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Blood and blood circulation
BLOOD
Blood consists of cells and liquid plasma which is 90% water and 10% dissolved nutrients, salts, proteins
and waste products. The nutrients are mainly the products of digestion, such as glucose, amino acids,
fatty acids and glycerol. Urea is the major waste product and comes from the de-amination of amino
acids in the liver. Other waste products include small quantities of ammonia and uric acid. The kidney
excretes these waste substances. Another important waste product is CO2, which is a by-product of the
internal respiration. Carbon dioxide dissolved in the blood plasma is carried from the cells to the lungs
where it leaves the body.
Among the proteins present in the plasma are immunoglobulins, which protect the body against
infection.
Other plasma proteins take part in blood clotting. Inorganic salts such as sodium (Na +), potassium (K+),
bicarbonate (HCO3-), phosphates (HPO4=), Sulphates (SO4 =) and many others are transported in the
plasma. Together with the protein components of the plasma, these salts help maintain the osmotic
concentration of the body constant and the pH of the blood at around 7.2.
Blood Cells
Blood cells are produced in the bone marrow and are of three types.
(a) The Red Blood Cells (RBCs)
Red blood cells are nonnucleate cells that contain haemoglobin, the compound that transports oxygen
in the body. Oxygen breaks down glucose in order to release energy.
The red cells, also known as erythrocytes, have a life span of approximately 4 weeks, after which they
are destroyed in the liver and spleen by another type of cell called the macrophage. Their destruction
releases haemoglobin, which is returned to the bone marrow to be used again.
People who have less than normal number of red blood cells suffer from a condition known as anaemia.
Anaemia may be caused by excessive loss of blood as in an accident, hookworm infection or malaria
infection, which destroys the red blood cells.
(b) The White Blood Cells (leukocytes)
Some white blood cells are called granulocytes because the have granules. These include neutrophils,
eosinophils and basophils. The granules contain chemical substances, including enzymes for digesting
bacteria. The other type consists of cells that do not have granules. These are called agranulocytes.
Among these are lymphocytes and monocytes.
Lymphocytes play a very important role in the protection of the body against infection by
microorganisms. Monocytes kill bacteria and other unwanted intruders by phagocytosis.
Another important type of a white blood cell is the platelet. This small, nonucleated agranular cell
tends to stick to damaged areas of the blood vessels where it stops bleeding. There are about 250,000
platelets per c.c. of blood.
Functions of the Blood
1. Transport of O2 from the lungs to the rest of the body and the transport of CO2 produced by the body
to the lungs.
2. Transportation of nutrients from the small intestine to the rest of the body
3. Transport of waste products to the kidneys
4. Distribution of heat throughout the body
5. Transport of hormones from the endocrine glands to the target organs
Blood Clotting
Following injury, chemical substances, including thromboplastin, are released by the platelets and the
cells of the damaged tissue. These chemical substances make the platelets become sticky and they
stick to the blood vessel at the site of injury. Then the plasma protein prothrombin is converted to
thrombin enzyme by the action of the thromboplastin in the pressure of calcium. Another plasma
protein, fibrinogen, is converted by prothrombin into fibrin, a compound that is made up of a mesh of
fibres that blocks the wound and stops bleeding.
Clotting can be prevented by the addition of anticoagulants, such as sodium oxalate and sodium
citrate, to the blood. These anticoagulants act by removing calcium from the blood. Another
anticoagulant, heparin, interferes with the conversion of prothrombin to thrombin. Blood will not clot
when prothrombin is absent. The saliva of blood sucking mosquitoes and leeches contains chemicals
that act as anticoagulants, thereby ensuring that blood does not clot.
Blood Groups
There are at least four blood groups used in hospitals for transfusion. These are A, B, AB and O. The
fifth is the Rh factor. Blood grouping is based on the type of antigens present in the surface of the red
blood cells. These antigens are A and B. Group A contains antigen A; group B contains antigen B and
group AB contains both antigens, that A and B. Group O has no antigens.
If the blood group is A, the blood plasma will have anti-B antibodies that can react with group B blood
and cause agglutination. What this means is that group A will not accept a donation of blood from
group B. Similarly, group B has anti-A antibodies and therefore will not accept a donation from A.
Group O blood does not have antigens but its plasma has both anti-A and anti-B antibodies. Group O
cannot accept a donation from A or B, but can donate to both of them. Group AB has both antigen A
and B but has no antibodies. Group can receive blood from all the other groups but can only donate to
another group AB. Group AB is described as a universal recipient, while group O is a universal donor.
The Rh Factor
The Rh factor is also an antigen found in red blood cells. A person with an Rh factor is positive and one
who does not have the factor is said to be negative. If an Rh-positive man marries a woman who is Rh
negative, their child will be Rh positive.
At the time of birth, some Rh-positive cells from the child may gain access to the mother’s blood
through broken blood vessels in the placenta. If this happens, the mother’s blood will develop anti-Rh
antibodies. During her second pregnancy her anti-Rh antibodies attack and destroy the baby’s red
blood cells because they have Rh-positive markers.
This problem can be avoided by giving an Rh-negative mother an injection containing Rh antibodies
immediately after giving birth to an Rh-positive child. The antibodies attack the child’s red blood cells
before they can stimulate a reaction in the mother.
Sickle-cell Anaemia
Sickle cell anaemia is due to a mutation in the genetic information that specifies the amino acid
sequence of one haemoglobin subunit. In this condition, one amino acid, glutamic acid, is replaced by
another amino acid, valine. Normal individuals have haemoglobin A (Hb A), while people with sickle
cell have HbS.
Individuals who inherit both HbA from their parents are normal, while those who inherit one Hb S gene
from one parent and one Hb A gene from the other parent are normal. They are, however, carriers and
capable passing the trait to their offspring. Only individuals who inherit both Hb S genes are sicklers.
Under conditions of reduced oxygen concentration, cells containing HbS change into elongated ‘sickle’
shaped because the protein crystallises into long fibres when it becomes deoxygenated. The sickle cells
clog small vessels and cut off oxygen supply to nearby tissues. Sickle cells are brittle and tend to be
destroyed more rapidly than normal red blood cells, leading to anaemia.
The symptoms include jaundice, fatigue, and shortness of breath and headaches. Crises can be
precipitated by infection, cold weather and many other causes, resulting in pain and damage to the
kidneys, lungs and intestines. Sicklers can be treated with supplements of folic acid.
Sickle cell anaemia is common in malarious areas, especially in Africa. Heterozygous carriers of the
sickle cell gene are usually more resistant to malaria than non-carriers.
Haemophilia
Haemophilia is a hereditary disease that is characterised by the failure of the blood to clot. In normal
conditions blood escaping from a wound should clot within minutes. However, in haemophilia blood
lacks one of the factors responsible for clotting. Consequently, people with haemophilia lose a lot of
blood whenever they get a cut.
Leukaemia
This is a cancer of the blood due to uncontrollable production of white blood cells many of them
immature. Because these cancerous cells are non-functional, the body’s defence to infection is
weakened. In addition, the cancerous cells multiply faster than the normal cells and use up most of the
nutrients and oxygen needed by the normal cells, leading to weight loss and fatigue.
BLOOD CIRCULATION
Microscopic organisms, such as Amoeba and Paramecium, are too small to require special circulatory
systems. Instead, nutrients and waste products diffuse in and out of their bodies freely across the
surface. However, diffusion alone would be totally inadequate where large animals are concerned.
Therefore, large animals are provided with circulatory vessels and pumps .
Open Circulatory System
An open circulatory system is one in which blood vessels open into the body cavity of the organism. In
insects, for example, the dorsal vessels carrying fluid towards the head open into the body cavity or
hemocoel.
After the fluid has bathed the tissues, it drains into a ventral vessel where it flows in the opposite
direction. The open circulatory system is present in molluscs and arthropods. These animals are said to
have incomplete circulatory systems.
Closed Circulatory System
In a closed circulatory system, the blood is confined inside vessels throughout. This form of circulation
is characteristic of earthworms and vertebrates. The system consists of a pump or a heart, and vessels,
which carry the blood to or away from the heart.
The earthworm’s circulatory system consists of dorsal and ventral blood vessels and five pairs of hearts
and branches, which form a capillary network. Unlike the insect’s blood, which has no colour, due to
the absence of haemoglobin, the earthworm’s blood has haemoglobin and is therefore red in colour.
Circulation in Vertebrates
(a) Arteries
Arteries are blood vessels that carry blood away from the heart and have thick tough walls that can
withstand the pressure of the blood. The blood that flows in the arteries is under pressure and because
of this, the arteries have thick, elastic walls that can expand and contract. The arterial wall is made up
of three layers. The outer layer is a connective tissue of elastic fibres; the middle layer is a smooth
muscle with more elastic fibres. The lumen of the vessel is lined by an endothelium which, in most
cases, consists of a single layer of cells.
The biggest blood vessel in the body, the aorta, is an artery that carries blood from the left auricle.
The pressure exerted by the blood is so high that the walls of the walls would burst if they were not
very thick.
(b) The Veins
Veins carry blood that is not under much pressure back to the heart. Therefore, their walls are thinner
and less elastic than those of the arteries are. They also have, on average, a bigger lumen than that of
the artery. Since the blood in the veins is not under much pressure, the veins are provided with valves
that ensure that the blood flows in one direction.
Circulation in the Fish
Fish have a two-chambered heart, made up of an atrium and a ventricle; an enlarged sinus venous that
opens into the atrium and conus arteriosus that takes blood away from the heart. When the ventricle
contracts, the blood flows to the gills where it picks up O2 and gives out CO2. The oxygenated blood
then flows to the rest of the body. Blood returning to the atrium is deoxygenated.
Circulation in Amphibians and Reptiles
Amphibians and reptiles have a three chambered heart, made up of two atria and a common ventricle.
When the ventricle contracts, some blood enters the lungs while some flows to the rest of the body.
This is a two-circuit pathway. This type of circulation is an improvement over that of the fish although
the amount of O2 carried is low because there is some mixing of deoxygenated and oxygenated blood.
Circulation in Birds and Mammals
The hearts of mammals and birds have four chambers consisting of two atria and two ventricles. There
is complete separation of the right part of the heart from the left so that there is no mixing of the
oxygenated and deoxygenated blood. The right side of the heart handles deoxygenated blood while the
left side deals with oxygenated blood. The pulmonary and systemic circulation, unlike those of other
vertebrates, is separate.
In a four chambered heart de-oxygenated blood from the rest of the body returns to the right atrium
via the anterior vena cava vein, also known as the superior vena cava, and posterior (inferior) vena
cava. Blood from the head, arms and chest regions returns to the heart via the anterior vena cava
while blood from the lower parts of the body returns by way of the posterior vena cava.
After receiving de-oxygenated blood, the right atrium contracts forcing blood into the right ventricle.
Contraction of the right ventricle sends blood to the lungs through the pulmonary artery. The blood
then picks up O2 in the lungs and gives up CO2. Oxygenated blood returns to the heart via the pulmonary
vein and enters the left atrium. From the left atrium, blood enters the left ventricle. The contraction
of the ventricle sends blood into the aorta.
The aorta is the largest blood vessel in the body, with very strong walls that are capable of resisting
high pressures exerted by the blood. The aorta distributes blood to the rest of the body, including the
heart muscles.
The figure above shows the internal structure of the heart. As you can see, there are four valves: two
arterioventricular valves, one pulmonary valve and one aortic valve. The left artioventricular valve
(tricuspid valve), and the right arterioventricular (bicuspid valve), opens when the ventricles relax.
After the ventricles fill with blood, they contract and both the tricuspid and bicuspid valves close.
Then the pulmonary and the aortic values open, and the blood flows to the lungs and the rest of the
body. The role of the valves is to prevent blood from flowing back into the heart and makes it flow in
one direction.
Once the blood is in the aorta it is distributed to the rest of the body through arteries which branch
into smaller arterioles and finally into tiny capillaries. Capillaries have thin walls that allow gases and
other waste products like urea to be exchanged easily between the blood and the tissues.
From the capillaries blood drains into small veins called venules. The speed of blood flow in the
venules is very slow. The blood in the venules contains mostly CO2 because much of the O2 it originally
carried from the lungs has been lost to the tissues. The venules empty into larger veins that carry blood
back to the heart.
By the time the blood reaches the veins, the hydrostatic pressure is so low that to prevent it from
flowing backwards the veins are provided with valves.
When the valves are damaged, the blood flows backwards and accumulates in the veins, especially of
the lower limbs, as vericose veins. Vericose veins are veins that are swollen with fluid.
The Human Heart
The human heart is a small organ averaging 300 gm in weight. The heart beats non-stop throughout a
person’s life. The brain cells need constant supply of glucose and oxygen and were the heart to stop,
for even a few minutes, the brain would be deprived of O2 and glucose and get damaged.
The heart muscle is described as myogenic because it contracts automatically without a stimulus. Even
when it is removed from the body, the heart will continue to beat for sometime before stopping. This
is especially so in a cold blooded animal, like the frog, which is not so sensitive to temperature.
The heartbeat is controlled by the autonomous nervous system. The sympathetic nerve speeds up the
rate of the heart, while the parasympathetic nerve slows it.
Heart contractions start at the pacemaker or sinoatrial node, which is a region in the wall of the right
atrium. This is situated near the point at which the anterior vena cava enters the heart.
When the pacemaker contracts, a wave of contractions spreads out until both atria are beating. On
reaching the atrioventricular node, a second node located at a point separating the atria from
ventricles, there is a momentary pause in contraction before the ventricles begin beating. The
momentary pause lasts for something like 0.1 of a second and allows the atria to drain before the
ventricles contract.
Heart contractions involve electric currents, which can be detected on the surface of the chest using
an instrument called an electrocardiogram (EKG).
Cardiovascular Diseases
Contraction of the atria is called diastole and that of the ventricles is systole. Human blood pressure is
measure by a sphygmomanometer in millilitres of mercury. A systolic reading of 120 and a diastolic
value of 80, usually taken on the brachial artery of the upper arm, are considered normal for most
persons
Diseases of the heart and blood vessels constitute cardiovascular diseases. One of these is
atherosclerosis, a chronic disease caused by the narrowing of arteries by deposits of fat and
cholesterol.
If, in addition to fat and cholesterol, calcium is also deposited on the arterial vessels, the walls harden
and become inelastic, resulting in a disease called arteriosclerosis. Blood clots or detached deposits or
plaques from the walls can easily block the narrowed blood vessels.
Moreover, the deposits make the inner walls of the blood vessels to become rough. This attracts the
adhesion of platelets to such walls and may thus trigger blood clotting.
High blood pressure or hypertension can promote atherosclerosis and the risk of heart attacks and
strokes. Smoking, diet rich in fats and cholesterol and lack of exercise increase the risks of
cardiovascular disease in a person.
A heart attack occurs when the coronary vessels supplying the heart muscle are damaged or blocked by
clots so that the heart is starved of oxygen and nutrients. On the other hand, a stroke is due to the
blockage of an important blood vessel supplying the brain, depriving the brain of glucose
THE LYMPHATIC SYSTEM
Some of the blood plasma leaks from the capillaries into the surrounding tissue. This tissue fluid which
contains fats, leukocytes and salts enters the lymphatic capillaries which are always close to the blood
capillaries throughout the body. Once this tissue fluid is in the lymphatic capillaries, it becomes lymph.
The lymphatic capillaries join together to form lymphatic veins which is turn join up with other
lymphatic veins from other parts of the body to form two large lymph ducts the right lymphatic duct
and the thoracic duct. These two drain into the subclavian veins in the neck region.
The lymphatic vessels are made up of thin walls, consisting of a single layer of cells and there are gaps
between the cells, which allow plasma and white blood cells to pass through. The flow of the lymph is
not under pressure but relies on the pressure due to the movement of the muscles surrounding the
vessels and the presence of the valves that ensure that the lymph flows in one direction.
The lymphatic system consists of lymph nodes. The lymph nodes are small round structures that are
composed of white blood cells that destroy bacteria and other harmful substances present in the
lymph. Usually the toxic substances released by the invading bacteria inflame the lymph node cells,
which then become swollen and painful.
Elephantiasis is common disease in the tropics that affect the lymphatic system, especially of the lower
limbs and the male genitalia. It is caused by a filarial worm called Wuchereria bancrofti that lives
coiled inside lymph vessels. If they are many, they can block the flow of lymph leading to the
accumulation of fluid in the limbs including the genital region. These structures become swollen with
fluid, making them look ugly and grotesque.
Functions of the lymphatic system
1 1.By returning “leaked” plasma back to the blood system, the lymphatic system ensures the
maintenance of a constant osmotic concentration of the body.
2. 2.The tissue fluid transports nutrients to the cells and waste products back to the blood.
3. 3.In addition, the tissue fluid acts as a medium for the exchange of O2 and CO2 between blood
vessels and the cells.
4. 4.The lymphatic nodes are the home of white blood cells that purify the body by getting rid of
harmful bacteria and their debris by phagocytosis.
5.The lymphatic system contains lymphocytes that are central in the immune response.