THE BLOOD CIRCULATORY SYSTEM

By the end of this unit, students must be able to;

(a) Define blood circulation
(b) Identify blood cells
(c) Explain the role of the circulatory system in mammals
(d) Describe the structure and function of the heart
(e) Identify blood vessels
(f) Describe the role of the lymphatic system
(g) Describe the abnormal conditions of the circulatory system
The circulatory system refers to the movement of blood around the body. The general movement
of the blood occurs between the heart and the rest of the body.
The organs of the circulatory system are heart, blood and blood vessels.
IMPORTANCE OF THE CICULATORY SYSTEM
It is responsible for the transportation of food nutrients and oxygen between organs that are far
from each other.
It helps to remove metabolic waste products from deeper organs of the body and carry them to the
excretory organs such as kidneys, lungs.
It transports food molecules and oxygen to organs very close to the surface of the body.
FUNCTIONS OF THE BLOOD CIRCULATORY SYSTEM IN MAMMALS
 To transport oxygen gas to all parts of the body. Oxygen gas is carried from the lungs. It
diffuses from the alveoli to the red blood cells due to affinity exerted by the haemoglobin.
The oxygen gas attaches to haemoglobin and forms oxyhaemoglobin through which it is
then transported to all parts of the body.
 To transport carbon dioxide gas from all parts of the body to the lungs for excretion. In
blood plasma carbon dioxide exists in the form of carbonate ions, the dry carbon dioxide
is broken by the carbonic anhydrase enzyme. When being moved out carbonate ions attach
to haemoglobin and form carboxyhaemoglobin which is carried to the lungs for excretion.
 To transport absorbed food molecules from the small intestine to the liver and later to the
rest of the body.
  To transport hormones (chemical messengers) from the endocrine glands to the target
organs. The target organs are body parts where the hormones work.
 To distribute heat energy to all parts of the body. Heat energy is produced during respiration
in body cells and must be carried by blood around the body. Distribution of heat energy is
important as it helps to maintain body temperature which in effect maintains homeostasis
(maintaining a constant rate of metabolic processes).
 The circulatory system transports nitrogenous waste products such as urea, ammonia from
the liver to the kidneys for excretion.
 To defend the body against diseases. The white blood cells also called leucocytes provide
immunity to the body.
 To form a blood clot on the wound. The platelets also called thrombocytes form a fibrous
mesh network on the wound as a blood clot to prevent loss of blood from the body.
 To transport metabolic waste products from all parts of the body to the excretory organs to
be expelled out of the body.

BLOOD CELLS
RED BLOOD CELLS
The red blood cells are produced in the bone marrow of short bones such as sternum, ribs and
spines of the vertebral column.
They are produced from the combination of iron (Heme which means red) and a protein called
globin). This combination is completed by enzyme Ferrochelatase and forms a red pigment in red
cells called haemoglobin. This pigment attracts oxygen.
They are small in size but occur in large numbers, a small volume of blood may contain millions
of red blood cells.
They lose the nucleus upon maturing, this allows red cells to develop a biconcave or disc shape.
Their production in the bone marrow is activated by the enzyme Erythropoeitin from the kidneys.
This the reason red blood cells are also called erythrocytes, a term which means transporting cells.
They live for a 120 days or four months after which they are carried to the liver where they are
broken down to release biliverdin from which bilirubin is formed.
Red blood cells move out of the bone marrow through openings called bone pores through which
blood capillaries reach the inner bone.
They have a round shape unlike white blood cells.

FUNCTIONS OF RED BLOOD CELLS

1.To transport oxygen gas to all parts of the body.
2.To transport carbon dioxide from all parts of the body to the lungs for excretion.
ADAPTATIONS OF THE RED BLOOD CELLS FOR THEIR FUNCTIONS
They have haemoglobin which exerts affinity or attraction for oxygen.
They are numerous, this creates a wide surface for transporting oxygen.
They have disc or biconcave shape which increases the surface for transporting oxygen.
WHITE BLOOD CELLS
They are called Leucocytes which means fighting cells, this is because white blood cells fight
pathogens thereby providing immunity to the body.
They are produced in the bone marrow of long bones.
Unlike red blood cells white blood cells maintain the nucleus and have irregular shape.
They have a long life span as compared with the red blood cells.
Apart from the bone marrow some white blood cells are produced in the thymus and the spleen,
the spleen contains white pulp where lymphocytes are produced and activated. The spleen also
contributes to immunity by filtering blood to remove old red blood cells and pathogens. The
thymus is a small organ positioned in front of the heart. It produces T cells or T lymphocytes.
White blood cells from the bone marrow mature here.

TYPES OF WHITE BLOOD CELLS

PHAGOCYTES; These white blood cells physically engulf and digest pathogens. They have lobed
nucleus that adapts them to engulfing germs. This process is called phagocytosis. Common
phagocytes include monocytes, neutrophylls and macrophages.
LYMPHOCYTES; These are the white blood cells that fight germs by the use of antibodies, this
is a chemical fight. Antibodies are proteins that bind germs and suffocate them to death. The
antibodies also mark the pathogen so that killer T cells can easily detect and kill them. Common
lymphocytes include;
B-Plasma Cells; They produce antibodies which recognize and neutralize pathogens. They
promote the production of memory B cells which enhance long term immunity by during re-
exposure to the antigen of a particular disease.
Killer T cells also called cytotoxic T cells. They defend the body by destroying infected cells or
damaged cells or cancerous cells. They produce cytokines which activates other lymphocytes.
Helper T cells; They are also known as CD4cells. They make up the strongest part of immunity
by activating other lymphocytes such as cytotoxic, B cells and macrophages. They produce a
protein called cytokines which activates other lymphocytes.

PLATELETS
They are called thrombocytes which means clotting cells. This because their function is to form a
blood clot on the wound to reduce loss of blood from the body and entry of pathogens into the
body.
FORMATION OF A BLOOD CLOT
At the site of injury or wound platelets get exposed to air and they break and release chemical
substances which called thromboplastin or thrombokinase, which is protein in nature.
This protein converts an enzyme called prothrombin into thrombin. The enzyme thrombin converts
a plasma protein called Fibrinogen into fibrin, this occurs in the presence of vitamin K
(phylloquinone) and a mineral element called calcium.
The fibrin forms a fibrous mesh network on the wound as a blood clot.

THE HEART
The heart is made of a cardiac muscle. This is an involuntary muscle that works without influence
from the nervous system. The heart muscle does not get tired, take note that in humans the heart
starts functioning from five to six months of pregnancy.
The heart is myogenic in that it works on its own unlike skeletal muscles that work under control.
It is surrounded by a protective membrane called pericardial membrane. This membrane produces
a pericardial fluid that keeps the heart moist and free from friction.
CHAMBERS OF THE HEART
The heart has four chambers, two chambers are on the upper part and are called auricles or atria.
Their function is to receive blood. The left atrium receives oxygenated blood from the lungs
through a blood vessel called pulmonary vein. The right atrium receives deoxygenated blood from
all parts of the body through a blood vessel called vena cava.
The lower chambers are called ventricles and their function is to pump blood out of the heart.
The left ventricle pumps oxygenated blood to all parts of the body. It has a thick muscular wall in
order to withstand the pressure of pumping blood. The right ventricle pumps deoxygenated blood
to the lungs where the blood loses carbon dioxide and gains oxygen thereby becoming oxygenated.
The left and the right chambers of the heart are separated by a muscle tissue called septum. The
septum ensures that oxygenated blood does not mix with oxygenated blood.
VALVES IN THE HEART
The heart is composed of certain valves and the major ones occur between the atria or auricles and
the ventricles from which the term atrioventricular valves is derived.
1. Biscuspid valve; this valve is located between the left atrium and the left ventricle. Its
function is to prevent the backward flow movement of blood from the left ventricle back
to the left atrium.
2. Tricuspid valve; this valve is located between the right atrium and the right ventricle. Its
function is to prevent backward flow movement of the blood from the right ventricle to the
right auricle.
3. Semi- lunar valves; these valves are located in the arteries that are connected to the heart
such as in the pulmonary artery and in the aorta. These valves prevent the backward flowing
of the blood that has been pumped out of the heart going back into the heart. This ensures
that the blood that has been pumped out of the heart should make a continuous movement
along the arteries.
THE HEARTBEAT AND BLOOD PRESSURE
The heartbeat refers to the regular rhythmic contraction and relaxation of the heart muscle.
The number of heartbeats per minute is called pulse rate and in an adult at rest it is 72 beats per
minute. It comprises the heart cycle as it is made up of the systole which is the contraction of the
heart muscles particularly ventricles which pump blood out of the heart and a diastole which is the
relaxation of the heart muscles allowing the heart chambers to be filled with blood. This separates
the blood pressure into systolic pressure and diastolic pressure.
SYSTOLIC PRESSURE
1. The ventricles contract while auricles relax. This allows auricles to receive blood.
2. The volume of the ventricles decreases.
3. The bicuspid and tricuspid valves close.
4. The pressure of blood in ventricles increases.
5. Blood is finally pumped out of the heart to the lungs and to the rest of the body.
NB: At this point a tissue in the heart called atrioventricular node, a group of cells below the
auricles, causes a slight delay to the contraction of the ventricles. This allows blood to flow down
from the auricles to the ventricles and that ventricles generate adequate cardiac output. Poor
functioning of the atrioventricular node can result in either very high or very low pressure of blood.
Within the ventricles are purkinje fibres that coordinate the rapid and steady contraction of the
ventricles thereby pumping blood out of the heart.
The blood pressure of an adult at rest is 120/80 millimetres of mercury (mmHg) where 120 is the
systolic pressure while 80 is diastolic pressure.
DIASTOLIC PRESSURE
1. The ventricles relax while auricles contract. This allows blood to move down into the
ventricles.
2. The volume of the ventricles increases.
3. Bicuspid and tricuspid valves open.
4. Blood pressure in the ventricles decreases while in the auricles the pressure is high.
5. Blood moves from auricles down to the ventricles.
The heartbeat can be detected at pressure points in the body. The pressure points are body parts
where arteries can be pressed to stop the flow of blood and the beating action of the heart can be
felt, these parts include the neck, wrist joint and many other parts.
BLOOD VESSEL

The blood vessel is a path through which blood travels or moves.

TYPES OF BLOOD VESSELS
 A. ARTERIES; they carry blood out from the heart. The aorta carries oxygenated blood to all
the parts of the body. The pulmonary artery carries deoxygenated blood to the lungs.
CHARACTERISTICS OF ARTERIES
1. They have narrow lumen.
2. They are made of thick elastic muscular walls. The innermost muscular layer is called
tunica intima while the middle one is tunic media and finally their outermost layer is tunica
adventitia.
3. They carry blood with high pressure due to the pumpi
4. ng action from the heart.
5. They pulsate during the flow of blood. To pulsate is to contract and relax.
6. They do not contain valves as in the veins.
7. They carry oxygenated blood except the pulmonary artery which carries oxygenated blood
to the lungs.
8. They branch into arterioles as they move away from the heart.
B. VEINS; They carry blood back from all parts of the body to the heart. The vena cava, a
main vein, carries deoxygenated blood from all parts of the body to the heart. The
pulmonary vein carries oxygenated blood from the lungs back to the heart.
CHARACTERISTICS OF VEINS
1. They have a wide lumen.
2. They do not pulsate as blood flows along them.
3. They contain valves to prevent backward movement of blood.
4. They are made of thin inelastic walls.
5. They carry blood with low pressure due to absence of the heart`s pumping action.
6. They carry deoxygenated blood except the pulmonary vein which carries oxygenated blood
from the lungs back to the heart.
BLOOD CAPILLARIES
The blood capillaries refer to a complex network of very narrow, thin walled tubes that carry blood
to every cell of the body thereby allowing rapid transfer of substances between cells and the blood
and this is the reason they have one cell thick walls. They are the smallest blood vessels in the
body.
CHARACTERISTICS OF BLOOD CAPILLARIES
 a. Their walls are made of one cell thick endothelium.
b. Their walls are not elastic and allow a slow flow of blood and they cannot pulsate.
c. They transport blood between arterioles and venules hence they have no valves.
d. They form a dense network which gives the greatest total cross sectional area ideal for the
exchange of substances.
e. They are highly permeable to water, ions and small molecules, this allows the movement
of oxygen, carbondioxide, nutrients and waste products between the blood and tissues.
f. They facilitate the exchange of materials which are supplied by the arterioles and drained
by venules.

FUNCTIONS OF BLOOD CAPILLARIES

1. They facilitate the exchange of substances between the blood and the body tissues.
2. They maintain the balance of fluids by removing excess fluids from body tissues.
3. They help in the transportation of hormones to the target organs.
4. They help in regulation of body temperature by absorbing heat energy from body tissues
and delivering it to the environment.
THE LYMPHATIC SYSTEM
It refers to the flow movement of lymph along the lymphatic vessels.
Lymph refers to the blood plasma without red cells and large blood proteins filtered out of blood
vessels.
HOW LYMPH IS FORMED
Lymph is formed during the flow of blood to the far parts of the body where it reaches arterioles.
In the arterioles blood exerts a high pressure which causes filtration of small molecular substances
out of the blood vessels. These substances become tissue fluid. The tissue fluid contains substances
such as water, food nutrients, oxygen, vitamins and mineral salts.
Body cells obtain these raw materials from the tissue fluid in a process called assimilation.
The entry of raw materials into body cells increases a pressure gradient which forces metabolic
waste products to diffuse out of the cells to the tissue fluid and later into venules through which
they move to the excretory organs such as kidneys and lungs.
The remaining tissue fluid drains into the lymphatic vessels as lymph.

COMPONENTS OF THE LYMPHATIC SYSTEM

1. Thymus; it is an organ that is located in front of the heart and just behind the chest bone or
sternum. The spleen produces large numbers of T- lymphocytes which move out of the
thymus to various parts of the body where they fight germs. Through production of
lymphocytes thymus is considered a lymphatic organ.
2. Spleen; it is a banana shaped organ behind the stomach. It filters blood and initiates immune
responses. Splenic artery supplies oxygenated blood to the spleen while splenic vein
carries deoxygenated blood away.
3. Lymph nodes; they contain large numbers of lymphocytes that attack and kill germs that
are carried by lymph.
4. Tonsils; they are the organs of the lymphatic system that contribute to the immune system.
They help to filter bacteria, viruses that enter the body through the mouth and nose. They
produce lymphocytes and antibodies which fight infections. They help the body to
recognize and remember specific pathogens making it easy to respond to infections.
IMPORTANCE OF THE LYMPHATIC SYSTEM
1. It filters lymph and trap bacteria, viruses and other pathogens and expose them to immune
cells for destruction.
2. It collects interstitial fluid from body tissues and drain it back into the blood streams
thereby bringing about fluid balance.
 3. Lymphatic vessels such as lacteals absorb dietary fats and transport them into the blood
streams.
4. It removes waste products and cellular debris from body tissues.
DIFFERENCES BETWEEN RED BLOOD CELLS AND WHITE BLOOD CELLS
RED BLOOD CORPUSCLES WHITE BLOOD CORPUSCLES
They are produced in the bone of short bones They are produced in the bone marrow of long
bones, thymus and spleen
They are small in size They are large in size
They have round shape They have irregular shape
They are numerous in number They are few in number
They transport oxygen gas They fight diseases and provide immunity
They have disc shape or biconcave shape They have round irregular shape
They contain haemoglobin They do not contain haemoglobin
They lack nucleus They contain nucleus

DIFFERENCES BETWEEN LYPMHATIC SYSTEM AND THE BLOOD CICULATORY

SYSTEM
1. Blood contains red blood cells, white blood cells, platelets and plasma while lymph is a
clear fluid with few proteins.
2. Blood is pumped by the heart while lymph depends on muscular contractions.
3. Blood moves in vessels such as arteries, veins and capillaries while lymph moves in
lymphatic vessels.
4. Blood moves within closed blood vessels while lymph moves from body tissues towards
the heart.
5. Blood moves with a high pressure system while lymph moves in a low pressure system.
6. Blood transports necessary materials to body organs such as food, oxygen, hormones and
removes waste matters while lymph maintains balance of fluids, helps in immune response
and fats absorption.
PROBLEMS ASSOCIATED WITH THE HUMAN BLOOD CIRCULATORY SYSTEM
1. Heart attack; It is also known as myocardial infarction and is caused when blood flow to
the heart muscle is blocked. The heart muscle becomes deprived of nutrients and oxygen
so it stops functioning. The following are the common causes of heart attack;
a. Atherosclerosis; this is the accumulation of fats in the coronary arteries resulting in
narrowing them and difficulty of blood to move.
b. Blood clots that can block the coronary artery.
c. Coronary artery spasms that can cause temporary narrowing of coronary artery, this
reduces blood flow to the heart.
d. High blood pressure which can cause damage of the arteries to the heart, may increase the
risk of heart attack.
e. Risky conditions like diabetes, smocking may increase the risk of heart attack.
2. Cardiac arrest; this is when the heart suddenly stops beating causing blood to stop flowing
to important organs. It is caused by; poor balance of mineral salts, severe trauma due to
injury, drug overdose, hypoxia or lack of oxygen.
3. Heart failure; it is a condition in which the heart fails to pump adequate blood to body parts.
This denies the body of nutrients and oxygen. This problem is caused by; coronary artery
disease, hypertension, a cardial disease that can make the heart muscle become stiff, thick
or enlarged thereby reducing its ability to pump blood, diabetes, obesity, excessive intake
of alcohol.
4. Fainting; it is also called syncope and is temporary loss of consciousness caused by reduced
flow of blood to the brain. It can be caused by; severe heat, pain, stress, low blood pressure,
dehydration and medications.
5. Varicose veins; is a condition in which the valves in the veins are inflamed or swollen. The
valves become weak or damaged. This can be due aging, pregnancy, hormonal changes.
6. Angina pectoris; it is a sharp pain in the chest due to reduced blood flowing to the heart
muscle. The arteries supplying nutrients to the heart muscle become blocked due to fats.
This condition is characterized by nausea and dizziness, shortness of breath, fatique, chest
pains, pain of arms, shoulders, neck or stomach.
7. Arteriosclerosis; it is the thickening and hardening of the arteries due to deposition of fats
in the arteries.
8. Elephantiasis; it refers to the severe swelling of the legs, genitals and arms. It occurs when
lymphatic vessels are blocked by filarial worms from culex mosquito. This condition is
characterized by severe swelling and thickened or hardened skin.
9. High blood pressure; It refers to the increased pumping force exerted by the heart on the
blood. Hypertension is caused due to;
 High concentration of salt in the blood. Too much salt in blood makes blood become
viscous or thick, this makes it difficult for the heart to pump it hence increasing extra
pumping force on the blood.
 High level of fats along the arteries; too much fats along the arteries cause arteries to
become narrow and hard, this forces the heart muscle to generate an extra pumping
force.