Download METABOLISM

HUMAN PHYSIOLOGY
Physiology is the study of how organisms work or function whereas anatomy is the study
of the structure of living organisms. The functioning of an organism involves various
changes, which maintain the stability of the internal environment (homeostasis) and keep
the organism alive. Homeostasis is the maintenance of constant internal conditions (such
as blood chemistry, temperature, and blood pressure) by the body’s control systems. A
homeostatic system is constantly reacting to external forces so as to maintain limits set by
the body’s need.
Physiology is concerned with the changes that take place, where in the organism
these changes occur and how they are regulated. Therefore, there is a need to understand
physiology for one to understand nutrition better.
Levels of human biological organization
Human beings are as simple as a collection of atoms and as complex as a single
organism.
Beginning as tiny units, atoms combine millions of times to form living
organisms. First molecules form, then many molecules combine to form cells. Cells
combine and become tissue, which connects to form organs. The interaction of organs
forms a coordinated system, called an organism. The human body is just such a
coordinated unit of many organ systems.
Atoms
system
molecules
cells
tissues
organs
body
organism
Human cell
Details of structure of a typical animal cell.
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Structure and function of cellular components
COMPONENT
STRUCTURE
Cell/plasma membrane Membrane composed of
phospholipid and protein
molecules.
Cytoplasm
Fluid, jellylike substance in
which organelles are suspended.
Endoplasmic
System of interconnected
reticulum
membrane-forming canals and
tubules.
Ribosomes
Golgi apparatus
Mitochondria
Granular particles composed of
protein and RNA
Cluster of flattened membrane
sacs
Lysosomes
Membranous sacs with folded
inner partitions
Membranous sacs
Peroxisomes
Spherical membranous vesicles
Centrosome
Nonmembranous mass of two
rodlike centrioles.
Membranous sacs
Vacuoles
Fibrils and
microtubules
Cilia and flagella
Nuclear membrane
Nucleolus
Nucleus
Chromatin
Thin, hollow tubes
Minute cytoplasmic extensions
from cell
Membrane surrounding nucleus,
composed of protein and lipid
molecules
Dense, nonmembranous mass
composed of protein and RNA
molecules.
A spherical stracture surrounded
by its own membrane
Fibrous strands composed of
protein and DNA molecules.
FUNCTION
Gives form to cell and controls passage
of materials in and out of cell.
Serves as matrix substance in which
chemical reactions occur.
Smooth endoplasmic reticulum
metabolizes nonpolar compounds and
stores Ca++ in striated muscle cells;
rough endoplasmic reticulum assists in
protein synthesis.
Synthesize proteins
Synthesizes carbohydrates and packages
molecules for secretion; secretes lipids
and glycoproteins.
Release energy from food molecules
and transform energy into usable ATP.
Digest foreign molecules and worn and
damaged cells.
Contain enzymes that produce hydrogen
peroxide and use this for oxidation
reactions.
Helps organize spindle fibers and
distribute chromosomes during mitosis.
Store and excrete various substances
within the cytoplasm.
Support cytoplasm and transport
materials within the cytoplasm.
Move particles along surface of cell or
move cell.
Supports nucleus and controls passage
of materials between nucleus and
cytoplasm.
Forms ribosomes
Carries the genetic information needed
to determine the exact nature of the
protein that will be synthesized.
Controls cellular activity for carrying on
life processes.
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METABOLISM
Metabolism is the sum total of all the chemical reactions that go on in living cell;
energy metabolism includes all the reactions by which the body obtains and spends the
energy from food.
Metabolism involves two types of reaction (catabolism and anabolism).
Catabolism refers to reactions in which large molecules are broken down to smaller ones.
Catabolic reactions usually release energy.
. kata = down
Anabolism refers to reactions in which small molecules are put together to build larger
ones. Anabolic reactions require energy.
. ana = up
However, there are other pathways (progression of chemical reaction from the starting to
the ending process) that connect anabolism and catabolism, described as amphibolic
pathways. These include pathways that serve both catabolic and anabolic purposes, such
as the citric acid cycle and oxidative phosphorylation.
In any one cell of the body catabolic and anabolic processes are carried out
simultaneously. Each chemical reaction requires a catalyst. Enzymes are protein
substances, produced in cells, which act as biological catalysts (speed up chemical
reactions without taking part). Enzymes are found throughout the body but are present in
particularly large amounts in the digestive system.
NB. There is a difference between an enzyme and a hormone.
A hormone is a chemical substance produced in an endocrine gland and
transported by the blood to other tissues where it influences function and metabolic
activity. OR, hormones are chemical messengers. These are secreted by a variety of
glands in response to altered conditions in the body. Each hormone travels to one or more
specific target tissues or organs, where it elicits a specific response to maintain
homeostasis. In general, a gastrointestinal hormone is called an enterogastrone.
Enzymes vs. hormones
All enzymes and some hormones are proteins, but an enzyme is not a hormone.
Enzymes facilitate the making and breaking of bonds in chemical reactions; hormones act
as chemical messengers, sometimes regulating enzyme action.
In metabolism there are two of the most important metabolic processes, which
take place in cells. These are energy production and protein synthesis.
Energy production (respiration)
Carbohydrates, fats and proteins contain chemical energy “locked up” in the
molecules. Respiration is the breakdown of these substances and the subsequent release
of energy. In order to remain alive, both animals and plants must respire. Respiration
normally involves the uptake of oxygen and the release of carbon dioxide and it has
become synonymous with breathing. For clarity, therefore, the external signs of
respiration (breathing) are termed external respiration and the breakdown processes
within the cells internal respiration.
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Energy from carbohydrates
Glucose and glycogen (storage form of glucose in the liver and muscles) are
broken down in the cells of the body into carbon dioxide and water. The breakdown is a
complex process, which takes place as a series of reactions involving many intermediate
products. One of the main intermediate products is pyruvic acid. The following equation
represents the breakdown of glucose into pyruvic acid.
C6H12O6
Glucose
2 CH3COCOOH + 4 [H]
energy pyruvic acid
hydrogen
This part of the respiratory process is anaerobic. The hydrogen atoms are not released as
free hydrogen but are transferred to co-enzymes that act as hydrogen acceptors. (Coenzyme are small organic molecules that work with enzymes to facilitate the enzymes’
activity. Many co-enzymes have vitamin Bs as part of their structures – they form an
integral part of co-enzymes. Co = with). Eventually the hydrogen atoms combine with
oxygen to form water. The breakdown of pyruvic acid is aerobic.
energy
2CH3COCOOH + 5O2
pyruvic acid
oxygen
6CO2 + 4H2O
carbon water
dioxide
This stage releases more energy than the first stage above.
During periods of strenuous exercise there is insufficient oxygen reaching muscle
cells and as a result pyruvic acid is broken down in the anaerobic conditions into lactic
acid. A build-up of lactic acid causes muscle fatigue.
The energy produced by the oxidation of glucose and other nutrients is not
released immediately. Certain organic phosphate compounds are able to store energy
until it is required. Two of these compounds of particular importance are: adenosine
diphosphate (ADP), a compound with two phosphate groups; and adenosine triphosphate
(ATP), a compound with three phosphate groups.
The addition of one more phosphate group to ADP, to convert it into ATP,
requires a relatively large amount of energy. Conversely, the removal of a phosphate
group from ATP releases a large amount of energy.
The energy released during the oxidation of nutrients is used to produce ATP
from ADP. When the cell needs energy, ATP is converted into ADP.
Energy from fats
Fat is first of all transferred from the adipose tissue to the liver. In the liver it is
hydrolyzed to glycerol and fatty acids. The glycerol is converted into pyruvic acid and
oxidized by the method already described above. The fatty acids are broken down into
acetic (ethanoic) acid (CH3COOH), which joins in with the series of reactions involving
the oxidation of pyruvic acid.
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Energy from proteins
Proteins in the diet also yield energy, although their main function is growth and
maintenance of cells. Amino acids not required for protein synthesis are deaminated in
the liver. (Deamination is the reaction that removes the nitrogen-containing amino group
from an amino acid). The deaminated molecules are converted into pyruvic acid and
other intermediate products, which are oxidized, and energy is released.
The summary of energy production is illustrated as follows:
Proteins
Amino acids
Carbohydrates
glucose
Fats
glycerol
fatty acids
ADP
ATP
Urea
pyruvic acid
acetic acid
ADP
ATP
CO2 + H2O
B vitamins roles in metabolism
B vitamins busily work in metabolic pathways all over the body. Metabolism is the
body’s work, and the B vitamin coenzymes are indispensable to every step. These
coenzymes depend on the following vitamins:
-NAD and NADP: niacin
-TPP: thiamin
-CoA: pantothenic acid
-B12: vitamin B12
-FMN and FAD: riboflavin
-THF: folate
-PLP: vitamin B6
-Biotin
Thiamin Pyrophosphate (TPP) is a coenzyme that includes the thiamin molecule as part
of its structure.
Flavin Mononucleotide (FMN) is a coenzyme that includes the riboflavin molecule as
part of its structure.
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Flavin Adenine Dinucleotide (FAD) is a coenzyme that includes the riboflavin molecule
as part of its structure.
Nicotinamide Adenine Dinucleotide (NAD+) and Nicotinamide Adenine Dinucleotide
Phosphate (NADP+). NADP has the same structure as NAD but with a phosphate group
attached to the O instead of the H.
Reduced NAD+ (NADH). When NAD+ is reduced by the addition of H+ and two
electrons, it becomes the coenzyme NADH.
Pyridoxal phosphate (PLP) and pyridoxamine phosphate. These coenzymes are
necessary for transamination (the transfer of an amino group from one amino acid to a
keto acid, producing a new nonessential amino acid and a new keto acid) and other
important processes.
Tetrahydrofolic acid, the active coenzyme form of folate. This active form has four
added hydrogens. An intermediate form, dihydrofolate, has two added hydrogens.
Coenzyme A (CoA). This molecule is made up in part of pantothenic acid.



To break down glucose to pyruvate, the cells must have certain enzymes. For the
enzymes to work, they must have the niacin coenzyme NAD. To make NAD, the
cells must be supplied with niacin (or enough of the amino acid tryptophan to
make niacin). They can make the rest of the coenzyme without dietary help.
The next step in glucose catabolism is the breakdown of pyruvate to acetyl CoA.
The enzymes involved in this step require NAD plus the thiamin coenzyme TPP.
The cells can manufacture the TPP they need from thiamin, if thiamin is in the
diet.
Another coenzyme needed for this step is CoA. Predictably, the cells can make
CoA except for an essential part that must be obtained in the diet – pantothenic
acid. Another coenzyme requiring biotin serves the enzyme complex involved in
converting pyruvate to a compound that can combine with acetyl CoA in the TCA
cycle. These and other coenzymes are involved throughout all the metabolic
pathways.

When the diet provides riboflavin, the body synthesizes FAD – a needed
coenzyme in the TCA cycle.

Vitamin B6 is an indispensable part of PLP – a coenzyme required for many
amino acid conversions, for a crucial step in the making of the iron-containing
portion of hemoglobin for red blood cells, and for many other reactions.

Folate becomes THF – the coenzyme required for the synthesis of new genetic
material and therefore new cells.

The vitamin B12 coenzyme, in turn, regenerates THF to its active form; thus
vitamin B12 is also necessary for the formation of new cells. Therefore each of the
B vitamin coenzymes is involved, directly or indirectly, in energy metabolism.
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Some are facilitators of the energy-releasing reactions themselves; others help
build new cells to deliver the oxygen and nutrients that permit the energy
pathways to run.
Metabolic functions of the liver
The following are just some of the many jobs performed by the liver.
Carbohydrates:





Converts fructose and galactose to glucose
Makes and stores glycogen
Breaks down glycogen and releases glucose
Breaks down glucose for energy when needed
Makes glucose from some amino acids and glycerol when needed.
Lipids:





Builds and breaks down triglycerides, phospholipids, and cholesterol as needed
Breaks down fatty acids for energy when needed
Packages extra lipids in lipoproteins for transport to other body organs
Manufactures bile to send to the gallbladder for use in fat digestion
Makes ketone bodies when necessary
Proteins:




Manufactures nonessential amino acids that are in short supply
Removes from circulation amino acids that are present in excess of need and
deaminates them or converts them to other amino acids
Removes ammonia from the blood and converts it to urea to be sent to the kidneys
for excretion.
Makes other nitrogen-containing compounds the body needs (such as bases used
in DNA and RNA).
 Makes plasma proteins such as clotting factors.
Other:
 Detoxifies alcohol, other drugs, and poisons; prepares waste products for
excretion
 Helps dismantle old red blood cells and captures the iron for recycling
 Stores most vitamins and many minerals
 Forms lymph.
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TISSUES
Although some single cells are self-sufficient organisms, multicellular organisms such as
the human body contain specialized cells, each of which has a particular function. Some,
for example, are capable of contraction, others can secrete substances from the blood and
so on. Groups of identical cells, which together perform a certain function, are called
tissues.
There are several types of tissues which vary according to the structure (size and shape)
of their cells, their position and their function.
In the body, there are five basic tissues namely: epithelial, connective, muscle, nervous,
and fluid.
1. Epithelial tissue
This consists of flat sheets of closely packed cells. It is found in the skin, lining
membranes and outer membranes of many organs. Because of the porosity of this
tissue, small molecules can pass through it; this occurs in the alveoli of the lungs and
in the small intestine. Some epithelial tissue possesses hair-like projections, called
cilia, which propel substances past the cells; this occurs in the trachea when mucus is
brushed upwards towards the mouth. Many epithelial cells produce secretions such
as mucus, which keep surfaces and linings moist.
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2. Connective tissue
This joins defferent kinds of tissue together, encloses groups of cells and holds organs
in place. It consists of specialized cells surrounded by non-cellular material called a
matrix. Much connective tissue contains collagen, a strong fibrous protein.
There are three main types of fibrous protein:
a) White collagen fibres – the most common, made of the protein collagen;
b) Reticular fibres – made of a protein called reticulin;
c) Yellow elastic fibres – composed of the protein elastin. These fibres will stretch
but recoil to resome thir original length.
The relative proportions of these fibres vary in the different types of connective
tissue.
1) Areolar tissue. This type of connective tissue is found under the skin and
surrounding internal organs, keeping them in place. It contains many
loosely packed collagen fibres.
2) Adipose tissue. This is the fatty tissue of the body. It is similar to areolar
tissue but the cells are swollen with droplets of fat. This fat represents the
main energy reserve of the body. Adipose tissue is also found under the
skin and around certain organs. It helps to reduce heat loss from the
surface of the body and protects organs such as the kidneys.
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3) Fibrous tissue. This is a form of connective tissue in which collagen
fibres are closely packed in bundles or sheets. It is tough and inelastic and
forms ligaments and tendons. Ligaments hold bones together at the joints
and tendons attach muscles to bones. Fibrous tissue also surrounds muscle
fibres and bundles of muscles fibres.
4) Cartilage. Is a strong connective tissue often found at the ends of bones
where it prevents friction. Cartilage contains a tough, rubbery organic
matrix. Some types contain white collagen fibres and some yellow fibres.
Outgrowths of cartilage form a large part of the nose and ear and it is
found in pads or discs between the vertebrae of the back bone. It is also
found in the wall of the trachea-windpipe.
5) Bone. Is another form of connective tissue, which gives the body shape
and firmness. It consists of a hard matrix containing large deposits of
calcium phosphate. Interspersed here and there are cells which are
connected to one another and are fed by blood vessels which pass into the
bone tissue.
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6) Elastic tissue. Is found in the walls of arteries, the stomach, and bladder
and forms a large part of the lung tissue.
3.Muscle tissue
Muscle is necessary for all movement in the body. The muscle cell is specialized for
contraction, which it does when stimulated to do so by nervous impulses. Nearly all
bodily processes depend on movement. The blood circulation and transport of
nutrients depend on the contraction of the heart. Intake of oxygen depends on
contraction of the respiratory muscles. Food is propelled along the digestive tract by
peristaltic contraction. Even our entrance into the world is brought about by the
powerful contractions of the uterus.
1) Skeletal (or voluntary) muscle is made up of bundles of fiblres running
parallel to one another, which are bound together with connective tissue.
The fibres are elongated multinucleated cells, which are covered with
visible cross-markings or striations, which give the muscle an alternative
name, striated muscle. These striations consist of thick and thin
filaments containing two proteins called actin and myocin respectively.
Contraction occurs in response to stimuli from branches of neurons. Each
fibre shortens as the filaments slide over one another; this may continue for a
few seconds or relaxation may follow. As the fibre becomes fatigued quickly,
the muscle cannot be held in a state of contraction for long. Energy for
contraction comes from the ATP in the cells.
When large amounts of energy are required, for example for a quick sprint,
the glycogen stored in the muscle is converted into lactic acid, releasing
energy. Only one quarter of the energy thus supplied is in the form of
mechanical energy and the remaining three-quarters is given off as heat, a
normal product of the muscular activity. Skeletal muscle contracts much fast
than other types of muscle, but as it tires easily, it works best in short bursts.
Skeletal muscle makes up the fleshy part of the body and is found on the
limbs and trunk. As it is under the control of the will (the cerebral cortex) it is
also called voluntary muscle.
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2) Smooth (or involuntary) muscle. This consists of sheets of spindleshaped cells (wide at the middle and narrow at each end). Each cell
contains a single nucleus. Smooth muscle has no striations and is very
elastic. It is not under voluntary control but is controlled by neurons of
the sympathetic and parasympathetic nervous system. Smooth muscle is
found on the walls of hollow organs and blood vessels. It causes
contractions of the arteries, alimentary canal, uterus and bladder; the
contractions brought about are long and slow, unlike the stronger quick
contractions of the voluntary muscles.
3) Cardiac muscle consists of short, irregularly striated fibres. Each is a true
cell, which interlocks with the next, forming parallel lines. Its structure is
similar to that of skeletal muscle except for some branching of fibres.
Cardiac muscle is capable of initiating its own contractions. The sinoartial nodes, which are specialized groups of muscle cells, stimulate the
contractions of the heart. Although some sympathetic nerves pass into the
heart, they serve only to speed up or slow down the heart beat. With each
beat of the heart, all of the available energy in the heart muscle is utilized.
As there is no energy reserve, loss of oxygen, even for a few seconds
could have fatal consequences.
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4. Nervous tissue
Nervous tissue consists of nerve cells or neurones which have the ability to respond to
stimuli (change in the environment) and to transmit impulses to other tissues, e.g. to
muscles.
A neurone has a cell body with many thread-like projections or processes. There is
usually a large number of branched processes called dendrites which conduct
impulses to the cell body and a single, long process, the axon, which conducts the
impulse away. The axon may be a metre or more in length.
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5. Fluid tissue
Blood and lymph may be classified as fluid tissue. They consist of cells which move
in a liquid intercellular matrix.
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OVERVIEW OF ORGANS
An organ is a structure composed of at least two, and usually all types of primary tissues.
For example, organs of the skeletal system (head, trunk, girdles, limbs) contain
connective tissue (bone, cartilage, areolar); fluid tissue; voluntary muscle and nerve.
The organs of the digestive system (mouth – [including teeth, tongue and salivary
glands]; oesophagus; stomach; liver; pancreas; small intestine; large intestine) contain
epithelial (glandular); connective (areolar); fluid (blood); involuntary muscle; and nerve
(autonomic).
The organs of respiratory system (nose, larynx, trachea, bronchi, lungs) contain
epithelial; connective (bone, cartilage); fluid (blood); voluntary muscle; nerve.
The organs of the circulatory system (heart, arteries, veins, capillaries) contain
epithelial; connective (areolar); fluid (blood); heart/cardiac muscle (involuntary); nerve.
The organs of the urogenital (often regarded as two systems) – kidneys, bladder,
gonads (testis or ovary), other sex glands, uterus, external organs (penis or vulva) contain
epithelial (glandular); connective (areolar); fluid (blood); involuntary muscle; and the
nerve.
The largest organ in the body, in terms of its surface area, is the skin.
Functions of the skin
1. To protect the inner tissues
2. To excrete waste substances
3. As a temperature regulator
4. As a sensory organ
5. To manufacture vitamin D
6. To act as an insulator by storing fat
These functions therefore, will serve in this section to illustrate how primary tissues
cooperate in the service of organ physiology.
The cornified epidermis protects the skin against water loss and against invasion
by disease-causing organisms. Invaginations of the epithelium into the underlying
connective tissue dermis create the exocrine glands of the skin. These include hair
follicles (which produce the hair), sweat glands, and sebaceous glands. The secretion of
sweat glands cools the body by evaporation and produces odors that, at least in lower
animals, serve as sexual attractants. Sebaceous glands secrete oily sebum into hair
follicles, where it is transported to the surface of the skin. Sebum lubricates the cornified
surface of the skin, helping to prevent it from drying and cracking.
The skin is nourished by blood vessels within the dermis. In addition to blood
vessels, the dermis contains wandering white blood cells and other types of cells that
protect against invading disease-causing organisms, as well as nerve fibers and fat cells.
Most of the fat cells, however, are grouped together to form the hypodermis (a layer
beneath the dermis). Although fat cells are a type of connective tissue, masses of fat
deposits throughout the body – such as subcutaneous fat – are referred to as adipose
tissue.
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Sensory nerve endings within the dermis mediate the cutaneous sensations of
touch, pressure, heat, cold, and pain. Some of these sensory stimuli directly affect the
sensory nerve endings. Others act via sensory structures derived from non neural primary
tissues. The pacinian corpuscles in the dermis of the skin, for example, monitors
sensations of pressure. Motor nerve fibers in the skin stimulate effector organs, resulting
in, for example, the secretions of exocrine glands and contractions of the arrector pili
muscles, which attach to hair follicles and surrounding connective tissue (producing
goose bumps). The degree of constrictions or dilations of cutaneous blood vessels – and
therefore the rate of blood flow – is also regulated by motor nerve fibers.
The epidermis itself is a dynamic structure that can respond to environmental
stimuli. The rate of its cell division – and consequently the thickness of the cornified
layer - increases under the stimulus of constant abrasion. This produces calluses (areas of
thick hardened skin). The skin also protects itself against the dangers of ultraviolet light
by increasing its production of melanin pigment, which absorbs ultraviolet light while
producing a tan. In addition, the skin is an endocrine gland that produces and secretes
vitamin D (derived from cholesterol under the influence of ultraviolet light), which
functions as a hormone.
The architecture of most organs is similar to that of the skin. Most are covered by
an epithelium immediately over a connective tissue layer. The connective tissue contains
blood vessels, nerve endings, scattered cells for fighting infection, and possibly glandular
tissue as well. If the organ is hollow – as in the digestive tract or in blood vessels- the
lumen is also lined with an epithelium immediately over a connective tissue layer. The
presence, type, and distribution of muscular and nervous tissue vary in different organs.
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THE RESPIRATORY SYSTEM
The function of the respiratory system is to supply the body with oxygen and to get rid of
carbon dioxide.
The respiratory system is divided into a respiratory zone, where gas exchange
between air and blood occurs, and a conducting zone, which conducts the air to the
respiratory zone. The exchange of gases between air and blood occurs across the walls of
tiny air sacs called alveoli, which are only one cell across in thickness to permit very
rapid rates of gas diffusion.
The term respiration includes three separate but related functions:
1) Ventilation (breathing);
2) Gas exchange
 Which occurs between the air and blood in the lungs and
 Between the blood and other tissues of the body;
3) Oxygen utilization by the tissues in the energy-liberating reactions of cell
respiration.
Ventilation and the exchange of gases (oxygen and carbon dioxide) between the air and
blood are together called external respiration.
Gas exchange between the blood and other tissues and oxygen utilization by the
tissues are together known as internal respiration.
Ventilation is the mechanical process that moves air into and out of the lungs.
Since air in the lungs has a higher oxygen concentration than in the blood, oxygen
diffuses from air to blood. Carbon dioxide, conversely, moves from the blood to the air
within the lungs by diffusing down its concentration gradient. As a result of this gas
exchange, the inspired air contains more oxygen and less carbon dioxide than the expired
air. More importantly, blood leaving the lungs (in the pulmonary vein) contains a higher
oxygen and a lower carbon dioxide concentration than the blood delivered to the lungs in
the pulmonary arteries. This results from the fact that the lungs function to bring the
blood into gaseous equilibrium with the air.
Gas exchange between the air and blood occurs entirely by diffusion through lung
tissue. This diffusion occurs very rapidly because there is a high surface area within the
lungs and a very short diffusion distance between blood and air.
The pathway of air
Air flowing in and out of lungs passes through the respiratory tract. The conducting zone
of the respiratory system, in summary, consists of the mouth, nose, pharynx, larynx,
trachea/windpipe, primary bronchi, and all successive branching of the bronchioles up to
and including the terminal bronchioles. In addition to conducting air into the respiratory
zone, these structures serve additional functions:
 Warming and humidification of the inspired air and
 Filtration and cleaning
When the inspired air reaches the respiratory zone it is at a temperature of 37 0C (body
temperature), and it is saturated with water vapor. Therefore, the warming function is
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needed to maintain a constant internal body temperature, and humidification is needed to
protect delicate lung tissue from desiccation (drying up).
Mucus secreted by cells of the conducting zone which is moved along at a rate of
1-2 centimeters per minute by cilia projecting from the tops of epithelial cells that line the
conducting zone, serves to trap small particles in the inspired air and thereby performs a
filtration function.
As a result of this filtration, particles larger than about 6  m do not normally
enter the respiratory zone of the lungs. The disease called black lung, which occurs in
miners who inhale too much carbon dust and therefore develop pulmonary fibrosis,
evidences the importance of this function.
As these functions are concerned, breathing through the mouth is not as
satisfactory because the air is neither filtered nor conditioned and lung infection may
ensue through bacterial action or inflammation caused by dryness.
The alveoli themselves are normally kept clean by the action of macrophages
(literally, “big eaters”) that reside within them. The cleansing action of cilia and
macrophages in the lungs has been shown to be diminished by cigarette smoke.
Surfactant and the Respiratory Distress Syndrome
Alveolar fluid contains a phospholipid known as dipalmitoyl lecithin, probably attached
to a protein, which functions to lower surface tension (surface tension is created by the
fact that water molecules at the surface are attracted more to other water molecules than
to air. As a result, attractive forces from underneath pull the surface water molecules
tightly together. The surface tension of an alveolus produces a force that is directed
inward and, as a result, creates pressure within the alveolus). This compound is called
lung surfactant, which is a contraction of the term surface active agent. Because of the
presence of surfactant, the surface tension in the alveoli is lower than would be predicted
if surfactant were absent. Further, the ability of surfactant to lower surface tension
improves, as the alveoli get smaller during expiration. This may be because the
surfactant molecules become more concentrated as the alveoli get smaller. Surfactant
thus prevents the alveoli from collapsing during expiration. Even after a forceful
expiration, the alveoli remain open and a residual volume of air remains in the lungs.
Since the alveoli do not collapse, less surface tension has to be overcome to inflate (fill
with air) them at the next inspiration.
Type II alveolar cells in late fetal life produce surfactant. Since surfactant does
not start to be produced until about the eight month, premature babies are sometimes born
with lungs that lack sufficient surfactant, and their alveoli are collapsed as a result. This
condition is called respiratory distress syndrome. It is also called hyaline membrane
disease, because the high surface tension causes plasma fluid to leak into the alveoli,
producing a glistening “membrane” appearance (and pulmonary edema). This condition
does not occur in all premature babies; the rate of lung development depends on
hormonal conditions (thyroxine and hydrocortisone primarily) and on genetic factors.
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EXCRETORY SYSTEM
The waste substances produced during the metabolic processes must be removed
regularly to enable the body to function properly. Such wastes are removed by the
excretory organs: the lungs, the skin, the kidneys, and the liver (the liver acts as an
excretory organ by altering and storing some waste substances). The large intestine is an
organ of elimination rather than excretion.
The CO2 produced by cellular respiration leaves the cells and is carried to the lungs by
red blood cells. At the lungs, red blood cells release their load of CO2, which is then
exhaled into the environment. These red blood cells then pick up oxygen for delivery to
the cells for use in cellular respiration. In addition to CO2, the lungs excrete a significant
amount of H2O through evaporation.
Another site for the excretion of wastes is the skin. Here perspiration or sweat causes the
loss of H2O, urea, and some minerals.
The kidney is the primary site for excretion of waste products from protein metabolism,
such as urea. The kidney works in two phases. In the first phase, water and dissolved
substances are filtered out of the blood into the tubules of the kidney. In the second
phase, some H2O and dissolved substances that are useful to the body, such as glucose,
are reabsorbed from the filtrate. Dissolved wastes, along with some H2O and other
substances that are not reabsorbed, are excreted in the urine.
Urea synthesis
When amino nitrogen is stripped from amino acids, ammonia is produced. The liver
detoxifies ammonia before releasing it into the bloodstream by combining it with another
waste product, carbon dioxide, to produce urea.
H
‫׀‬
H–N– H
ammonia
O
N
║
‫׀‬
+ C + H– N – H
║
ammonia
O
H–O-H
water
H
O H
‫׀ ║ ׀‬
H–N–C–N -H
urea
19
Urea excretion
The liver and kidneys both play a role in disposing of excess nitrogen. This is the reason
why the person with liver disease has high blood ammonia, while the person with kidney
disease has high blood urea.
The reabsorption of H2O must be regulated to prevent dehydration. The amount of H2O
excreted in the urine and the amount reabsorbed are under hormonal control, so
homeostasis is maintained.
Osmoregulation
Water lost by urine, feces, sweat and lungs must be balanced by the intake of H2O in food
and drink. The brain controls this delicate balancing of the H2O level of the body.
Sensory receptors in the hypothalamus stimulate the pituitary gland just below to produce
the antidiuretic hormone (ADH) (it is a hormone released by the pituitary gland in
response to highly concentrated blood. The kidneys respond by reabsorbing water, thus
preventing water loss. It is also called vasopressin because it elevates blood pressure).
Anti=against; dia=through; ure=urine; vaso=vessel and press= pressure. This on
reaching the kidneys, stimulates the tubules to return more water to the blood, thereby
increasing the concentration of urine. This would occur after considerable water loss
occurred, e.g. after sweating or diarrhea. Likewise when the blood becomes too dilute
20
(when large quantities of liquid have been drunk), the amount of ADH is suppressed and
the tubules return less water, increasing the quantity of urine excreted.
Inability to secrete this hormone causes diabetes insipidus, the symptoms of which are
thirst and frequent heavy urine.
Blood volume and blood pressure
Water balance maintains the blood volume, which in turn influences blood pressure. If
the body loses too much water, blood volume and blood pressure fall.
ADH and water retention: whenever the blood becomes too concentrated, or whenever
blood volume or blood pressure falls too low, the hypothalamus signals the pituitary
gland to release the antidiuretic hormone (ADH). ADH stimulates the kidneys to
reabsorb water, rather than excrete it. Consequently, the more water you need, the less
your kidneys excrete.
Angiotensin and blood vessel constriction: Cells in the kidneys respond to low blood
pressure by releasing an enzyme called renin. Through a complex series of events, renin
causes the kidneys to reabsorb sodium. Sodium reabsorption, in turn, is always
accompanied by water retention, which helps to restore blood volume and blood pressure.
Renin also activates the blood protein angiotensinogen to angiotensin. Angiotensin is a
powerful vasoconstrictor: it narrows the diameters of blood vessels, thereby raising the
blood pressure.
Aldosterone and Sodium retention: angiotensin also mediates the release of the
hormone aldosterone from the adrenal glands. Aldosterone causes the kidneys to retain
more sodium (and thus more water). Again, the effect is that when more water is needed,
less is excreted.
In summary, in response to low blood volume or highly concentrated blood, these three
actions combine to effectively restore homeostasis as shown in the following diagram:
 ADH causes water retention
 Angiotensin constricts blood vessels
 Aldosterone causes sodium retention.
Renin is an enzyme from the kidneys that activates angiotensin.
N.B.: Renin and Rennin are two different enzymes. The latter is an enzyme that
coagulates milk, found in the gastric juice of cows, but not human beings except for
infants.
Angiotensin is a hormone involved in blood pressure regulation. Its precursor protein is
called angiotensinogen.
Vasoconstrictor is a substance that constricts or narrows the blood vessels.
Aldosterone is a hormone secreted by the adrenal glands that stimulates the reabsorption
of sodium by the kidneys; aldosterone also regulates chloride and potassium
concentrations.
21
How the body regulates water excretion
The kidneys respond to
reduced blood flow by
releasing the enzyme renin.
The hypothalamus responds
to high salt concentrations in
the blood by stimulating the
pituitary gland.
Renin
Renin initiates the activation
of the protein angiotensinogen
to angiotensin.
The pituitary gland releases
antidiuretic hormone (ADH)
Angiotensin
Adrenal glands
secrete aldosterone.
ADH
Blood vessels
constrict, raising
pressure
Aldosterone
Kidneys retain sodium and water, thus increasing blood volume.
22
NERVOUS SYSTEM
In order to control the various body processes and enable the body to work as a unit, there
must be a system, which coordinates all the others. In fact there are two systems – the
nervous system and the endocrine system.
The nervous system is composed of neurons, which produce and conduct
electrochemical impulses, and neuroglial cells, which support the functions of neurons.
The function of the nervous system is to control and coordinate the activities of the body
by enabling the body to perceive changes in the environment (stimuli) and to respond
accordingly.
The nervous system is divided into the central nervous system (CNS), which
includes the brain and spinal cord, which contain nuclei and tracts (collections of nerve
fibers that interconnect regions of CNS) and the peripheral nervous system (PNS),
which includes the cranial nerves arising from the brain and the spinal nerves arising
from the spinal cord, and ganglia (collections of neuron cell bodies located outside CNS).
Functions of the spinal cord are as follows:
1. It carries impulses to and from the brain
2. It is the center for simple reflex actions (e.g. lifting one’s fingers from a very hot
object).
Neurons
Although neurons vary considerably in size and shape, they generally have three
principal regions:
1. a cell body
2. dendrites
3. an axon
Dendrites and axon can be referred to generically as processes, or extensions from the
cell body.
The cell body, or perikaryon (peri = around; karyon = nucleus), is the enlarged
portion of the neuron, which contains the nucleus and serves as the “nutritional center”
of the neuron where macromolecules are produced. The perikaryon also contains
granular, densely staining material known as Nissl bodies, which are not found in the
dendrites or axon. The Nissl bodies are composed of granular (rough) endoplasmic
reticulum, an organelle involved in protein synthesis. The cell bodies within the CNS are
frequently clustered into groups called nuclei (not to be confused with the nucleus of the
cell). Cell bodies in the PNS usually occur in clusters called ganglia.
Dendrites (dendron = tree branch) are thin-branched processes that extend from
the cytoplasm of the cell body. Dendrites serve as a receptive area that transmits
electrical impulses to the cell body.
The axon, or nerve fiber, is a longer process that conducts impulses away from
the cell body. Axons vary in length from only a millimeter to a meter or more (for axons
that extend from the CNS to the foot). The origin of the axon near the cell body is called
23
the axon hillock, and side branches that may extend from the axon are called axon
collaterals.
Proteins and other molecules are transported through the axon at faster rates than
could be achieved by simple diffusion. This rapid movement is produced by two
different mechanisms: axoplasmic flow and axonal transport. Axoplasmic flow, the
slower of the two, results from rhythmic waves of contraction that push the cytoplasm
from the axon hillock to the nerve endings. Axonal transport, which is more rapid and
more selective, may occur in a reverse (retrograde) as well as a forward (orthograde)
direction. Indeed, retrograde transport may be responsible for the movement of herpes
virus, rabies virus, and tetanus toxin from the nerve terminals into cell bodies.
Comparison of axoplasmic flow with axonal transport
Axoplasmic flow
Axonal transport
-Transport rate comparatively
slow (1-2 mm/day)
-Molecules transported only
from cell body
-Transport rate comparatively
fast (200-400mm/day)
- Molecules transported from
cell body to axon endings and in
reverse direction.
-Transport of specific proteins,
mainly of membrane proteins
and acetylcholinesterase.
-transport dependent on cagelike
microtubule structure within
axon and on actin and Ca++
-Bulk movement of proteins
in axoplasm, including micro
filaments and tubules
-Transport accompanied
by peristaltic waves of
axon membrane
Classification of Neurons and nerves
Neurons may be classified according to their structure or function.
The functional classification is based on the direction that they conduct impulses.
When a nerve fiber is stimulated, usually at one end, a reaction causes a change in the
next part of the fiber. This, in turn, causes another section to react and in this way the
impulse travels along the length of the fiber (it could be compared to a lighting fuse).
The speed of the reaction is roughly 120 meters per second. A supply of oxygen is
necessary for the transmission of impulses.
The functional classification is as follows:
1. Sensory, or afferent neurons conduct impulses from sensory receptors into the
CNS.
2. Motor, or efferent neurons conduct impulses out of the CNS to effector organs
(muscles and glands)
3. Association neurons, or interneurons, are located entirely within the CNS and
serve the associative, or integrative, functions of the nervous system.
24
There are two types of motor neurons: somatic and autonomic.
a) Somatic motor neurons provide both reflex and voluntary control of
skeletal muscles.
b) Autonomic motor neurons innervate (distribute the nerves to) the
involuntary effectors-smooth muscle, cardiac muscle, and glands. The cell
bodies of the autonomic neurons that innervate these organs are located
outside the CNS in autonomic ganglia.
There are two subdivisions of autonomic motor neurons: sympathetic and
parasympathetic. Autonomic motor neurons, together with their central control centers,
comprise the autonomic nervous system.
i) The sympathetic nervous system makes sudden action possible by increasing blood
supply to the heart and lungs and reducing the supply to the organs of digestion and
excretion. It speeds up conversion of glycogen to glucose.
ii) The parasympathetic nervous system reverses the effects of the sympathetic
system, restoring the body to normal. It decreases heart beat and blood supply to
lungs and restarts digestive and excretory organs. It also exercises control on the
degree of the contractions, which occur.
The structural classification of neurons is based on the number of processes that extend
from the cell body of the neuron.
1. Bipolar neurons have two processes, one at either end; this type is found in the
retina of the eye.
2. Multipolar neurons have several dendrites and one axon extending from the cell
body; this is the most common type of neuron (motor neurons are good examples
of this type).
3. A pseudounipolar neuron has a single short process that divides like a T to form
a longer process. Sensory neurons are pdeudounipolar – one end of the process
formed by the T receives sensory stimuli and produces nerve impulses; the other
end of the T delivers these impulses to synapses within the brain or spinal cord.
The long process that extends from the sensory receptor to the stalk of the T
(toward the cell body) is a dendrite, while the process that extends from the stalk
to the CNS is the axon. The cell bodies of these sensory neurons are located
outside the CNS in the dorsal root ganglia of spinal and cranial nerves.
Three different types of neurons according to their structures
25
A nerve is a bundle of axons outside the CNS. Most nerves are composed of both motor
and sensory fibers and are thus called mixed nerves. Some of the cranial nerves,
however, contain only sensory processes. These are the nerves that serve the special
senses of sight, hearing, taste, and smell.
The senses of sight, taste and smell play an important role in the appreciation of
food and in stimulating the secretion of digestive juices.
The retina has two types of nerve endings: rods and cones. These are both
sensitive to light and impulses are conveyed from them, along the optic nerve, to the
brain. The rods contain the pigment rhodopsin (visual purple) and are concerned with
black and white vision in dim light. A deficiency of vitamin A depletes the supply of
rhodopsin and night blindness results. The cones contain other pigments and are
concerned with color vision in daylight.
The receptors of taste are the taste buds, which are situated on the tongue and soft
palate, the back part of the roof of the mouth. Each taste bud is a round cluster of
spindle-shaped cells from which nerve fibres lead to the brain.
The sensation of taste depends on water, either from food or from saliva, to carry
the stimulating substances to the taste buds. The taste buds are capable of recognizing
four basic or fundamental tastes, namely, sweet, sour, salt and bitter. These different
tastes are transmitted along the nerves by different patterns of impulse. Some parts of the
tongue are more sensitive than others to particular tastes.
The four basic tastes
Basic taste
Sweet
Sour
Salt
Bitter
Substances responsible
Sugars, saccharin, aspartame…
Acids, e.g., citric, malic, tartaric…
Chlorides, especially sodium chloride
(common salt)
Alkaloids, e. g., quinine (tonic water);
caffeine (coffee)
Area of tongue where detected
Tip and Top front
Sides
Tip and Sides
Back
The olfactory organs, the organs responsible for the sense of smell, are found in the upper
part of the nose and are somewhat similar in structure to the taste buds.
26
ENDOCRINE SYSTEM
As mentioned earlier, there are two systems concerned with the control and coordination
of the activities of the body. These are the nervous system and the endocrine system.
The latter is made up of various glands, which release secretions, known as hormones,
into the blood stream. The word hormone comes from the Greek hormaein meaning “I
activate”. Endocrine glands are ductless and are distinct from exocrine glands, which
release their secretions (e.g. sweat, digestive juices) into a duct to be used locally.
Although hormones are slow to cause change, their effects last longer than the
effects of nervous impulses. They regulate such long-term processes as growth, sexual
maturity and ageing.
The amount of hormone secreted is extremely important as an imbalance leads to
various disorders. Normally, the amount of hormone is adjusted to suit the body’s
current requirements.
Many hormones are neutralized in the liver and eventually excreted by the
kidneys.
Pituitary Gland
This “master” gland influences the activity of many endocrine glands. It is a peasized structure at the base of the brain, which is divided into two parts, the anterior
pituitary and the posterior pituitary.
The pituitary gland is closely associated with the hypothalamus, which lies just
above it. The hypothalamus is very sensitive to changes in the body and can respond by
stimulating the pituitary into the production of hormones.
Anterior pituitary. The anterior pituitary secretes at least seven hormones:
1. Human growth hormone (HGH) influences the growth of cells. Excess causes
gigantism, while its deficiency results in dwarfism.
2. Lactogenic hormone (LTH) stimulates milk production in females after birth.
3. Thyroid stimulating hormone (TSH) or Thyrotropic hormone stimulates the
thyroid gland to secrete its hormone thyroxine.
4. Adrenocorticotropic hormone (ACTH) stimulates the adrenal cortex to release
its hormones.
5. Follicle stimulating hormone (FSH) acts on the gonads (sex organs). In the
female it stimulates egg ripening and oestrogen production; in males it stimulates
the development of the seminiferous tubules and sperm production.
6. Luteinising hormone (LH) stimulates production of sex hormones –
progesterone in females and androgens in males.
7. Melanocyte stimulating hormone (MSH) affects the pigmentation of the skin.
27
Posterior lobe.
1. Oxytocin stimulates the contraction of the uterus thus beginning labour in
childbirth. When birth is artificially induced it is injected into the bloodstream.
2. Antidiuretic hormone (ADH) stimulates reabsorption of water from kidney
tubules. It also affects blood pressure by constricting the walls of small arterioles.
Thyroid Gland
This is the largest endocrine gland. It is a double structure situated in the neck in front of
the trachea and is richly supplied with blood. Thyroxine, its hormone, controls the rate
of growth in the young: deficiency of this in children can cause dwarfism and mental
retardation (known as cretinism). If the condition is diagnosed quickly, administering
thyroxine can cure it.
In adults thyroxine affects the rate of metabolism: excess amounts raise the
metabolic rate, causing thinness and hyperactivity, while a deficiency lowers the basal
metabolic rate causing obesity, lethargy and mental confusion.
Any disturbance in the activity of the gland causes it to enlarge, resulting in
goiter. The hormone thyroxine contains iodine, and if there is a deficiency of iodine in
the diet, goiter will result.
Parathyroid Glands
Imbedded in the tissues at the back of the thyroid are four small glands known as
parathyroids. They release a hormone parathormone (PTH), which controls the level of
calcium in the blood. Excess PTH causes brittle, badly formed bones and may cause
kidney stones, while deficiency can be fatal unless calcium is administered.
Adrenal Glands
These are two small structures, which lie one on each kidney.
Adrenal cortex. This is the outer layer of the glands. It produces at least three types
of hormones:
1. The glucocorticoids – steroids including cortisone, which is involved with
metabolism of carbohydrates, stimulate the laying down of glycogen in the liver.
They are also involved with general body metabolism. The glucocorticoids
reduce inflammation; this is why cortisone is important in the treatment of many
diseases. They also take over from adrenaline after its initial stimulation has worn
off, helping the body to adapt to stress.
2. Aldosterone, which regulates salt/water balance in the blood
3. Sex hormones, particularly male androgens (the steroid sex hormones that exert
masculinizing effects, and they promote protein anabolism and growth). Excess
of these hormones causes male characteristics in women. A general deficiency of
the hormones of the adrenal cortex causes Addison’s Disease, which results in
28
muscular weakness, apathy and eventually death. Administration of cortical
hormones is usually successful, however.
Adrenal medulla. This is the interior of the adrenal gland. It secretes two hormones,
adrenaline and noradrenaline. The former is secreted in large quantities when the body
suffers stress, anger, fright or injury; it causes the heartbeat to quicken, diverts blood
from less important organs to those concerned with “fright or flight”, speeds up the rate
of breathing, raises the blood pressure and blood sugar and increases the rate of
metabolism. All of these changes prepare the body for violent physical action. The latter
hormone, noradrenaline, is secreted soon after adrenaline in order to sustain these
reactions by causing a further increase in blood pressure.
Pancreas
Apart from its function in the digestion of food, the pancreas contains groups of
endocrine cells called Islets of Longerhans. These secrete insulin, a hormone which
controls carbohydrate metabolism and hence the level of sugar in the blood. A rise in
blood sugar releases insulin into the blood. This passes through the portal vein to the
liver where it hastens the conversion of glucose to fat and glycogen, quickly returning the
blood sugar level to normal.
Insufficient insulin causes diabetes mellitus. As the body is unable to convert
extra glucose to glycogen, large amounts circulate in the blood and pass into the urine.
Protein and fat are converted into glucose and through osmosis, large amounts of water
remain in the tubules to be excreted as urine. A low carbohydrate diet is often effective
in coping with mild cases of diabetes, and injections of animal insulin control the disease
in more serious cases so that diabetics can lead a normal life. Excessive insulin can
reduce the level of glucose in the blood to such an extent that the brain cells are deprived
of food, and shock and even coma may result.
Gonads
These are the male and female reproductive organs. As well as producing the sperms and
ova, they are endocrine glands, which secrete important hormones controlling
reproduction and sex drive.
Testes
These produce the male sex hormone testosterone, which influences the development of
secondary male characteristics such as the deepening of the voice and growth of hair on
the face and body. The LH from the pituitary gland stimulates it.
Ovaries
These secrete two hormones in response to stimulation by the pituitary gland:
Oestrogen, which causes various changes to take place at puberty, e.g. breast
development. This hormone also initiates and controls menstruation.
29
Progesterone controls the changes, which occur during pregnancy and lactation.
Deficiency causes miscarriage.
Other organs with endocrine functions
The stomach secretes a hormone gastrin into the blood stream when we eat and
when this returns to the stomach it stimulates the production of gastric juices.
The duodenum produces secretin which, when it reaches the pancreas in the
bloodstream, activates the production of pancreatic enzymes.
The kidneys produce a hormone concerned with blood pressure.
Feedback
Many hormones operate a self-regulating feedback system whereby the release of one
hormone into the bloodstream triggers off a response in another hormone, causing it to
suppress production. This ensures that excessive amounts of any one hormone are not
released. For example, presence of thyroxine inhibits TSH production; TSH stimulates
the formation of thyroxine.
30
REPRODUCTIVE SYSTEM
Reproduction is the means whereby a species is perpetuated. Human life begins
as a single-celled embryo formed by the fusion of a sperm from the testes of a male and
an ovum (egg) from the ovaries of a female. In humans the reproduction also satisfies
emotional needs and drives such as love, protection and security.
Male reproductive organs
The reproductive organs of the male lie outside the body because the cooler
temperature is more conducive to the production of sperm. They consist of the testes,
two oval organs which lie in a loose covering of skin called the scrotum. Each testis
contains a large mass of seminiferous tubules, the walls of which contain cells that
manufacture sperm. The mature sperm pass into a duct, which leads from inside the
testes to another coiled tube, this time outside each testis (the epididymis). From this a
larger tube, called the vas deferens or sperm duct, passes out of the scrotum. Near the
bladder it braches into two, one branch leading to the coiled seminal vesicle, the other
opening into the urethra at the base of the bladder. Near this point are two glands, the
prostate gland and Cowpers gland. Together with the seminal vesicles, these produce
the seminal fluid in which the spermatozoa (sperm) are suspended. The urethra travels
downward into the penis, which is composed of connective tissue and in which there are
large numbers of spaces resembling the holes in a sponge. These fill with blood during
intercourse.
At puberty the pituitary hormone LH stimulates production of testosterone which
brings about secondary male characteristics (e.g. hair growth, deepening of the voice,
etc.) and also initiates sperm production.
The seminal vesicles and prostate are androgen-dependent accessory sexual
organs – they atrophy if androgen is withdrawn by castration. The seminal vesicles
secrete fluid containing fructose (which serves as an energy source for the spermatozoa),
citric acid, coagulation proteins, and prostaglandins. The prostate secretes a liquefying
agent and the enzyme acid phosphatase, which is often measured clinically to assess
prostate function.
Erection, emission, and ejaculation
Erection, accompanied by increases in the length and width of the penis, is
achieved as a result of blood flow into the “erectile tissue” of the penis. These erectile
tissues include two paired structures – the corpora cavernosa- located on the dorsal side
of the penis, and one unpaired corpus spongiosum on the ventral side. The urethra runs
through the center of the corpus spongiosum. The erectile tissue forms columns
extending the length of the penis, although the corpora cavernosa do not extend all the
way to the tip.
Erection is achieved as a result of parasympathetic nerve – induced vasodilation
of arterioles that allows blood to flow into the penis. As the erectile tissues become
engorged with blood and the penis becomes turgid, venous outflow of blood is partially
occluded, thus aiding erection. The emission refers to the movement of semen into the
31
urethra, and ejaculation refers to the forcible expulsion of semen from the urethra out of
the penis. Emission and ejaculation are stimulated by sympathetic nerves, which cause
peristaltic contractions of the tubular system, contractions of the seminal vesicles and
prostate, and contractions of muscles at the base of the penis. Sexual function in the male
thus requires the synergistic action (rather than antagonistic action) of the
parasympathetic and sympathetic systems.
Two portions of the central nervous system- the hypothalamus in the brain and the
sacral portion of the spinal cord control erection. Conscious sexual thoughts originating
in the cerebral cortex act via the hypothalamus to control the sacral region, which in turn
increases parasympathetic nerve activity to promote vasodilation and erection in the
penis. Conscious thought is not required for erection, however, because sensory
stimulation of the penis can more directly activate the sacral region of the spinal cord and
cause an erection.
Female reproductive organs
The two ovaries lie within the pelvis, towards the back and underneath the
kidneys. They are small, oval organs, each about the size of a bean, composed of
connective tissue and containing many thousands of immature ova or eggs. The oviduct
or Fallopian tube with its funnel-shaped opening commences beside each ovary and
passes into the uterus or womb, a pear-shaped muscular organ. The uterus is about 8 cm
long and opens at the base where there is a strong band of muscle called the cervix.
From this a narrow muscular tube, the vagina, passes outwards and opens between the
labia into the groin. The bladder, which lies to the front of the uterus, releases its urine
through the urethra, which opens close to the vagina.
Ovulation
Thousands of potential ova (egg cells) are present in the ovaries of a girl even
before birth. These lie dormant until puberty when, upon stimulation of the ovaries by
the pituitary hormone FSH, the ova begin to mature one at a time about once a month.
One by one, each ovum grows larger and a sheath of cells containing fluid rich in
nutrients develops around it. This Graafian or ovarian follicle, as it is called, grows
larger as it ripens and pushes the wall of the ovary outwards until it eventually bursts,
discharging the ovum into the funnel-shaped entrance to the Fallopian tube. The process
just described is ovulation. It occurs in the 2nd week after a period and is the time during
which fertilization is most likely to occur.
As the ovum travels along the Fallopian tube to the uterus, the follicle in the ovary
continues to grow, eventually forming the corpus luteum. This secretes the hormone
progesterone, which begins to prepare the body for pregnancy by thickening the uterine
lining and enlarging the breasts. If fertilization does not occur the corpus luteum shrivels
up and the progesterone level falls rapidly. If the ovum is fertilized the corpus luteum
enlarges, secreting even greater amounts of progesterone. This increases the blood
supply to the uterus, stimulates further thickening of the uterine wall and halts the
development of follicles in the ovaries.
32
Menstruation
The menstrual cycle is directly related to the hormonal influence on the uterine
lining. This lining is thin following menstruation. As the follicle ripens it releases
oestrogen, which causes thickening of the lining and an increase in the blood flow to the
uterus. After ovulation, the corpus luteum secretes progesterone, which causes further
thickening of the lining. When fertilization does not take place, production of
progesterone slows down and stops, the corpus luteum shrinks, the uterine lining
disintegrates and as a result of contractions of the uterus, its contents are expelled through
the vagina.
Menstruation – or a period as it is usually called – occurs about fourteen days
after ovulation. The cramps often experienced on the first or second day of a period are
due to uterine contraction; some people also experience pre-menstrual tension, which is
thought to be related to the withdrawal of progesterone.
Menstruation begins at puberty, usually between the ages of eleven and fifteen,
and continues every month (except during pregnancy) until menopause, when it becomes
more erratic and finally ceases altogether. During the menopause, which generally
occurs between the mid-forties and mid-fifties, some women experience unpleasant
symptoms (e.g. depression, hot flushes) because of the withdrawal of the female
hormones. In some cases these symptoms can be alleviated by the use of hormone
replacement therapy (HRT).
Fertilization
During sexual intercourse the blood vessels in the penis dilate and an extra supply
of blood fills the spaces, causing the penis to become erect. This facilitates its entry into
the vagina and upon further sexual stimulations the urethra contracts, causing ejaculation
of the semen. In the female the walls of the vagina relax during intercourse and secrete
lubricating mucus.
Each sperm consists of a “head” containing a nucleus and a long “tail”. The
strong, lashing movements of the tail enable the hundreds of millions of sperm released
to swim upward through the cervix into the Fallopian tubes. Many millions die before
they reach the tubes, but others survive and if an ovum is present it is likely that
fertilization will take place. Several sperm surround the ovum but only one penetrates its
protective covering. Soon afterwards the nucleus of the sperm enters the ovum and
unites with the nucleus of the ovum. This is the moment of fertilization.
Pregnancy
The fertilized ovum, now called a zygote, divides in two. Each cell then divides
again and thus the growth of the embryo begins. The cells multiply rapidly as the
embryo moves along the Fallopian tube until within a week it reaches the uterus. Here it
sinks into the thick uterine lining (endometrium) and sends small projections (villi) into
the lining to hold it in position and facilitate absorption of nutrients from its walls. The
area around the villi develops into the placenta, which is attached to the abdomen of the
foetus by the umbilical cord.
33
The placenta
This is concerned with providing the embryo with nutrients and oxygen from its
mother’s blood. It also receives waste products such as carbon dioxide and urea from the
blood of the embryo. Although the capillaries in the placenta closely connect the blood
of mother and embryo, they do not mix; rather the placenta acts as a filter through which
certain substances are allowed to pass and by which many harmful products are held
back. It does not prevent the passage of all dangerous substances, however: some drugs
and viruses such as German measles can pass through and may have unfortunate
consequences for the foetus. For this reason it is inadvisable to take any form of drugs,
even aspirin, or to smoke during pregnancy.
The placenta stimulates increased production of oestrogen and progesterone and
eventually begins to produce its own progesterone. These hormones are responsible for
the many changes which occur during pregnancy, preparing the body for birth and
lactation:
1. Enlargement and thickening of the uterus
2. Accumulation of fat and fluid
3. Inhibition of contraction of the uterus
4. Enlargement of the mammary glands.
The uterus grows larger to accommodate the growing embryo and the cells in the embryo
begin to form tissues. Within eight weeks the embryo, now called a foetus, has a
recognizable form with a head, trunk and limbs. The foetus is surrounded by a water sac
or amnion (containing amniotic fluid), which protects it from injury. It continues to
develop and by about six months all the organs are fully formed, but it remains in the
uterus until it is strong enough to survive independently. The whole period of pregnancy,
known as the gestation period, is about forty weeks.
Birth
By the time the baby is ready to be born it lies with its head down, facing the
cervix. Secretion of progesterone diminishes and, stimulated by the hormone oxytocin,
labor begins. The uterus begins to contract rhythmically and the contractions gradually
increase in strength and frequency. After some time the cervix dilates, the membrane of
the amniotic sac ruptures and the amniotic fluid flows out. In the final stages of labor,
vigorous contractions of the uterus and abdomen expel the baby out through the cervix
and vagina. The baby now begins to breathe on its own. The umbilical cord is tied and
severed and soon afterwards, as a result of further contractions of the uterus, the placenta,
now called the afterbirth, is expelled.
Within a couple of days the mammary glands, stimulated by the lactogenic
hormone from the pituitary gland, begin to secrete milk with which to suckle the baby.
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