accumulated in the body to the extent that it begins to
have an adverse effect on health
Oestrogen
(estrogen) steroid hormone produced mainly in the follicles of the ovary, which
causes the development of female secondary sexual characteristics during
puberty and the development of the endometrium during the menstrual cycle
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GLOSSARY
Osmotic pressure
(OP) a measure of the tendency of a solution to take in water; if blood has a high
OP because it contains a high concentration of glucose, it will tend to remove water
from tissues with lower solute concentration
Ovulation
mature follicle ruptures to release the ovum into the fallopian tube
Pancreas
the organ that is located close under the stomach which supplies digestive
enzymes to the small intestine and hormones into the blood
Parasympathetic
part of the autonomic nervous system which is responsible for 'rest and digest'
functions, generally slowing processes
Pedigree chart
a diagram that shows the occurrence of the phenotypes of a particular gene from
one generation to the next
Peripheral Vascular Disease (PVD)
an obstruction by atherosclerosis of large arteries other than those in the vicinity
of the heart or brain
P generation
the parents at the start of any pedigree chart
Phenotype
the expression of a gene in an individual in terms of appearance, behaviour or
biochemistry
Phenylketonuria
(PKU) a genetic disorder caused by a recessive mutation to the autosomal gene
which produces the enzyme that converts phenylalanine to tyrosine; the build-up
of phenylalanine after birth seriously slows brain development
Pituitary gland
endocrine gland, attached to the hypothalamus at the base of the brain, which
releases nine hormones involved in homeostasis
Platelets
cell fragments which are formed in the bone marrow; they lack a nuclues and are
involved in both blood clotting and tissue regeneration
Polysaccharide
large carbohydrate molecules consisting of repeating units, e.g. glycogen, which
is made up of glucose units
Precursor
a compound that is part of the chemical reactions that produces another
compound
© H ERIOT-WATT U NIVERSITY
GLOSSARY
Primates
the class of vertebrate animals to which humans belongs, which includes species
ranging in size from tiny lemurs to the gorilla; humans belong to the Hominidae
family along with the great apes
Progesterone
steroid hormone produced by the corpus luteum and the placenta, which promotes
the vascularisation of the endometrium and maintains it during pregnancy, and
also decreases the mother's immune response during pregnancy
Puberty
sequence of physical changes by which the human body develops from that of a
child into that of an adult capable of reproduction
Recessive
an allele that is not expressed in the phenotype of the heterozygote
Red blood cells
cells which lack a nucleus; they are formed in the red bone marrow and transport
oxygen around the body in the blood, taking it in when the oxygen concentration
is high and giving it out when the surrounding oxygen concentration is low
Respiratory substrates
molecules which may act as the raw materials of respiration, e.g. glucose, amino
and fatty acids
Rhesus
one of the blood group systems which refers to the D-antigen; Rh+ indicating the
presence of the D-antigen and Rh- its absence
Saturated fats
fat containing only fatty acids which have no double bonds
Semilunar valve
valve in the opening of the major arteries leaving the heart which prevent backflow
into the ventricles during diastole
Seminal fluid
fluid released during ejaculation by the male which contains sperm, water,
proteins, amino acids, acids and minerals
Seminiferous tubules
site of meiosis in the testes
Septum
wall separating the left and right sides of the heart
Sex-chromosome
one of the pair of chromosomes which determine the sex of the individual (XX in
the female and XY in the male)
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GLOSSARY
Sex-linked genes
genes carried on the X-chromosome
Siblings
offspring of the same parents, although the term can also be applied children who
share a single parent (half- as opposed to full-siblings)
Somatic
cells of the body other than the germline cells
Stroke
a loss of brain function resulting from an interruption of blood supply caused by a
blockage (thrombosis) or a haemorrhage
Sucrose
a discaccharide sugar composed of glucose and fructose
Superovulation
the development of several follicles and ova in a single menstrual cycle as a result
of the taking of fertility drugs
Sympathetic
part of the autonomic nervous system which is responsible for 'fight or flight'
functions, generally speeding up processes
Systole
contraction of the heart muscle, atrial systole preceding ventricular
Testis
male reproductive organ
Testosterone
steroid hormone, released by the interstitial cells, which stimulates the
development of secondary sexual characteristics in puberty, sperm maturation,
prevention of osteoporosis and many other effects
Thrombosis
the formation of a blood clot in an artery, which may break away to form an embolus
- this may block the artery, causing a stroke or heart attack
Thrombus
a blood clot
Tissue fluid
(interstitial fluid) liquid which leaves the capillaries and bathes all cells; differs from
blood in lacking red blood cells and the largest protein molecules
Twins
two individuals produced in the same pregnancy
© H ERIOT-WATT U NIVERSITY
GLOSSARY
Type 1 diabetes
formerly known as juvenile diabetes, usually developing in childhood, it is caused
by autoimmune destruction of the β-cells in the pancreas which produce insulin
Type 2 diabetes
also known as non-insulin-dependent or adult-onset diabetes, it is caused by the
development of insulin resistance or, less often, by failure of insulin production
Vasoconstriction
narrowing of the smaller arteries and arterioles, restricting blood supply
Vasodilation
opening up of the smaller arteries and arterioles, increasing blood supply
Ventricles
lower chambers of the heart
White blood cells
cells of various types which are formed in the bone marrow and act as part of the
immune system, combatting disease
Zygote
cell formed as the result of fertilisation, the fusion of sperm and egg
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ANSWERS: TOPIC 1
Answers to questions and activities
1 Reproductive organs, gametes and fertilisation
Gametes: Questions (page 3)
Q1: Meiosis reduces the chromosome number from diploid (2n) to haploid (n) so that
when fertilisation occurs, the diploid number is re-established.
(In the formation of homologous pairs, meiosis also ensures that all genes are present
in the gamete and therefore in the zygote.)
Q2: Sperm are very small, long and thin, with no food stores. All these features make
them more efficient swimmers.
The large round egg is not concerned about moving through its medium and carries a
food store to support the zygote in its later development.
The male reproductive system: Questions (page 5)
Q3:
© H ERIOT-WATT U NIVERSITY
ANSWERS: TOPIC 1
Q4:
Q5: Seminiferous tubules
Q6: Interstitial cells
Q7: The seminal fluid provides a liquid medium in which the sperm can swim most
efficiently so energy is not wasted. It also provides food, especially sugar, which supplies
the sperm with energy so they do not have to carry energy stores.
The female reproductive system: Questions (page 9)
Q8:
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ANSWERS: TOPIC 1
Q9:
1. In response to increasing levels of FSH, a dormant 'primordial' follicle begins to
grow.
2. The follicles matures.
3. At the point of maturation it is known as a Graafian follicle.
4. The Graafian follicle bursts, releasing the ovum from the ovary. This is ovulation.
5. The Graafian follicle develops under the influence of LH into the corpus luteum.
6. If the ovum is not fertilised, the corpus lutem degenerates.
After ovulation: Questions (page 10)
Q10: The yolk is used up, providing energy for cell division.
Q11:
a) Mitochondrial DNA is not carried on the chromosomes in the nucleus, so is not
involved in meiosis.
b) The Y-chromosome (unlike the X) has no homologous partner and so does not
undergo crossing over during meiosis I.
Extended response question: Development of ova (page 12)
Suggested marking scheme
Each line represents a point worth one mark. The concept may be expressed in other
words. Words which are bracketed are not essential. Alternative answers are separated
by a solidus (/); if both such answers are given, only a single mark is allocated. In
checking the answer, the number of the point being allocated a mark should be written
on the answer paper. A maximum of eight marks can be gained.
A) Development of ova in the ovary (maximum of 6 marks):
1.
2.
3.
4.
5.
6.
7.
8.
The ovary contains many immature ova.
Each ovum develops in a follicle.
Follicles protect the ova.
Follicles secrete hormones / oestrogen.
Every 28 days, a (mature) follicle moves to the surface of the ovary.
An ovum is released into the oviduct.
The follicle develops into the corpus luteum. . .
. . .which releases hormones / progesterone.
B) After ovulation (maximum of 2 marks):
i
ii
iii
iv
The ovum may be fertilised to form a zygote.
The fertilised ovum / zygote divides many times to form the blastocyst.
The blastocyst implants in the endometrium.
Unfertilised ova pass out of the body .
© H ERIOT-WATT U NIVERSITY
ANSWERS: TOPIC 1
193
End of Topic 1 test (page 12)
Q12: Gametes are produced from germline cells which are found in the testes and
ovaries. These cells divide first by mitosis and then by meiosis. Sperm are produced
in the seminiferous tubules of the testes and ova within the follicles in the ovaries.
Seminal fluid contains secretions from the prostrate gland and the seminal vesicles
which maintain the motility and viability of the sperm. Hormones are released from
the interstitial cells, the follicles and the corpus luteum. Ovulation takes place
approximately every 28 days, releasing an ovum into the oviduct. Fertilisation takes
place in the oviduct, forming a zygote which then undergoes a series of divisions to
form the blastocyst.
Q13:
Cell type
Difference
Division
Germline
Somatic
mitosis and meiosis
mitosis
gametes
all other cells of the body
Develop into
Q14: A, C
Q15: Interstitial cells
Q16: Secrete / release testosterone
Q17: X: immature ovum in developing follicle.
Y: maturing follicle, releasing hormone/oestrogen.
Z: follicle ruptures at surface of ovary; ovulation/ovum released
Q18: Corpus luteum
Q19: Release of hormone / progesterone
Q20:
1. If it is unfertilised it passes out of the body.
2. If it is fertilised it forms a zygote.
Q21: By repeated divisions of the zygote to form a ball of cells.
Q22: It implants in the endometrium.
Q23: 21.1%
Q24: 13:2
Q25: By taking several samples from each man.
Q26: 1, 3, 4, 5
Q27: 2, 3, 4
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ANSWERS: TOPIC 2
2 Hormonal control of reproduction
Onset of puberty: Question (page 17)
Q1:
Hypothalamus
produces releaser hormone.
Pituitary gland
secretes FSH and LH.
FSH acts on the ovaries
follicles develop.
FSH acts on the testes
seminiferous tubules start to produce
sperm.
Follicles release oestrogen
under the influence of FSH and LH.
After ovulation
LH causes the release of progesterone.
The corpus luteum
releases progesterone.
LH acts on the interstitial cells
causing release of testosterone.
The menstrual cycle: Questions (page 20)
Q2: Warm / dry weather, abundant food supply for young and mother, few predators /
abundant prey available for predators.
Q3:
More reliable all-year food supply, shelter reduces exposure to weather.
The follicular phase: Questions (page 22)
Q4:
Low oestrogen and progesterone levels
pituitary releases high level of FSH.
High level of FSH causes
development of follicle.
Developing follicle releases oestrogen
increasing oestrogen levels suppress
FSH release.
Oestrogen acts on the endometrium
causing proliferation.
Follicles release oestrogen
under the influence of oestrogen.
© H ERIOT-WATT U NIVERSITY
ANSWERS: TOPIC 2
195
Q5: Development order:
1. oestrogen level rises;
2. oestrogen reaches critical level;
3. pituitary releases surge of LH;
4. LH acts on mature follicle;
5. follicle releases egg into oviduct.
The luteal phase: Questions (page 24)
Q6:
Q7:
Hormone changes
Organ responses
Low level of LH secreted by pituitary:
corpus luteum degenerates
Progesterone production falls:
endometrium breaks down
Low level of oestrogen in blood:
pituitary increases secretion of FSH
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ANSWERS: TOPIC 2
Extended response question: Negative feedback control (page 27)
Suggested marking scheme
Each line represents a point worth one mark. The concept may be expressed in other
words. Words which are bracketed are not essential. Alternative answers are separated
by a solidus (/); if both such answers are given, only a single mark is allocated. In
checking the answer, the number of the point being allocated a mark should be written
on the answer paper. A maximum of ten marks can be gained.
A) Testosterone production (maximum of 3 marks):
1. The pituitary gland releases LH/ICSH.
2. LH stimulates the interstitial cells of the testes to release testosterone.
3. Increasing level of testosterone inhibits/reduces the production/release of LH.
4. Pituitary gland releases less LH. . .
5. . . .so testes/interstitial cells release less testosterone.
B) The luteal phase of the menstrual cycle (maximum of 7 marks):
i
After ovulation, the high level of LH causes the follicle to develop into the
corpus luteum.
ii
The corpus luteum secretes progesterone and oestrogen.
iii During this phase the secretion of oestrogen and progesterone rise to a
maximum and then decline.
iv The high levels of oestrogen inhibit the pituitary from secreting FSH.
v
The high levels of progesterone inhibit the pituitary from secreting LH.
vi The resulting low level of FSH suppresses the development of further follicles.
vii The low level of LH causes the corpus luteum to degenerate. . .
viii . . .and progesterone secretion to fall to a minimum.
ix The falling level of progesterone at the end of the cycle triggers the start of
menstruation.
x
The low level of oestrogen at the end of the cycle causes the pituitary to
increase secretion of FSH.
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End of Topic 2 test (page 27)
Q8:
Hypothalamus:
a releaser hormone released from here triggers puberty.
Pituitary:
responds to the releaser hormone by releasing other
hormones.
FSH and LH:
controls the production of gametes.
Negative feedback
cycle:
FSH and LH form part of it.
FSH:
stimulates the cells lining the seminiferous tubules.
LH/ICSH:
stimulates the interstitial cell.
Testosterone:
activates the prostate and seminal vesicles.
Inhibit FSH and LH
release:
high levels of testosterone production.
Follicular phase:
menstrual cycle up to ovulation.
Luteal phase:
mestrual cycle after ovulation.
Q9:
At the start of the menstrual cycle, the pituitary releases high levels of FSH.
The hormone which stimulates the development of follicles FSH.
Proliferation of the endometrium is stimulated by oestrogen.
High levels stimulate the surge in the release of LH oestrogen.
High levels stimulate the development of the corpus luteum LH.
Hormone which causes further vascularistation of the endometrium progesterone.
High levels oestrogen and testosterone inhibit the production FSH and LH.
Degeneration of the corpus luteum is caused by low levels of LH.
Falling levels trigger the start of menstruation progesterone.
Stimulates the increased production of FSH by the pituitary oestrogen.
Q10: The hypothalamus produces a releaser hormone.
Q11:
Hormone
Effect on male
Effect on female
FSH
Seminiferous tubules start
to produce sperm
Follicles start to mature in
the ovary and release
oestrogen
Interstitial cells start to
release testosterone
Follicles release oestrogen
LH
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Q12: Follicle
Q13: Ovum
Q14: Oestrogen
Q15: Causes a decrease in the production of FSH...
...and an increase in the production of LH.
Q16: Structure B grows in size, becoming a dense yellow structure.
Q17: The corpus luteum releases a high level of oestrogen...
...which inhibits the pituitary from producing FSH.
Q18: Interstitial cells
Q19: Seminiferous tubule
Q20: D/interstitial cells
Q21: Testosterone
Q22:
1. The pituitary releases LH, which stimulates interstitial cells to produce testosterone;
2. an increased level of testosterone causes the pituitary to reduce LH secretion;
3. interstitial cells reduce testosterone production so the pituitary increases LH
release.
Q23:
Hormone
Name
Oestrogen
A
Progesterone
B
Produced
Follicle
Corpus luteum
C
Follicle stimulating hormone
Pituitary
D
Luteinising hormone
Pituitary
Q24: Day 15
Q25: The high concentration of oestrogen (hormone A) stimulates the pituitary to
release a high concentration of LH which stimulates ovulation.
Q26: Description: concentration increases from day 1 to 4, then falls from day 4 or 5 to
day 12.
Explanation:
1. the pituitary releases an increasing concentration of FSH on days 1 to 4;
2. the increase from day 1 to 4 is caused by a low level of oestrogen/A;
3. the decrease from day 5 to 12 is caused by an increase in secretion of oestrogen/A;
4. which causes a reduction in the secretion of FSH/C.
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Q27: Suppresses production of FSH and therefore development of follicles.
Q28: Hormone B is progesterone which promotes the further proliferation of the
endometrium.
Q29:
1. B is progesterone which is released by the corpus luteum.
2. The corpus luteum is only present after ovulation.
3. The corpus luteum grows from day 16 to 22 then degenerates from day 23 to 28.
4. This is caused by an increase then a decrease in secretion of LH by pituitary.
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ANSWERS: TOPIC 3
3 The biology of controlling fertility
Fertile periods: Questions (page 35)
Q1: ICSH and testosterone interact in a negative feedback loop: as testosterone
secreted by the interstitial cells increases in concentration in the blood, the pituitary
responds by decreasing its release of ICSH, causing a fall in the production of
testosterone. As a result, the level of testosterone remains fairly constant. Given that
testosterone stimulates the production of sperm by the seminiferous tubules, this results
in sperm production also remaining constant.
Q2:
A surge in secretion of LH by pituitary. (the actual mechanism is complex)
Q3:
Any from:
• women can have irregular cycles;
• not allowing sufficient time before or after ovulation;
• not abstaining from intercourse during the fertile period.
Causes of infertility: Question (page 37)
Q4:
Example of factor for:
Factor affecting fertility
men
women
Age
Gradual reduction over lifetime,
with reduction in quality and
concentration of sperm
Maximum fertility at about age 25, a
steady reduction to 35, then a faster
reduction until menopause (45-55)
Genetics
Y microdeletions, Klinefelter’s
syndrome
Turner syndrome and many others
Disease
Mumps, chlamydia, testicular
cancer
Gonorrhoea, eating disorders
Lifestyle
Smoking tobacco, drug and
alcohol abuse, anabolic steroids
Smoking tobacco or cannabis,
chemotherapy
Artificial insemination: Question (page 38)
Q5:
Any from:
• sperm samples are much easier to transport than live animals such as bulls;
• sperm from one male can be used to fertilise large numbers of females;
• sperm can be transported long distances from one country to another without the
complications of importing live animals;
• farmers don’t have to buy a high quality animal in order to access its genetic
qualities.
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Stimulating ovulation: Question (page 39)
Q6: The development of follicles is stimulated by FSH secreted by the pituitary, but the
developing follicle releases oestrogen which inhibits the release of FSH, preventing the
development of other follicles.
Blocking oestrogen receptors in the pituitary will prevent this inhibition so that FSH
secretion will remain high.
Artificially increasing FSH levels will maintain the concentrations so that follicle
development continues to be stimulated.
In vitro fertilisation: Question (page 40)
Q7:
a) Woman given FSH injection.
b) Superovulation.
c) Harvesting of ova.
d) Sperm are washed.
e) Ova incubated with sperm.
f) Eight cell stage at 3 days.
g) Two cells removed from embryo.
h) Chromosomes and DNA analysed.
i)
Blastocyst at eighth day.
j)
Embryo transferred into uterus.
Treatments for infertility: Question (page 41)
Q8:
Treatment
Full name
Appropriate for
AI
artificial insemination
low sperm count;
conception not possible by
sexual intercourse
IVF
in vitro fertilisation
blocked fallopian tubes;
low sperm count
ICSI
intracytoplasmic sperm
injection
low sperm motility;
low sperm count
PGD
pre-implantation genetic
diagnosis
inherited genetic disorder;
chromosome abnormality
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Calendar-based methods: Question (page 42)
Q9: Day 14 in the menstrual cycle. Sperm can survive for up to three days in the
woman's system and the ovum is able to be fertilised for up to two days after ovulation.
Contraception: Question (page 43)
Q10:
Sterilisation is available to men and women and is irreversible.
The mini-pill causes thickening of cervical mucus and contains progesterone.
Barrier methods stop sperm reaching uterus and are available for men and women.
The morning-after pill contains high levels of synthetic oestrogen and progesterone
and suppresses ovulation.
IUDs are placed in the uterus and prevent fertilisation.
The rhythm method requires careful monitoring of temperature and cervical mucus
and can be used without medical supervision.
The pill contains synthetic oestrogen and progesterone is taken every day.
Extended response question: Fertile periods (page 46)
Suggested marking scheme
Each line represents a point worth one mark. The concept may be expressed in other
words. Words which are bracketed are not essential. Alternative answers are separated
by a solidus (/); if both such answers are given, only a single mark is allocated. In
checking the answer, the number of the point being allocated a mark should be written
on the answer paper. A maximum of ten marks can be gained.
A) Men (maximum of 4 marks):
1. Men are fertile from puberty to death.
2. Fertility does decrease with age.
3. Men are continuously fertile / produce sperm constantly.
4. Testosterone stimulates sperm production.
5. Testosterone production is stimulated by ICSH from pituitary.
6. Testosterone and ICSH interact in a negative feedback loop.
B) Women (maximum of 6 marks):
i
Women are fertile from puberty to the menopause / age 45 - 55.
ii
Women are cyclically fertile.
iii Only fertile for a few days each month around time of ovulation.
iv Ovulation is stimulated by a surge in LH secretion.
v
LH is secreted by the pituitary gland.
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vi Oestrogen and progesterone from ovary interact in a negative feedback
loop. . .
vii . . .with FSH and LH from the pituitary.
viii Time of ovulation can be determined by recording body temperature and
cervical mucus.
ix Fertile period lasts from a few days before to a few days after ovulation.
End of Topic 3 test (page 46)
Q11:
A woman is capable of conceiving
during the fertile
period.
Men are continuously fertile
from puberty to
death.
Cyclically fertile means that women are fertile
for a few days a
month.
A change in a woman's cervical mucus occurs
during ovulation.
An example of a barrier method is
the diaphragm.
A method of reducing the motility of sperm is
the intra-uterine
device.
The procedure that involves cutting the vas deferens is
sterilisation.
The morning after pill contains synthetic oestrogen and
progesterone.
The mini-pill causes thickening of
cervical mucous.
Q12: Men and women can both be made infertile by factors such as age, genetics,
disease and lifestyle.
Artificial insemination is particularly used when a man has a low sperm count.
Ovulation is stimulated when a woman is ovulating irregularly.
Super-ovulation may result in multiple births.
IVF uses super-ovulation as it provides several eggs that may be collected.
Ovulation is enhanced by stimulating the secretion of FSH or supplementing hormone
levels with synthetic mimics.
IVF involves removal of eggs after super-ovulation.
Embryos are transferred when they reach the 8-16 cell stage.
Pre-implantation genetic screening identifies genetic disorders.
ICSI differs from IVF in that a single sperm is injected into an ovum.
Q13: Interstitial cells
Q14: LH
Q15: Testosterone
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ANSWERS: TOPIC 3
Q16:
1. Testosterone stimulates sperm production.
2. Increased testosterone inhibits LH secretion which reduces testosterone level and
sperm production.
3. This reduced testosterone allows for an increase in LH secretion, increasing
testosterone levels and sperm production.
Q17: Women are fertile for a few days each month.
Q18: A couple of examples are given below. Any suitable form of infertility with its cause
would be accepted.
Form of infertility: no sperm.
Cause: cancer / Klinefelter's syndrome.
Form of infertility: low sperm count.
Cause: advancing age, smoking tobacco.
If you have other answers, show them to your teacher for marking since you may still be
correct.
Q19:
Form of infertility: failure to ovulate.
Cause: eating disorder.
Form of infertility: blocked fallopian tubes.
Cause: gonorrhoea.
If you have other answers, show them to your teacher for marking since you may still be
correct.
Q20: The artificially induced production of several ova at one time.
Q21: Either of:
• blocking oestrogen receptors - causing FSH levels to remain high stimulating follicle
development;
• increasing hormone levels with synthetic FSH - causing FSH levels to rise
stimulating several follicles to develop.
Q22: For men with a low sperm count or unable to carry out sexual intercourse.
Q23: Similarity: both fertilise egg outside the mother/in a petri dish, both require eggs
to be collected by superovulation;
Difference: intracytoplasmic sperm injection involves the direct injection of the sperm
nucleus into the egg, but IVF allows sperm to fertilise egg on its own.
Q24: Chromosomes / DNA taken from the embryo.
Q25: 8-cell stage.
Q26: Genetic disorders AND chromosome abnormalities.
Q27: Where the family history showed a high incidence of an inherited disorder.
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Q28: Day 16
Q29: Rise
in
temperature
thinner/watery/slippery.
(0.5 ◦ C)
AND
cervical
mucus
becomes
Q30: Days 14 - 18 (13 - 19 allowed)
Q31: Any five from:
• Contraceptive pills regulate the negative feedback control of ovulation.
• Contraceptive pills contain synthetic/artificial forms of oestrogen and progesterone.
• The pill contains both oestrogen and progesterone.
• The pill suppresses ovulation but maintains menstruation.
• The mini-pill contains progesterone only.
• The mini-pill prevents fertilisation by causing thickening of cervical mucus.
• The morning-after pill contains high levels of oestrogen and progesterone.
• The morning-after pill suppresses ovulation / inhibits implantation.
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ANSWERS: TOPIC 4
4 Antenatal and postnatal screening and care
Antenatal care: Question (page 54)
Q1:
Condition
Check/Test
Renal failure
Urine test
Obesity
BMI
Cystic fibrosis
Medical history
Pre-eclampsia
Blood pressure test
Diabetes
Blood sugar test
Biochemical tests: Question (page 57)
Q2: Anomaly scan - abnormally high level of fluid around the neck of the fetus,
absence of bone in nose.
Biochemical test - abnormally low levels of estriol and alpha-fetoprotein in the mother's
blood.
Diagnostic testing: Question (page 57)
Q3:
Soon after her pregnancy is confirmed, a woman is given a range of routine tests to
assess the possibility of complications to the pregnancy. These are screening tests.
Examples are blood pressure, blood pressure and urine tests.
These are followed a dating scan at 8 - 14 weeks which determines due date. A second
scan at 18 - 20 weeks seeks to identify physical problems and is called an anomaly
scan. Both these scans use ultrasound imaging.
All the tests mentioned so far identify the possibility of a disorder being present and
are referred to as screening tests. Tests which identify the presence of a disorder very
precisely are called diagnostic tests. Examples of such tests are amniocentesis and
CVS.
Chorionic villus sampling: Question (page 58)
Q4: A screening test, such as ultrasound imaging or a blood test, detects symptoms
which indicate the possibility of particular abnormalities being present.
A diagnostic test, such as karyotyping after amniocentesis or CVS, identifies the
presence or absence of a specific condition.
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The moral dimension: Question (page 59)
Q5: Initial visit to GP: height and weight, blood pressure, blood and urine tests, medical
history.
Dating scan: 8 - 15 weeks to determine exact age of fetus.
If the couple feel the serious nature of the disorder justifies the risk, then a diagnostic
test would be carried out - either CVS or an early amniocentesis with DNA analysis.
If the tests prove positive, is the pregnancy to proceed? If so, special care must be
prepared for the child and the parents must adjust their lives to cope with this.
Alternatively, the decision may be to terminate the pregnancy. This will also require
appropriate aftercare for the mother.
Rhesus antibody testing: Question (page 60)
Q6: If the fetus is Rhesus positive and the mother Rhesus negative, the mother's
system will become sensitised to the Rhesus positive antigen.
A second pregnancy will result in a large immune response by the mother, which will
cause antibodies to pass across the placenta and destroy the fetal red blood cells,
resulting in jaundice after birth.
The treatment is to give a Rhesus negative woman injections of anti-Rhesus antibodies
(immunoglobulins) before and after birth. These destroy the fetal red blood cells before
the mother's immune system can respond.
Postnatal screening: Question (page 61)
Q7: PKU is Phenylketonuria , a genetic disorder caused by a recessive mutation to the
autosomal gene which produces the enzyme which converts phenylalanine to tyrosine.
The build-up of phenylalanine after birth seriously slows brain development.
PKU is tested by the heel-prick test, which samples blood within three days of birth. The
dried blood is tested for several inherited disorders.
PKU is treated by having a diet which does not contain phenylalanine.
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ANSWERS: TOPIC 4
Extended response question: Tests which can be carried out during pregnancy
(page 64)
Suggested marking scheme
Each line represents a point worth one mark. The concept may be expressed in other
words. Words which are bracketed are not essential. Alternative answers are separated
by a solidus (/); if both such answers are given, only a single mark is allocated. In
checking the answer, the number of the point being allocated a mark should be written
on the answer paper. A maximum of ten marks can be gained.
A) Screening tests (maximum of 6 marks):
1. Screening indicates possibility of a condition.
2. Ultrasound dating scan at 8-14 weeks. . .
3. . . .indicates age of fetus / likely due date.
4. Ultrasound anomaly scan at 18-20 weeks. . .
5. . . .indicates possible unusual development, associated with e.g.
syndrome.
Down's
6. Biochemical tests on blood samples.
7. Levels of marker chemicals inappropriate to stage of pregnancy, e.g. α-feto
protein. . .
8. . . .indicate possible presence of Down's syndrome.
B) Diagnostic tests (maximum of 4 marks):
i
Diagnostic tests confirm the presence of condition.
ii
Amniocentesis removes cells from the amniotic fluid. . .
iii . . .and are carried out at about 18 weeks
iv Chorionic villus sampling removes cells from the placenta. . .
v
. . .and are carried out at 10-12 weeks.
vi Karyotyping - examination of fetal chromosomes.
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End of Topic 5 test (page 65)
Q8:
Tested as part of a general health check:
blood pressure.
A procedure that may be involved in IVF:
pre-implantation genetic
diagnosis.
Usually taken 8-14 weeks after pregnancy:
first ultrasound imaging.
Used to detect the possibility of physical problems:
second ultrasound imaging.
Timing of tests taken into account when assessing:
blood marker chemicals.
Used to indicate the likelihood of a disorder:
screening tests.
Used to confirm the presence of a disorder:
diagnostic tests.
Screening tests include:
ultrasound and blood tests.
Diagnostic tests include:
amniocentesis & chorionic
villus sampling.
The name for an image of a foetus' chromosomes:
karyotype.
Involves removing cells from the amniotic fluid:
amniocentesis.
Involves removing cells from the placenta:
chorionic villus sampling.
Q9: A Guthrie heel-prick test is used to collect a blood sample from a newborn
child, which screens for several inherited disorders. Phenyketonuria is an autosomal
recessive disorder causing high levels of phenylalanine which restricts brain
development. Individuals with PKU are given a restricted diet lacking phenylalanine.
Q10: Blood pressure
Q11: Cardiovascular disease, diabetes and hypoglycaemia.
Q12: Family history
Q13: A screening test, such as ultrasound imaging or a blood test, detects symptoms
which indicate the possibility of particular abnormalities being present.
A diagnostic test, such as karyotyping after amniocentesis or CVS, identifies the
presence or absence of a specific condition.
Q14: Either of:
• dating scan;
• anomaly scan.
Q15: Dating scan: 10 - 14 weeks; to determine the age of the fetus/due date.
Anomaly scan: 18 - 20 weeks; to detect possible physical problems.
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Q16: A marker chemical may indicate the presence of a fetal disorder if its level is above
or below the level in the mother's blood. . .
. . .which is normally expected at that stage in the pregnancy.
Q17: A marker chemical may indicate the presence of a disorder when it is absent. . .
. . .if the age of the fetus is not correctly determined.
Q18: To produce an image of the fetus in the uterus.
To guide the needle and avoid damage to the fetus / placenta.
Q19:
1. Tissue sampled is placenta.
2. Fetus much smaller.
Q20: A karyotype is an image of an individual's chromosomes, arranged in homologous
pairs.
A karyotype is used to identify anomalies in numbers or structure of chromosomes (as
well as the sex of the individual).
Q21: An autosomal recessive mutation.
Q22: It results in high levels of phenylalanine which severely restrict brain development.
Q23: A restricted diet which lacks phenylalanine.
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211
5 Patterns of inheritance
The monohybrid cross: F1 generation (page 72)
Q1: BB (for the male) and bb (for the female).
Q2: The parents produce haploid gametes with the genotypes B (from the male) and
b (from the female). This means that the offspring in the F 1 generation all have the
genotype Bb. All of the offspring will be brown because B is completely dominant to b.
The monohybrid cross: F2 generation (page 73)
Q3: 3:1
Q4: Both parents have the genotype Bb. This means that the genotypes of the
gametes will be B or b (in equal proportion). There are four possible genotypes for the
F2 generation, as shown by the Punnet square below, of which only one (bb) will confer
a white colour. Consequently, the ratio of brown coloured to white coloured bobbits in
the F2 generation will be 3:1.
Male gametes
Female gametes
B
b
B
BB
Bb
b
Bb
bb
Cystic fibrosis: Questions (page 74)
Q5:
N
n
N
NN
Nn
n
Nn
nn
Q6: 25
Q7: 25
Q8: The chances are zero because any child will receive a normal allele from the
homozygous normal parent.
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ANSWERS: TOPIC 5
Dominance: Question (page 75)
Q9:
H
h
H
Hh
hh
h
Hh
hh
Q10: 50
Q11: 100
Q12: Because the symptoms of Huntington's do not manifest until middle age, affected
individuals tend to have reproduced before they are affected. Therefore, the allele will be
passed onto the next generation. (Normally lethal alleles such as these affect individuals
before they reach reproductive age, which means that they usually die before they
can reproduce - such an abnormal allele would therefore quickly disappear from the
population.)
Incomplete dominance: Questions (page 77)
Q13: In heterozygous (HS) individuals, the S allele provides protection against malaria.
Therefore, natural selection favours these individuals because they are more likely to
survive and reproduce. Individuals who are homozygous for the normal H allele (HH)
have a greater chance of dying from malaria.
Q14: There is no selective advantage in having the HS genotype as protection against
malaria is not necessary.
Sex-linked inheritance: Questions (page 78)
Q15:
Chromosome
Gametes
Woman
Man
XX
XY
X
X
X
Y
Q16:
Male gametes
Female gametes
X
Y
X
XX
XY
X
XX
XY
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ANSWERS: TOPIC 5
Q17: The mother produces gametes (egg cells) containing one X-chromosome. The
father produces gametes (sperm cells) which contain either an X or Y-chromosome,
each of which is equally likely to occur in a sperm cell. Consequently, there is an equal
chance of a male (XY) or female (XX) being produced in the offspring.
Q18: b) 50%
Q19:
Q20:
Q21:
• 50%
• 50%
• 0%
Q22:
• 25%
• 25%
• 25%
• 25%
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ANSWERS: TOPIC 5
Q23:
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Q24: A male cannot pass an X-chromosome to a son, only to a daughter. The daughter
of a male with a recessive sex-linked disorder will always be a carrier, but the affected
male cannot have a son who is affected (assuming that the female partner is not a
carrier). There is a 50% chance that the son of a female carrier will be affected with the
disorder, assuming that the male partner is not affected.
End of Topic 4 test (page 83)
Q25:
Diploid:
a cell containing two of each chromosome.
Gamete:
a cell containing one of each chromosome.
Alleles:
alternative forms of a gene.
Genotype:
a pair of alleles carried by a gene.
Phenotype:
the expression of the alleles of a gene.
Homozygous:
a genotype with two identical alleles.
Dominant:
an allele which is always expressed in the phenotype if
present.
Recessive:
allele which is only expressed by a homozygous person.
Incomplete
dominance:
causes three different phenotypes for a gene.
Autosome:
a chromosome which does not affect gender.
X:
a chromosome which carries sex-linked genes.
Siblings:
brothers and sisters.
Q26:
Number of alleles a man has for an autosomal gene on his Y chromosome: 0
Genotype of a woman heterozygous for tongue-rolling: Tt
Genotype of a man who will not develop Huntingdons' disease (caused by a dominant
autosomal allele): hh
Genotype of a woman who shows sickle cell trait as a result of incomplete dominance:
HS
Number of alleles a man carries for a gene with multiple alleles: 2
Genotype of a woman with type O blood group: OO
Not true of homologous chromosomes: same alleles
Percentage chance of a girl inheriting a sex-linked allele from her father: 100%
Gametes of the daughter of a father unaffected by recessive condition and a
heterozygous mother: XB and Xb
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ANSWERS: TOPIC 5
Proportion of heterozygotes among the children of parents who are heterozygous for a
condition which is lethal in early pregnancy: 2 in 3
Q27: An individual who possesses two identical alleles controlling a characteristic is
said to be homozygous for that characteristic.
Q28: An individual's genotype is their genetic makeup with respect to a particular
characteristic.
Q29: Characteristics which have three phenotypes are caused by alleles which show
incomplete dominance.
Q30: d) 100%
Q31: b) 50%
Q32: b) 50%
Q33: b) 1:2:1
Q34: c) 2:1
Q35: b) 50%
Q36: a) 25%
Q37: b) 50%
Q38: a) 0%
Q39: d) 100%
Q40: c) 50%
Q41: a) 0%
Q42: c) 50%
Q43: b) 25%
Q44: Since sex-linked genes are found on the X–chromosome, the daughter will receive
the sex-linked gene from the father's X-chromosome (she will be a carrier).
Because she is a carrier, she will not show the condition, but will have a 50% chance of
passing the affected allele on to her children.
Any son who receives the affected allele will show the condition, because he possesses
only the affected allele (because he will receive the Y-chromosome from his father).
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6 Blood vessels
Why have a cardiovascular system: Questions (page 91)
Q1: Living cells need to take in a respiratory substrate to provide ATP to power
metabolism. They also require raw materials such as amino acids and fats. Waste
materials such as carbon dioxide and products like hormones must be removed.
Q2:
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
The heart contracts, increasing the pressure of the blood
Blood flows through major arteries
Muscular walls contract to maintain blood pressure
Arteries divide to form smaller arterioles
Blood in arterioles is at higher pressure that fluid in tissues
Fluid is forced from capillaries into tissues
Cells exchange substances with tissue fluid
Tissue fluid returns to capillaries
Capillaries join to form venules
Venules join to form veins to carry blood back to heart
Arteries: Question (page 92)
Q3:
The centre of all vessels is the
lumen.
All vessels are lined by
endothelium.
The outer layer of the artery wall is
connective tissue with elastic fibres.
Smooth muscle and elastic fibres make
up the
middle layer of the artery wall.
The surge of blood from the heart is
accommodated by the
elastic walls of the arteries.
Capillaries: Question (page 93)
Q4:
A capillary
exchanges materials with the tissues.
A capillary wall
consists of two layers of cells: endothelium and
connective tissue.
Vasoconstriction
involves the contraction of the smooth muscle in artery
walls.
Vasodilation
allows blood to flow into the capillary bed.
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Veins: Question (page 95)
Q5:
Veins carry the blood
back to the heart under low pressure.
Valves cause the blood
to flow through veins in only one direction.
Veins have a much thinner
muscular wall than arteries.
Veins have an outer layer of
connective tissue with elastic fibres.
Lymph vessels: Question (page 97)
Q6:
Fluids leave capillaries because
the pressure is higher than the tissue
fluid.
Fluids re-enter the capillaries because
the osmotic pressure exceeds the
hydrostatic pressure.
Lymph is formed from
tissue fluid, which enters the lymphatic
capillaries.
Lymph vessels return lymph to
veins near the heart.
The waste and other products of cells
return to the
capillaries in the tissue fluid.
Tissue fluid is similar to blood plasma
but lacks
large protein molecules.
Tissue fluid supplies cells with
oxygen, glucose and other materials.
Extended response question: The exchange of materials between the blood and
the body tissues (page 99)
Suggested marking scheme
Each line represents a point worth one mark. The concept may be expressed in other
words. Words which are bracketed are not essential. Alternative answers are separated
by a solidus (/); if both such answers are given, only a single mark is allocated. In
checking the answer, the number of the point being allocated a mark should be written
on the answer paper. A maximum of ten marks can be gained.
A) Movement of fluid in and out of the blood (maximum of 6 marks):
1. Exchange only occurs in the capillaries / capillary beds.
2. Fluid moves because of pressure differences.
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ANSWERS: TOPIC 6
3. Fluid leaves blood because hydrostatic pressure exceeds osmotic pressure.
4. Fluid re-enters blood because osmotic pressure exceeds hydrostatic
pressure.
5. Fluid leaves capillaries by pressure filtration.
6. The capillary walls determine what may pass through them.
7. Fluid entering the tissues is called tissue fluid.
8. Tissue fluid is similar to blood plasma but lacks plasma proteins.
B) Substances transferred (maximum of 4 marks):
i
All substances in the blood except cells and large protein molecules pass out
into tissue fluid.
ii
Tissue fluid supplies cells with oxygen, glucose, amino acids, fatty acids,
hormones, mineral ions. (any two for one mark, any four for two marks)
iii Waste materials from cells are passed back into the blood + any example,
e.g. CO2 , lactic acid. (full mark requires an example)
iv Products of cells are passed back into the blood + any example, e.g.
hormones. (full mark requires an example)
v
Excess / 10% tissue fluid passes into the lymphatic system.
End of Topic 6 test (page 100)
Q7:
The blood circulation carries blood from the heart through arteries to the tissues and
back through veins to the heart. Blood pressure decreases with distance from the
heart. All blood vessels have a central lumen lined by the endothelium.
Capillaries allow exchange of substances with the tissues. The wall of the capillary
is very thin, consisting of a layer of endothelium cells and a surrounding layer of
connective tissue. The pressure difference between the blood in the capillary and
the fluid in the surrounding tissue causes plasma to flow out of the capillary. In most
capillaries, the escaping fluid consists of water with a variety of dissolved substances
including glucose, oxygen, hormones, amino acids and mineral ions. This escaping
fluid becomes tissue fluid, which is similar to blood plasma but does not contain plasma
proteins.
Tissue fluid returning to the capillaries carries with it carbon dioxide and metabolic
wastes for excretion.
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ANSWERS: TOPIC 6
Q8:
Thick muscular walls are a feature of
arteries.
Arteries are made from an outer layer of connective tissue with
elastic fibres.
The middle layer of artery walls is made from
smooth muscle.
Contraction of the smooth muscle of the artery wall is known as
vasoconstriction.
The process that increases the diameter of the artery lumen is
vasodilation.
Thin walls with little smooth muscle are a feature of
veins.
Blood flow in only one direction is ensured by
valves.
The blood flow in veins is caused by the contraction of
surrounding
muscle.
90% of tissue fluid returns to the
capillaries.
A substance that is returned to veins near the heart is
lymph.
Q9:
Blood travels from the heart to the tissues in arteries.
As blood gets further from the heart, the pressure in the vessels decreases.
The centre of all blood vessels is lined by the endothelium.
Compared to veins, arteries have thicker muscular walls.
Q10: Elastic fibres allow walls to stretch and recoil. . .
. . .to accommodate the surge of blood after each contraction of the heart.
Q11: Vasoconstriction
Q12: It reduces the diameter of the lumen of the artery.
It restricts blood flow into the capillaries.
Q13: One of:
• to reduce heat lost when hands / feet / extremities are very cold;
• blood is directed from the skin to muscles when a person is angry or scared
Q14: Capillaries
Q15: Walls have lumen only one cell thick.
Q16: Any three from:
• veins contain valves;
• valves prevent backflow / only allow flow back to heart;
• muscles in vicinity of vein contract;
• muscles become shorter and fatter, so squeezing vein.
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ANSWERS: TOPIC 6
Q17: (Hydrostatic) pressure caused by contraction of heart. . .
. . .is greater that osmotic pressure caused by albumin/plasma proteins.
Q18: Tissue fluid
Q19: It lacks plasma proteins.
Q20: Any two from:
• carbon dioxide;
• hormones;
• lactic acid;
• urea.
Q21: 90%
Q22: Lymph capillaries
Q23: Subclavian veins near heart.
Q24: Lymph
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ANSWERS: TOPIC 7
7 Structure and function of the heart
The structure of the heart: Questions (page 105)
Q1:
Q2:
They are found between the atria and ventricles in the heart.
Q3:
To prevent backflow of blood.
Q4: They prevent the atrio-ventricular valves from being inverted when the ventricles
contract.
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The human circulatory system: Questions (page 107)
Q5:
Q6:
Organ/area of body
Associated artery
Associated vein
heart
coronary artery
coronary vein
head
carotid artery
jugular vein
renal artery
renal vein
hepatic artery
hepatic vein
gut arteries
hepatic portal vein
kidney
liver
small intestine
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ANSWERS: TOPIC 7
Effect of exercise on cardiac output: Question (page 110)
Q7:
State of body
Stroke volume
(ml)
Heart rate
(bpm)
Cardiac output
(l/min)
at rest
60
60
3.6
during gentle
exercise
70
120
8.4
during strenuous
exercise
80
180
14.4
The cardiac cycle: Questions (page 113)
Q8: Systole is contraction of the heart muscle while diastole is relaxation of the heart
muscle.
Q9: The atrio-ventricular (bicuspid and tricuspid) valves are open and the semilunar
valves are closed.
Q10: The pressure is greater during ventricular systole because the ventricle walls have
contracted thus putting the blood under increased pressure. This forces the blood out
into the arteries.
The cardiac conducting system: Questions (page 117)
Q11: In the wall of the right atrium (close to the entrance of the superior vena cava).
Q12: Contraction is initiated at the bottom of the ventricles so that they empty from the
bottom up. This is the most efficient way to completely empty the ventricles.
Blood pressure: Questions (page 122)
Q13: b) During diastole
Q14: b) 130/85
Q15: 140
Q16: d) Hypertensive
Q17: Systole is when the heart contracts and pumps blood out of the heart and diastole
is the relaxation of the heart. Therefore blood is being pumped into the arteries during
systole, increasing the pressure.
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Q18:
Q19: When the heart contracts the blood pressure increases, forcing blood into the
arteries. Then the heart relaxes before the next heartbeat so the pressure drops.
Extended response question: The cardiac cycle (page 127)
Suggested marking scheme
Each line represents a point worth one mark. The concept may be expressed in other
words. Words which are bracketed are not essential. Alternative answers are separated
by a solidus (/); if both such answers are given, only a single mark is allocated. In
checking the answer, the number of the point being allocated a mark should be written
on the answer paper. A maximum of ten marks can be gained.
A) Atrial systole (maximum of 3 marks):
1. During atrial systole, the sino-atrial node / pacemaker, sends waves of nerve
impulses across the walls of the atria. . .
2. . . .which makes them contract simultaneously.
3. The pacemaker is situated in the wall of the right atrium.
4. Blood is forced through the atrio-ventricular/AV valves into the ventricles.
B) Ventricular systole (maximum of 5 marks):
i
The SA node also stimulates the atrio-ventricular (AV) node. . .
ii
. . .which is situated between the atria and the ventricles.
iii The AV node sends nerve impulses down the septum and into the walls of
the ventricles. . .
iv . . .making them contract simultaneously. . .
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ANSWERS: TOPIC 7
v
. . .from the bottom up, which is the most efficient way of emptying them.
vi As the ventricles contract the AV valves close. . .
vii . . .preventing backflow of blood (into the atria).
C) Diastole. (maximum of 2 marks):
I
During diastole the atria and ventricles relax.
II
The semi-lunar valves close.
III The AV valves open to allow blood to pass into the ventricles (again).
End of Topic 7 test (page 127)
Q20:
Atria:
upper chambers of the heart.
Ventricles:
lower chambers of the heart.
Right ventricle:
chamber which pumps blood to the lungs.
Pulmonary artery:
vessel which carries blood from the heart to the lungs.
Left ventricle:
chamber which pumps blood round the body.
Aorta:
artery which carries blood from the heart to the body.
Atrioventricular
valves:
structures between upper and lower chambers of the heart.
Semilunar valves:
structures which prevent backflow of blood to the heart.
Coronary artery:
vessel which supplies blood to the heart muscle.
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227
Q21:
Autonomic:
part of the nervous system that controls heart rate.
Medulla:
area of the brain that controls heart rate.
Sympathetic:
part of the nervous system that increases heart rate.
Parasympathetic:
part of the nervous system that decreases heart rate.
Antagonistic:
action of the nervous system that controls heart rate.
Noradrenaline:
secreted by nerves to increase heart rate.
Acetylcholine:
secreted by nerves to decrease heart rate.
Adrenaline:
hormone that increases heart rate.
Stroke volume:
volume from each ventricle during one heartbeat.
Heart rate:
number of heartbeats per minute.
Cardiac output:
calculated by multiplying stroke volume by heart rate.
Q22:
Cardiac cycle:
contraction and relaxation of the heart during one complete
beat.
Atrial systole:
contraction of the muscle of the atrium.
Ventricular systole:
contraction of ventricle muscle.
Diastole:
relaxation of heart muscle.
Atrioventricular:
valves which open when the atria contract.
Semilunar:
valves which open when the ventricles contract.
Myogenic:
muscle which can contract without external stimulation.
Sinoatrial node:
co-ordination of the heart muscle contraction.
Ventricular:
contraction controlled by the atrioventricular node.
Electrocardiogram:
record of impulses generated by the SAN and AVN.
Q23:
The contraction and relaxation of the heart muscle during the cardiac cycle cause large
variations in blood pressure in the arteries which are detectable as the pulse.
Blood pressure is measured using a sphygmomanometer. An inflatable cuff stops
blood flow and gradually deflates. As systolic pressure is reached, blood flow restarts
and the pulse is felt. Once blood is flowing freely, no pulse is felt and diastolic pressure
is measured.
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Blood pressure is expressed as systolic pressure / diastolic pressure, a typical reading
being 120/70 mm Hg.
Hypertension, which is indicated by blood pressure reading of over 140/90mm Hg, is a
major risk factor for coronary heart disease.
Q24: The volume of blood pumped by the left and right ventricles is the same.
Q25: The cardiac output of a person with a heart rate of 80 bpm and stroke volume of
75 ml is 6 L/min.
Q26: Either of:
• during atrial systole, the muscle of the right atrium is contracting;
• during atrial systole, the muscle of the right ventricle is relaxing.
Q27: During diastole, blood flows from the vena cava into the right atrium.
At this stage, the atrio-ventricular valves valves are open and the semilunar valves
are closed.
During ventricular systole, the semilunar valves are open and the atrio-ventricular
valves valves are closed.
Blood therefore flows from the left ventricle into the aorta and the right ventricle into the
pulmonary artery.
Q28:
Cardiac muscle cells are myogenic.
The cells of the pacemaker are autorhythmic.
The rate at which cardiac muscles contract is set by the sinoatrial node.
The contraction of the ventricles is triggered by the AV node.
Impulses in the heart can be recorded on an electrocardiogram.
Q29: The two branches of the autonomic nervous system are said to be antagonistic.
They are controlled by the medulla of the brain.
The sympathetic nervous system increases the rate of the SAN by the secretion of
noradrenaline.
The parasympathetic nervous system decreases the rate of the SAN by the secretion
of acetylcholine.
Q30: The cardiac cycle
Q31: Sphygmomanometer
Q32: Diastolic
Q33: Hypertension
Q34: mmHg
Q35: Systolic blood pressure 145 mm Hg, diastolic blood pressure 100 mm Hg.
Q36: Coronary heart disease, stroke (ask your teacher if these are not your answers
because you may still be correct)
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229
8 Cholesterol and cardiovascular disease
Cholesterol: Questions (page 134)
Q1: Cholesterol is a waxy steroid compound (a fat). It is synthesised in all cells, but
the largest producer is the liver. Dietary sources of cholesterol are animal foods, such
as meat and dairy products.
A diet high in saturated fats may lead to increased cholesterol levels.
Cholesterol is eliminated from the body by the liver in the form of bile.
Q2: Blood plasma is mostly water, and cholesterol is insoluble in water.
Functions of cholesterol: Question (page 134)
Q3:
Function
Example
Location
Control permeability
Plasma membrane
Hydrogen/sodium ions
Transport into cell
Plasma membrane
Endocytosis
Precursor
Skin
Vitamin D
Precursor
Interstitial cells
Testosterone
High density and low density lipoproteins: Question (page 136)
Q4:
1. Cholesterol is transported by lipoproteins.
2. Excess cholesterol is carried from body cells to the liver by HDLs.
3. Cholesterol is transported from the liver to the body cells by LDLs.
4. LDL-receptors are found are found on most cells.
5. When a cell is making cell membrane, LDL-receptor production is increased.
6. Excess LDLs may deposit cholesterol in atheromas.
7. Atherosclerosis is reduced by a HDL/LDL ratio which is high.
8. LDL levels in the blood are increased by a diet high in fats which are saturated.
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ANSWERS: TOPIC 8
Controlling cholesterol levels: Question (page 137)
Q5:
Regular physical activity raises
HDL levels.
The concentration of cholesterol is reduced by
lowering
the fat content of the diet.
Reducing the unsaturated fat in the diet reduces
the level of LDLs.
Reducing the content of animal food in the diet
reduces
cholesterol intake.
Statins are drugs which inhibit
an enzyme in the liver.
Statins reduce
cholesterol production by
liver cells.
Familial hypercholesterolaemia: Question (page 138)
Q6:
i
FH is usually caused by a mutation of which gene? LDL-receptor
ii
What inheritance pattern does this mutation show? incomplete dominance
iii What type of chromosome is this gene found on? autosome
iv In FH patients, what is reduced on liver cell membranes? LDL-receptors
v
FH causes the early onset of what disease? cardiovascular
vi This results from increased levels of what in the blood? LDLs
vii In consequence, what is deposited in artery walls? cholesterol
viii What should be reduced in the diet of FH patients? fat
ix What drugs may be used to control FH? statins
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Extended response question: Cholesterol in the body (page 146)
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words. Words which are bracketed are not essential. Alternative answers are separated
by a solidus (/); if both such answers are given, only a single mark is allocated. In
checking the answer, the number of the point being allocated a mark should be written
on the answer paper. A maximum of ten marks can be gained.
A) Sources and removal of cholesterol (maximum of 3 marks):
1. Cholesterol is formed in all body cells.
2. The liver is the greatest producer.
3. Cholesterol is present in animal foods e.g dairy products / meat / poultry /
fish.
4. Cholesterol is eliminated from the body in the bile.
B) High density lipoproteins and low density lipoproteins (maximum of 7 marks):
i
Lipoproteins are made in the liver.
ii
They are responsible for the transport of cholesterol.
iii High density lipoproteins (HDLs) carry cholesterol from the body cells to the
liver.
iv Low density lipoproteins (LDLs) carry cholesterol from the liver to body cells.
v
LDLs attach to LDL-receptors on the cell membrane of most cells.
vi The production of LDL-receptors is controlled by negative feedback.
vii LDLs are carried into the cells to release their cholesterol.
viii Excess LDLs circulate in the blood and can become absorbed into the
atheromas in plaques on artery walls.
ix A high ratio of HDLs to LDLs lowers the level of cholesterol in the blood. . .
x
. . .reducing the development of atherosclerosis.
End of Topic 8 test (page 146)
Q7:
Cholesterol is a waxy steroid which is synthesised in all body cells, although the liver
produces most in the body. It is a component of cell membranes, where it controls
permeability. It is also a precursor of vitamin D and steroid hormones. Cholesterol is
eliminated from the body in the bile produced by the liver.
Cholesterol is present in animal foods, especially dairy products, meat and poultry. A
diet high in saturated fats may cause increased cholesterol levels.
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ANSWERS: TOPIC 8
Q8:
Lipoproteins:
synthesised in the liver and used to transport cholesterol.
High density
lipoproteins:
carriers of cholesterol from the body cells to the liver.
Low density
lipoproteins:
carriers of cholesterol from the liver to the body cells.
LDL-receptors:
present on the cell membrane of most cells.
Negative feedback:
control of LDL-receptor production.
Excess LDLs:
become incorporated into atheromas.
High HDL : low LDL:
reduces the development of atherosclerosis.
Regular physical
activity:
raises HDL levels.
Cholesterol intake:
reduced by less animal foods in diet.
Statins:
drugs used to reduce cholesterol levels.
Q9:
Atheroma:
build-up of fatty material beneath the endothelium.
Plaque:
thickening consisting of fibrous material, calcified tissue and
fat.
Increased blood
pressure:
result of narrower lumen and reduced elasticity of the artery
wall.
Atherosclerosis:
root cause of angina, myocardial infarction, stroke and PVD.
Fibrin:
formed by the effect of thrombin on fibrinogen.
Thrombus:
formed by a mesh of fibrin threads and red blood cells.
Embolus:
a clot which breaks loose in the blood stream.
Myocardial
infarction:
caused by a loose clot in the coronary artery.
Stroke:
caused by a loose clot in an artery in the brain.
Peripheral vascular
disease:
atherosclerosis in arteries other than in brain or heart.
Deep vein
thrombosis:
formation of a clot in a deep vein.
Pulmonary
embolism:
caused by an embolus lodged in the pulmonary artery.
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233
Q10:
Incompletely dominant
autosomal:
a type of mutation causing familial FH.
Fewer LDL-receptors on liver
cells:
the effect of a mutation on the LDL-gene.
High levels of LDL-cholesterol
in blood:
the result of a mutation to the LDL-gene.
Genetic screening:
a method of detecting FH.
A diet low in fat and saturated
fat:
the natural method of treating FH.
Statins:
the drugs used to treat FH.
Q11:
Liver:
the organ which makes and eliminates cholesterol.
Animal foods:
a type of food which is high in cholesterol.
High density
lipoproteins/HDLs:
transport cholesterol from body cells to the liver.
Cell membrane:
the cell structure that contains the most cholesterol.
Q12: Low density lipoproteins / LDLs carry cholesterol from liver in blood.
LDLs attach to LDL-receptors on cell membrane.
LDL-receptors pass into cell and release the cholesterol.
Q13: Natural: regular physical activity / reduced unsaturated fat in diet.
Artificial: statins / statin drugs.
Q14: Genetic screening
Q15: A diet that is low in fat and saturated fat.
Q16: Plaques form beneath the endothelium of arteries.
Plaques consist of fibrous material, atheroma and calcified tissue.
Reduces the lumen of the artery / reduces blood flow.
Q17: Any two from:
• angina;
• myocardial infarction / heart attack;
• stroke;
• peripheral vascular disease.
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Q18: Any four from:
• unstable plaque ruptures triggering the clotting process;
• prothrombin is converted to thrombin;
• thrombin catalyses conversion of (soluble) fibrinogen to (insoluble) fibrin fibres;
• fibrin traps red blood cells to form a clot;
• clot breaks loose to form the embolism.
Q19: It deprives cells of oxygen.
Q20: DVT is formation of a clot in a deep vein.
If the clot becomes detached it may lodge in the pulmonary artery.
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235
9 Pathology of cardiovascular disease
Blood glucose levels: Question (page 152)
Q1:
Glucose is the principal respiratory substrate in the body. It circulates in solution in
the blood plasma and is stored as an insoluble polysaccharide called glycogen. These
stores are found in the liver and the muscles, but only the store in the liver can be
mobilised to raise blood sugar levels.
Blood glucose levels are kept in a very narrow range, by a process of homeostasis. If
the glucose concentration falls below this range, the condition of hypoglycaemia sets
in with potentially fatal results. If the glucose concentration exceeds the normal range,
the condition is known as hyperglycemia.
Homeostatic control of blood glucose levels: Question (page 154)
Q2:
Blood glucose levels are controlled by hormones released from
cells in
the pancreas.
Blood glucose levels are detected on the surface of these cells
by
receptors.
The hormone stimulating conversion of glycogen to glucose is
glucagon.
The hormone released in response to low blood glucose levels
is
glucagon.
The hormone stimulating conversion of glucose to glycogen is
insulin.
The hormone released in response to high blood glucose levels
is
insulin.
The organ which both releases and stores glucose as glycogen
is
the liver.
The effect of exercise on blood glucose levels: Question (page 154)
Q3:
1. Lowering the blood sugar level causes the release of glucagon.
2. Vigorous exercise stimulates the release from the thyroid gland of thyroxine.
3. Excitement and stress cause the release of adrenaline.
4. Adrenaline inhibits the release of insulin.
5. Adrenaline has the same effect on the liver as glucagon.
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ANSWERS: TOPIC 9
Diabetes: Question (page 158)
Q4:
The condition of blood glucose levels consistently below the normal range is called
hypoglycaemia. Diabetes is the most common cause of hyperglycemia. A common
complication of diabetes is chronic vascular disease. Type 1 diabetes is also known as
known as insulin dependent diabetes. Type 1 diabetes typically develops in childhood
and is treated with insulin injections. Type 2 diabetes typically develops in adulthood
and results from insulin resistance caused by the reduced number of receptors.
Typically, Type 2 diabetes is associated with obesity and is treated by altering diet and
increasing exercise. Diabetics excrete glucose in their urine. The glucose tolerance
test is taken before and two hours after taking a drink containing 75g of glucose.
Blood glucose levels and vascular disease: Question (page 159)
Q5:
The endothelial cells which line the blood vessels are damaged by chronic high blood
glucose concentrations. As a result, the flow of blood is slowed, causing hypertension.
In small arteries the effects are particularly bad in certain tissues. In the eye, the retina
is affected, causing blurring and eventual loss of vision. In the peripheral nervous
system, conduction of nerve impulses is slowed or stopped. In the kidney, the damage
to the glomeruli eventually leads to kidney failure. The effects in large arteries are
atherosclerosis and peripheral vascular disease, which may ultimately lead to stroke
and myocardial infarction.
Obesity: Question (page 164)
Q6:
Obesity is defined as an excess of fat compared to lean tissue. It is a major risk factor
in cardiovascular disease and Type 2 diabetes.
BMI stands for Body Mass Index. A BMI in excess of 30 indicates obesity. Accurate
estimation of body fat content requires a measurement of body density.
To counter obesity, a person should reduce their dietary intake of fat and free sugar,
and increase their physical activity. Compared with carbohydrates, fats contain twice
as much energy per gram. Exercise increases energy expenditure and preserves lean
tissue. Exercise can reduce the risk of developing cardiovascular disease by helping
control weight, reducing stress and hypertension, and raising HDL levels.
Extended response question: Type 2 diabetes (page 167)
Suggested marking scheme
Each line represents a point worth one mark. The concept may be expressed in other
words. Words which are bracketed are not essential. Alternative answers are separated
by a solidus (/); if both such answers are given, only a single mark is allocated. In
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ANSWERS: TOPIC 9
checking the answer, the number of the point being allocated a mark should be written
on the answer paper. A maximum of ten marks can be gained.
A) Causes (maximum of 2 marks):
1. Type 2 diabetes results from reduced cellular sensitivity to insulin. . .. . .
2. . . .caused by a reduction in the number of insulin-receptors on liver cells.
3. It is associated mainly with increasing obesity in adulthood.
B) Effects (maximum of 6 marks):
i
Diabetes of all forms means that the body is unable to control the rise in blood
glucose levels after meals.
ii
Diabetics have chronically high blood glucose levels. . .
iii . . .which damage blood vessels walls.
iv This leads to the development of vascular diseases in large blood vessels. . .
v
. . .e.g. atherosclerosis / stroke / myocardial infarction.
vi In small blood vessels of vulnerable tissues it can cause haemorrhaging in
the retina, resulting in reduced vision / damage to the glomeruli, resulting in
renal failure / slowing down of impulses in the peripheral nervous system,
resulting in loss of control and sensation. (1 mark for naming a cause and 1
additional mark for naming the resultant effect)
C) Diagnosis and treatment (maximum of 2 marks)
I
Diabetes is diagnosed by a glucose tolerance test. . .
II . . .which involves blood tests taken before, and 2 hours after taking a drink
containing 75g of glucose. The patient should have fasted for 8 hours before
the test.
III Treatment is usually administered by adjusting diet. . .
IV . . .reducing energy intake and increasing exercise levels.
End of Topic 9 test (page 167)
Q7:
Blood glucose levels are homeostatically regulated by negative feedback. Receptors
in the pancreas detect the concentration of glucose in the blood. High blood glucose
levels stimulate the release of insulin by the pancreas. Insulin stimulates the liver
(and other tissues) to convert glucose to glycogen. Low blood glucose levels stimulate
the release of glucagon by the pancreas. Glucagon stimulates the liver to convert
glycogen to glucose.
Exercise and fight/flight situations stimulate the release of adrenaline from the adrenal
glands.
Adrenaline stimulates the liver (and other tissues) to convert glycogen to glucose.
Adrenaline also stimulates the release of glucagon and inhibits the release of insulin
by the pancreas.
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ANSWERS: TOPIC 9
Q8:
Hyperglycemia:
the inability to control a rise in blood sugar levels.
Chronic vascular
disease:
a complication of diabetes.
Insulin production:
Type 1 diabetes is caused by its failure.
Type 2 diabetes:
develops during adulthood.
Insulin receptors on liver
cells:
Type 2 diabetes is caused by their reduction in number.
Obesity:
frequently associated with Type 2 diabetes.
Positive urine test for
glucose:
a strong indicator of diabetes.
Glucose tolerance test:
two blood samples tested after drinking glucose
solution.
Q9:
Endothelial cells:
damaged by chronic high blood glucose levels.
Atherosclerosis:
result of chronic high glucose levels in larger blood vessels.
Peripheral nerve
dysfunction:
result of chronic high glucose levels in smaller blood
vessels.
Body mass index:
calculated by dividing mass (kg) by height (m) squared.
BMI > 30:
an indicator of obesity.
Fat:
contains twice as much energy per gram as protein.
Free sugars in diet:
their digestion requires no metabolic energy.
Exercise:
reduces stress and hypertension, and raises HDL levels.
Q10: Receptors in the pancreas.
Q11: Glucagon
Q12: Liver
Q13: It stimulates conversion of glycogen to glucose.
Q14: Adrenal glands
Q15: In fight or flight situations or during exercise.
Q16: It stimulates conversion of glycogen to glucose.
Q17: A rise in blood sugar levels.
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ANSWERS: TOPIC 9
Q18: Chronic vascular disease
Q19:
Type 1 diabetes typically appears in childhood.
It is treated with change of diet.
Type 2 diabetes is usually developed in adulthood.
It is more common in developed countries and in people who are over-weight.
Q20: Glucose
Q21: A glucose tolerance test
Q22: Any two for (1 mark) or all four for (2 marks) from:
1. eight hour fast;
2. blood samples;
3. one before and one two hours after;
4. drink containing 75g glucose.
Q23: Chronic high blood glucose levels lead to endothelial cells taking abnormally large
quantities of glucose, damaging the lining of the blood vessels.
Q24: Any from:
• atherosclerosis;
• cardiovascular disease;
• stroke;
• peripheral vascular disease.
Q25: Any from:
• haemorrhaging in the retina;
• renal failure;
• peripheral nerve dysfunction.
Q26: Obesity results from an excess of body fat in relation to lean tissue / muscle.
Q27:
• High fat diet;
• reduced physical activity.
Q28: Body Mass Index
Q29: Mass (kg) / (Height (m)2 )
Q30: 30
Q31:
• Increases energy used.
• Preserves lean tissue.
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ANSWERS: TOPIC 10
10 End of unit test
End of Unit 2 test (page 172)
Q1:
Sperm are produced in the seminiferous
tubules.
The ova are contained within
follicles.
The zygote divides repeatedly to become the
blastocyst.
FSH and LH production form part of a negative feedback
cycle.
Testosterone activates the seminal
vesicles.
The phase of the menstrual cycle after ovulation is called
luteal.
IVF involves the surgical removal of eggs after
superovulation.
IUDs are placed in the uterus and reduce the sperm
motility.
XC Xc is the genotype of a colour vision
carrier.
Screening tests indicate the likely presence of a
disorder.
CVS carries a higher risk of inducing a miscarriage than
amniocentesis.
Individuals with PKU are given a diet which lacks
phenylalanine.
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ANSWERS: TOPIC 10
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Q2:
Reduces the flow of blood into capillary beds:
vasoconstriction.
Action of the sympathetic and parasympathetic nervous
systems:
antagonistic.
Volume of blood leaving each ventricle in one minute:
cardiac output.
Measures blood pressure:
sphygmomanometer.
Major risk factor for coronary heart disease:
hypertension.
Component of cell membranes which controls permeability:
cholesterol.
Transport cholesterol from the liver to body cells:
low density
lipoproteins.
Drugs which reduce cholesterol levels:
statins.
Embolus lodged in the pulmonary artery:
embolism.
Release stimulated by low blood glucose levels:
glucagon.
Common cause of hyperglycemia:
diabetes.
A BMI (mass/(height squared)) in excess of 30 indicates:
obesity.
Q3: ICSH/LH
Q4: Interstitial cells
Q5: Any two from:
• sperm production;
• activation of prostate gland and/or seminal vesicles;
• development of secondary sexual characteristics
Q6: Any two points for 1 mark or all four points for 2 marks:
• the pituitary secretes hormone X/ICSH/LH;
• stimulating tissue Y / interstitial cells to secrete testosterone;
• an increasing level of testosterone inhibits secretion of X/ICSH/LH;
• the secretion of testosterone is reduced.
Q7: Hypothalamus
Q8: 25%
Q9: 1950
Q10: 1994
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ANSWERS: TOPIC 10
Q11: 45 million/cm3
Q12: Any two from:
• increased smoking;
• reduced exercise;
• increased obesity.
Q13: Age
Q14: Either of:
• drugs that prevent the negative feedback effect of oestrogen on FSH secretion /
block oestrogen receptors;
• artificial forms of FSH and LH taken.
Q15: Sperm defective / very low sperm count
Q16: The head/nucleus of a sperm drawn into a needle. . .
. . .and injected into the egg/ovum
Q17: Any two from:
• mimicking negative feedback, preventing release of FSH/LH;
• preventing implantation;
• causing thickening of the cervical mucus.
Q18: Autonomic
Q19: It slows it down
Q20: Noradrenaline/norepinephrine
Q21: AVN / atrioventricular node
Q22: Controls/co-ordinates contraction of the ventricles.
Q23: 48 bpm
Q24: 136/80
Q25: Systolic / diastolic (blood pressure)
Q26: Exercise increases both (systolic and diastolic) blood pressures
Q27: Same/second group of students treated in same way but with no exercise.
Q28: Any two from:
• same intensity of exercise;
• same time of activity;
• no caffeine immediately before;
• blood pressure taken immediately before and after activity.
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ANSWERS: TOPIC 10
Q29: Type 1 diabetes typically develops in childhood.
Q30: Type 2 diabetes is usually caused by an increase in insulin resistance.
Q31: Diabetes is diagnosed by a glucose tolerance test.
Q32: Statins
Q33: It inhibits the synthesis of cholesterol by liver cells.
Q34:
2 marks (1 mark each) for the axes with suitable scale, correct labels and units.
1 mark for points correctly plotted and single line joining points, either as a line of best
fit or as straight lines between each point.
Q35: 5 μg/g of liver/hour
Q36: Uptake at 0 unit/ cm3 is 1.8 μg/g of liver/hour.
Uptake at 25 units/cm3 is 4.8 μg/g of liver/hour.
4.8 / 1.8 = 2 2/3 = 8/3
Ratio is 1 : 8/3 (multiply both sides by 3 to get whole numbers)
Simple whole number ration is 3 : 8
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