Download The human life cycle

Unit 1A
Human Form & Function
Body systems
The human life cycle
Study Guide
Read:
• Our Human Species (3rd edtn)
Chapter 17
Complete:
• Human Biological Science
1A Workbook
Topic 11 – The human life cycle
Life cycle
• In biology the term ‘life cycle’ is used to
describe the events starting at fertilisation
in one generation to the same stage in the
following generation.
Human life stages
• The term ‘life cycle’ is often used to
describe the sequence of life stages we
pass through from fertilisation to death.
Life stages
• Fertilisation and gestation (the
pregnancy) culminate in the individual’s
birth.
After birth we progress through the
neonate period, infancy, childhood,
adolescence, adulthood and old age.
Eventually, death completes the cycle.
Gestation
• Gestation is the nine-month period of
pregnancy from conception to birth.
• Doctors measure a pregnancy from the
first day of the last menstrual period, which
is usually about two weeks before
fertilisation.
Gestation = conception → birth
Gestation
• The gestation period from fertilisation to
birth is about thirty-eight weeks.
• A pregnancy is divided into three equal
trimesters each of about three months.
• From the time of fertilisation until the end
of the eighth week of gestation the
developing baby is referred to as an
embryo – from the eighth week until birth
it is known as a fetus (foetus).
Fertilisation
• Fertilisation is also known as conception,
and marks the beginning of pregnancy and
the life cycle.
• Fertilisation results from the fusion of a
male gamete ( a sperm cell) with a female
gamete (an ovum or egg)
• The fertilised egg is referred to as a
zygote.
Sperm on surface of a human egg
Yorgos Nikas, Wellcome Images
Fertilised human egg
Sperm
Zygote
M Johnson, Wellcome Images
• Fertilisation occurs in a Fallopian tube.
• As the fertilised egg passes along the
Fallopian tube and into the uterus it
divides repeatedly to form a hollow ball of
cells known as a blastocyst.
Human blastocyst at days 4 & 5
Yorgos Nikas, Wellcome Images
Implantation
• Six days after fertilisation the hollow
blastocyst consists of about 150 to 200
cells.
• At about this time the blastocyst buries
itself in the wall of its mother’s uterus. This
is known as implantation.
Human embryo implanting in the uterine lining
Yorgos Nikas, Wellcome Images
First trimester
• Around 16 days after conception three
embryonic germ layers become
established. These layers - the
endoderm, ectoderm and mesoderm later develop into various body systems.
• Miscarriages are most common in the first
trimester and the baby is most easily
harmed by drugs, alcohol and pathogens.
Five week-old human embryo
Wellcome Photo Library
Second and third trimester
• By the end of the first trimester the
rudiments of all the major organ systems
are established.
• During the second trimester the organ
systems become well established.
• A baby born prematurely at 23 weeks has
a 60% chance of survival.
Stages of development of human fetus
Miles Kelly Art
Library,
Wellcome Images
Ultrasound of developing fetus at 16 weeks (left)
and 24 weeks (right)
Wellcome Photo Library
Childbirth
• Two to three weeks before labour begins
the baby rotates and moves lower down
so that its head settles into the bony
pelvis.
• The process of childbirth (also called
labour, or parturition) is divided into
three stages – the dilation, expulsion
and placental stages.
• Childbirth is initiated by a surge of the
hormone oxytocin.
Dilation
• Regular contractions become
progressively stronger and more frequent
(this can continue for several hours).
• The cervix dilates and the baby’s head
starts to move into the birth canal.
• The membranes holding the amniotic fluid
rupture (the ‘waters break’).
• A baby in the normal birth position pushing its head
through the mother's cervix as she gives birth.
Medical Art Services, Munich,
Wellcome Images
The expulsion and placental stages
Expulsion stage
• This is the actual delivery or childbirth
stage.
• The cervix is fully dilated and the baby is
forced through the birth canal.
Placental stage
• A final contraction expels the placenta
(afterbirth).
Delivery
A
B
A. Stage 1a – early dilation
B. Stage 1b – late dilation
C
C. Stage 2 - expulsion
Newborn – the neonate
Anthea Sieveking, Wellcome Images
The neonatal period
• The neonatal period lasts from birth to one
month.
• Following the birth, the respiratory system
starts to function and the digestive and
urinary systems become functional.
Important changes also occur in the blood
circulatory system.
• Newborn babies cannot regulate their
water loss or body temperature.
The digestive system
• Before birth the fetus receives nutrient-rich
blood from its mother via the placental
circulation.
• Although the fetus swallows amniotic fluid
before it is born, the digestive system does
not become fully functional until after birth.
The respiratory system
• Before birth the fetus receives oxygen-rich
blood from its mother via the placental
circulation.
• After birth breathing begins and the blood
circulation to the lungs is activated.
The circulatory system
At birth the following circulatory changes take
place:
– The umbilical arteries and vein stop
functioning.
– The blood supply to the liver (hepatic
circulation) becomes fully functional (the
ductus venosus closes).
– The blood supply to the lungs (pulmonary
circulation) becomes fully functional (the
ductus aeteriosus & foramen ovale close).
Changes in the circulation
Ductus arteriosus
Foramen ovale
Ductus venosus
Umbilical arteries
and vein
Infancy
Changes during infancy
• Infancy spans the first year of life.
• During the first 12 months the baby grows
and its organ systems develop.
• Growth is regulated by hormones, but
because tissues respond differently,
growth is not uniform, and the proportion
of the body parts gradually changes.
%
100
80
Proportion 60
of body
size
40
20
Age
0.5
3
7
13
18
(yr)
Changing body proportions
Childhood
During childhood body systems
develop at different rates
Performance Landmarks
AGE
(Months)
Landmarks
0-2
First smile
Recognises faces
2-3
Able to lift head and bear weight on its arms
3-6
Birth weight doubled
Cuts first teeth
Able to raise itself, shake a rattle and grasp
objects deliberately
6-12
Starts crawling
Able to sit and stand supported
12-18
Able to pull itself into standing position
Starts walking
Toilet training completed
Speech Landmarks
AGE
(Years)
1
Landmarks
Understands first words, may say first words.
(Speaking can start from between 8 months
and 2.5 years)
2
Can speak in short sentences
3
Has a 900 word vocabulary and is learning 10
new words each day. (Many adults have a
basic vocabulary of 1600 words)
4
Has mastered basic principles of grammar
Adolescence
Uplands Community College, Wellcome Images
Puberty and adolescence
• Puberty is the period during which sexual
maturity occurs and marks the start of
adolescence.
• Adolescence is not a natural biological
stage. It marks the social transition from
childhood to adulthood.
• Adolescence is particularly significant in
Western cultures.
Onset of puberty
• At the start of puberty there is a sharp increase
in the level of follicle stimulating hormone
(FSH) and luteinising hormone (LH) secreted
by the pituitary gland.
• The ovaries and testes respond to these
hormones by producing gametes (sperm and
eggs) and secreting sex hormones.
• The sex hormones (androgens in boys and
estrogens in girls) stimulate the development of
secondary sexual characteristics.
Secondary sexual characteristics
Boys
–
–
–
–
–
–
Starts between the ages of about 12 and 15
The voice breaks
Hair grows on the face and the body
The body becomes more muscular
The genitals develop
Sperm are produced
Girls
–
–
–
–
–
–
Starts between the ages of about 10 and14
The breasts develop
The hips and thighs widen
Pubic and underarm hair develop
The ovaries start to produce eggs
Menstruation begins
Chart showing how boys and girls develop at different rates
Adulthood
Adulthood
Development and the effects of aging
• body systems develop at different rates during
childhood and adolescence.
• All body systems are fully developed by the time
we reach young adulthood.
• Body systems function at peak efficiency from
our late teens to early 30s.
• In mid-life, body systems start to function less
efficiently.
• Many diseases are age-related.
• Health in old age can be affected by fitness as
well as genetic and lifestyle factors.
• Front views of five human skulls showing the
changes that occur during development and
aging. From left to right: newborn, 5 years, 14
years, adult and elderly.
Sarah-Jane Smith, Wellcome Images
Old age
Senescence
• Some mental abilities, such as sensory perception,
reaction time and reflexes, decline over time.
• A number of people experience a progressive loss
of mental abilities and develop senile dementia as
a consequence of aging.
• Our special senses deteriorate as we grow older.
Many people become more long-sighted and need
reading glasses. As people get older they are less
able to hear high-pitched sounds, and their senses
of smell and taste decline.
• As people age, cardiac output tends to
decrease and blood vessels lose their
elasticity.
• In some individuals there is an
accumulation of fatty plaque in the blood
vessels (atherosclerosis) – this can lead to
serious cardiovascular disease.
• As people get older their lungs become
less elastic and their vital capacity
decreases. They may also experience
some degree of emphysema, especially if
they have smoked cigarettes.
• The digestive system generally withstands
aging quite well although there is an
increased risk of dental decay, constipation
and the development of cancers.
• Aging is associated with an increase in the
number of urinary system problems. These
include kidney stones, incontinence and
enlargement of the prostate gland in men.
• The immune system becomes less
responsive with age and there is an
increased risk of infections, cancers and
autoimmune diseases.
Death
Charles Errard, NIH
Death
• A person is considered to be dead when there is no
longer any evidence of life processes.
• In the past, death was believed to coincide with the
onset of clinical death i.e. when there was no longer
evidence of blood circulation and breathing.
• Today, with sophisticated life support systems, it is
possible to keep people alive by artificial means for
many years.
• A person is now considered to be legally dead if
they are brain dead i.e. there is permanent and
irreversible brain failure.
• Brain death can be determined by an absence of
deep reflexes (e.g. pupil dilation), electrical activity
or blood flow in the brain and no response to pain.