TRANSPORT IN ANIMALS

The Human Circulatory System

The human circulatory system consists of (i) the heart, (ii) blood vessels, (iii) blood and (iv)
valves.

Blood is the transport medium i.e. it carries substances around the body.

Blood vessels are the passage of blood.

The heart pumps blood i.e. it creates the force which moves blood in blood vessels.

Valves are crucial for maintaining pressure in the circulatory system. They also prevent
backflow of blood for example from arteries back into the heart and in veins where blood
pressure is low.

The human circulatory system is a double circulatory system. This means blood passes
twice through the heart in one complete circuit.
The circulation through the lungs is called the pulmonary circulation; the circulation around
the rest of the body is called the systemic circulation.

Advantages of the double circulatory system

1. Ensures blood in the systemic circulation flows under high pressure.

2. Ensures blood in the pulmonary circulation has the correct pressure to prevent
bursting of capillaries surrounding alveoli due to high pressure.
3. Separates oxygenated blood from deoxygenated blood thus ensuring fast delivery of
enough oxygen to respiring tissues.
Functions of the circulatory system

The circulatory system also plays a very important role in immunity.

The Heart
Function of heart parts

Vena cava: brings deoxygenated blood from the body and pours it into the right atrium.

Right atrium: pumps blood into the right ventricle.

Right ventricle: pumps blood to the lungs.

Tricuspid valve: prevents backflow of blood from right ventricle into right atrium.

Pulmonary artery: carries deoxygenated blood from the right ventricle to the lungs.

Pulmonary vein: brings oxygenated blood from the lungs and pours it into the left atrium.

Left atrium: pumps blood into the left ventricle.

Left ventricle: pumps blood to the body.

Bicuspid valve: prevents backflow of blood from left ventricle into left atrium.

Aorta: carries oxygenated blood from the left ventricle to the body.

Semi-lunar valves: prevents blood in the pulmonary artery and aorta from flowing back into
their respective ventricles.

Muscle – Cardiac muscle: it makes up the walls of the heart and contracts to create the force
which moves blood along blood vessels.

Coronary artery: carries oxygenated blood into cardiac muscles.

The Cardiac cycle

Blood vessels

There are 3 types of blood vessels: artery, veins and capillary.

Arteries carry oxygenated blood away from the heart. However there is an exception, the
pulmonary artery which carries deoxygenated blood.

Artery branch into arterioles which then branch into capillaries.

Veins carry deoxygenated blood to the heart. However there are 2 exceptions, the pulmonary
vein which carries oxygenated blood and the hepatic portal vein which does not carry blood
to the heart but carries it from the digestive system to the liver.

Capillaries are the site of exchange of materials between blood and cells of the body.
Capillaries allow substances to exit and enter blood because their wall are one cell thick.

Capillaries unite to form venules which will unite to form veins.

Feature Artery Vein Capillary
elastic fibrous layer present, thicker present, thinner absent
muscle layer present, thicker present, thinner absent
lumen present, small lumen present, large lumen present, large lumen
relative to overall relative to overall relative to overall
diameter diameter diameter
red blood cells move
in a single file along
capillaries since the
lumen is large
enough to
accommodate the
width of a red blood
cell
valves absent present absent
blood oxygenated deoxygenated mixture of
oxygenated and
deoxygenated
blood pressure high low high
permeability no no yes, allows
substances to exit
and enter

Formation of tissue fluid

Tissue fluid is a liquid which surrounds all body cells.

Tissue fluid is similar in composition to blood plasma – the liquid component of blood.

When blood moves from arterioles into capillaries the lumen diameter decreases resulting in
an increase in blood pressure in the capillaries. This high blood pressure forces plasma out of
the capillaries (plasma = water, glucose, salts, amino acids, lipids, hormones, vitamins), these
substances now called tissue fluid, move into the spaces which surround body cells. The
substances move out of capillaries by diffusion.

Cells will then take whatever they need from the tissue fluid.
Cells will release waste products into the tissue fluid and the wastes will move into capillaries
or lymphatic vessels.

Blood

Coronary heart disease

In the lining of the large and medium arteries, deposits of a fatty substance, called atheroma,
are laid down in patches. This happens to everyone and the patches get more numerous and
extensive with age, but until one of them actually blocks an important artery the effects are
not noticed. It is not known how or why the deposits form.

Some doctors think that fatty substances in the blood pass into the lining. Others believe that
small blood clots form on damaged areas of the lining and are covered over by the atheroma
patches. The patches may join up to form a continuous layer, which reduces the internal
diameter of the vessel.

The surface of a patch of atheroma sometimes becomes rough and causes fibrinogen in the
plasma to deposit fibrin on it, causing a blood clot (a thrombus) to form. If the blood clot
blocks the coronary artery, which supplies the muscles of the ventricles with blood, it starves
the muscles of oxygenated blood and the heart may stop beating. This is a severe heart attack
from coronary thrombosis. A thrombus might form anywhere in the arterial system, but its
effects in the coronary artery and in parts of the brain (strokes) are the most drastic.

In the early stages of coronary heart disease, the atheroma may partially block the coronary
artery and reduce the blood supply to the heart. This can lead to angina, i.e. a pain in the chest
that occurs during exercise or exertion. This is a warning to the person that he or she is at risk
and should take precautions to avoid a heart attack.

Possible causes of coronary heart disease

Atheroma and thrombus formation are the immediate causes of a heart attack but the long-
term causes that give rise to these conditions are not well understood.

There is an inherited tendency towards the disease but incidences of the disease have
increased very significantly in affluent countries in recent years. This makes us think that
some features of ‘Western’ diets or lifestyles might be causing it.

The main risk factors are thought to be an unbalanced diet with too much fat, stress,
smoking, genetic disposition, age, gender and lack of exercise.
Diet

The atheroma deposits contain cholesterol, which is present, combined with lipids and
proteins, in the blood. Cholesterol plays an essential part in our physiology, but it is known
that people with high levels of blood cholesterol are more likely to suffer from heart attacks
than people with low cholesterol levels.

Blood cholesterol can be influenced, to some extent, by the amount and type of fat in the diet.
Many doctors and dieticians believe that animal fats (milk, cream, butter, cheese, egg-yolk,
fatty meat) are more likely to raise the blood cholesterol than are the vegetable oils, which
contain a high proportion of unsaturated fatty acids.

An unbalanced diet with too many calories can lead to obesity. Being overweight puts extra
strain on the heart and makes it more difficult for the person to exercise.

Stress

Emotional stress often leads to raised blood pressure. High blood pressure may increase the
rate at which atheroma are formed in the arteries.

Smoking

Statistical studies suggest that smokers are two to three times more likely to die from a heart
attack than are non-smokers of a similar age. The carbon monoxide and other chemicals in
cigarette smoke may damage the lining of the arteries, allowing atheroma to form, but there is
very little direct evidence for this.

Genetic predisposition

Coronary heart disease appears to be passed from one generation to the next in some families.
This is not something we have any control over, but we can be aware of this risk and reduce
some of the other risk factors to compensate.

Age and gender

As we get older our risk of suffering from coronary heart disease increases. Males are more at
risk of a heart attack than females: it may be that males tend to have less healthy lifestyles
than females.

Lack of exercise

Heart muscle loses its tone and becomes less efficient at pumping blood when exercise is not
untaken. A sluggish blood flow, resulting from lack of exercise, may allow atheroma to form
in the arterial lining but, once again, the direct evidence for this is slim.

Prevention of coronary heart disease

Maintaining a healthy, balanced diet will result in less chance of a person becoming obese.
There will also be a low intake of saturated fats, so the chances of atheroma and thrombus
formation are reduced.

There is some evidence that regular, vigorous exercise reduces the chances of a heart attack.
This may be because it increases muscle tone – not only of skeletal muscle, but also of
cardiac muscle. Good heart muscle tone leads to an improved coronary blood flow and the
heart requires less effort to keep pumping.
Treatment of coronary heart disease

The simplest treatment for a patient who suffers from coronary heart disease is to be given a
regular dose of aspirin (salicylic acid). Aspirin prevents the formation of blood clots in the
arteries, which can lead to a heart attack. It has been found that long-term use of low-dose
aspirin also reduces the risk of coronary heart disease.

Methods of removing or treating atheroma and thrombus formations include the use of
angioplasty, a stent and, in the most severe cases, by-pass surgery.

Physical activity and heart rate

During periods of physical activity, active parts of the body (mainly skeletal muscle) respire
faster, demanding more oxygen and glucose. Increased respiration also produces more carbon
dioxide, which needs to be removed. Blood carries the oxygen and glucose, so the heart rate
needs to increase to satisfy demand. If the muscle does not get enough oxygen, it will start to
respire anaerobically, producing lactic acid (lactate). Lactic acid build-up causes muscle
fatigue, leading to cramp. An ‘oxygen debt’ is created, which needs to be repaid after
exercise by continued rapid breathing and higher than normal heart rate.