Its function is to pump blood around the body. The whole heart is surrounded by the pericardium which has two layers between which is the pericardial fluid that reduce friction between them.
The heart is made of tissues called cardiac muscles which have the potential to contract rapidly.
It’s divided in to four chambers. The upper chambers are called atrium / auricle and the lower chambers are each called ventricle.

The heart is divided in to sections ie left and right by a muscular septum whose function is to prevent mixing of oxygenated and deoxygenated blood
Movement of blood in the heart is maintained in a single direction ie from the auricle to ventricle and then to blood vessels.
Blood flow in one direction in the heart is maintained by the presence of valves.
The auricles receive blood from all parts of the body while the ventricles pump blood to the body e.g. the left atrium receives oxygenated blood from the pulmonary vein and pump it to the left ventricle through the bicuspid valve.

The right atrium receives deoxygenated blood from the rest of the body from the vena cava and pumps it to the right ventricle via the tricuspid valve.
The ventricle walls are more muscular (have thicker walls) than those or the auricles because the auricle pump blood to shorter distance i.e. to the ventricle while the ventricles pump blood longer distances i.e. to body and lungs.
The walls of the left ventricle that pump blood in to the systemic circulation are thicker than those of the right ventricle which pump blood to pulmonary circulation.

Flow of blood through the heart
Blood flows in to the heart from the rest of the body via the vena cava to the right atrium which pumps it to the right ventricle via the tricuspid valve.

The right ventricle pumps blood to the pulmonary artery to the lungs and blood flows back to the left atrium via the pulmonary vein which pumps it to the left ventricle via the bicuspid valve and then finally pumped to the rest of the body via the aorta.


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This refers to the sequence of events by which the heart pumps and is refilled with blood. The cardiac cycle involves two phases:
Re-filling of the heart with blood
Pumping of blood

The pumping action of the heart consists of alternate contraction and relaxation of cardiac muscles in the walls of the heart. Contraction of cardiac muscles is called systole while relaxation is called diastole.
During diastole, the cardiac muscles in the walls of the atria relax and expand; blood from the vena cava and pulmonary vein enter the atria and becomes filled with blood. The walls of the ventricles relax and expand while those of the atria contract, forcing blood from the atria into ventricles via bicuspid and tricuspid valves as semilunar valves remain closed.


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During systole, cardiac muscles of the ventricles contract, forcing blood out of the heart via the semi lunar valves into the aorta and pulmonary artery. At this time, the atria relax and expand in order to be re-filled with blood. The cuspid valves close against high blood pressure to prevent the back flow of blood into the auricles. The closure of the valves produces the heart sound termed as lub.

After expelling blood, ventricles relax and their pressure lowers compared to aorta and pulmonary artery pressure.
This would cause back flow of blood to the heart but is prevented by sudden closure of the semi lunar valves. The closure of the semi lunar valves causes a second heart sound called dub.
The 2 sounds lub and dub are so close and often described as lub-dub and they form a single heartbeat.

Initiation and control of the heart beat
Contraction of the heart is initiated by heart, heart muscles / cardiac muscles themselves
Therefore the heart muscles are myogenic i.e. the rhythmic contraction a rise from within the tissue itself.
Heart beat is controlled by collection of cells in the right atrium called pacemakers located in the sino artrio node (SAN) which are controlled by nervous impulse from the medulla oblongata of the brain that change the rate of heart beat.

Factors affecting the heart beat rate
Lack of hormones in the body e.g. adrenaline
State of health and diseases e.g. malaria
Age i.e. its faster in infants than adults.
Body size i.e. it is faster in small organisms than large
Sex i.e. faster in female than in male.

NB: In normal adults at rest, heart contracts about 70 / 72 times per minute.

This is the force with which blood flows from one part of the body to another. The blood pressure is due to the pumping action of the heart as experienced by the blood vessels. The narrow blood vessels experience high blood pressure and wide vessels experience low blood pressure. Sometimes fats accumulate in the blood vessels making their rumens narrow. This increases blood pressure and it is the major cause of high blood pressure in fat people, however small people also experience high blood pressure. This is due to conditions like stress, anxiety, fear, etc. These conditions tend to increase the rate of heartbeat and more blood is pumped to the blood vessels causing high pressure in them.

Blood is a connective tissue made up of cells suspended in a fluid matrix called plasma.
There are two types of cells in blood i.e. White blood cells (leucocytes) and red blood cells (erythrocytes). The platelets (thrombocytes) are fragments of cells.
In an adult human being, there are five to six liters of blood with blood making up approximately 10% of the body weight.

Main components/nutrients of blood

  1. Red blood cells/erythrocytes/red corpusles
  2. White blood cells/eucocytes/white corpusles
  3. Platelets/thrombocytes
  4. Plasma
    General importance of blood in the bodies of animals
  1. It transports oxygen from the lungs to all parts of the body.
  2. It transports digested food from the ileum to other parts of the body for use.
  3. It transports Carbon dioxide from the tissues to the lungs.
  4. It transports nitrogenous wastes from the liver to the kidney where they are excreted.
  5. It transports hormones from their site of production to where they perform their functions.
  6. It distributes heat and aids in temperature control.
  7. It prevents infection by transportation of white blood cells.
  8. It regulates the amounts of chemicals such as glucose in the body.
  9. It prevents loss of fluids and cells through forming blood clots.

Characteristics of Red Blood Cells:

  • They have hemoglobin molecules which carry oxygen from the lungs to the tissues.
  • They lack nuclei
  • They have thin cell membranes which thinness reduces the diffusion distance for gases.
  • They are manufactured from the red bone marrow
  • On average, red blood cells last for four month after which they are destroyed by the liver to form bile pigment and the iron in haemoglobin is stored in the liver
  • They have a biconcave disk shape
  • They are approximately 5 million/mm3 of blood.

Importance of Red Blood Cells:
They transport oxygen from gaseous exchange surfaces to the tissues
They transport carbon dioxide from tissues to the gaseous exchange surfaces.

Adaptation of Red Blood Cells to carry out their function
They are biconcave in shape so as to avail a large surface area to volume ratio for absorption of oxygen.
They have hemoglobin molecules that bind to oxygen and transport it from the lungs to the tissues.
They have a thin membrane which reduces the diffusion distance for the respiratory gases in and out of the cells.
They lack nuclei which provides enough space for packaging of haemoglobin
They lack mitochondria and generate their ATP exclusively by anaerobic respiration to prevent them from using the oxygen they are carrying.
They have an enzyme, carbonic anhydrase which plays a role in carbon dioxide transport
They are numerous per mm3 to increase surface area for transportation of oxygen
They have flexible membranes which make them able to squeeze through capillary networks as they exchange materials they transport with the surrounding tissues.

NB: The concentration of red blood cells increases as one climbs up a mountain because the concentration of oxygen in the air reduces with increase in height above sea level. So the body adopts by producing more red cells to increase the available total surface area to bind and carry oxygen to the tissues regardless the reducing oxygen concentration main.

Structure of the red blood cell.

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Red blood cells are made from the red bone marrow of short bones in adults and in the foetus, red blood cells are made in the liver. They last for approximately four months after which they are taken to the liver or spleen for their destruction. They are more numerous than any other cells in the blood. Red blood cells are responsible for transporting oxygen in the body.

These are blood cells made from the white bone marrow of long bones. They are also made in the spleen and lymphatic system. They are responsible for defense of the body against infection. They are fewer in blood than the red blood cells.

Characteristics of white blood cells
i) They have no definite shape (they are amoeboid)
ii) They have a nucleus even at maturity.
iii) They are relatively few in blood but their number increases when the body is attached by an infection.
iv) They lack haemoglobin.
v) They feed on foreign particles by Phagocytes
White blood cells are divided into two major categories. These are;

  1. Phagocytes. These are white blood cells with a lobed nucleus. They ingest and destroy germs by phagocytes.
  2. Lymphocytes. These are white blood cells, which defend the body by producing antibodies.

Structure of a white blood cell

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Production of red and white blood cells
The red blood cells are manufactured form the red bone marrows in adults. Old red blood cells are taken to the liver for destruction.
White blood cells are manufactured from the white bone marrows of long bones. Some white blood cells are manufactured from the lymph nodes. Worn out white blood cells are also taken to the liver for destruction. In the foetus, the liver manufactures blood cells.

Action of white blood cells on the foreign particles
Some white blood cells attack and destroy the foreign particles directly by themselves. These are called phagocytes and they destroy the foreign particles by Phagocytosis. In this process the white blood cells form pseudopodia, which they use to engulf the foreign particle by Phagocytosis.
After engulfing the foreign particle, a food vacuole is formed into which digestive enzymes are produced. The enzymes break down the particle and the important materials are absorbed by the white blood cell while the wastes are excreted out of the cell through the contractile vacuole.

Illustration of Phagocytosis

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Some white blood cells destroy foreign particles by releasing antibodies, which destroy the particles. White blood cells, which produce antibodies, are called lymphocytes. There are four types of antibodies

1) Opsonins; these attach to the outer surface of the foreign particle and make it easier for phagocytic white blood cells to ingest them.
2) Agglutinins; these cause the foreign particles to stick together. In this condition the foreign particles cannot invade the tissues.
3) Lysins; these destroy bacteria by dissolving their outer coats.
4) Anti-toxins; these combine with and so neutralize the toxins produced by foreign particles.

These are blood cells formed as fragments in the bone marrows during the formation of red blood cells. They are responsible for blood clotting.

Characteristics of platelets

  1. They are cell fragments.
  2. They are spherical in shape.
  3. They do not have a nucleus.
  4. They do not have haemoglobin.

They play a role in blood clotting which protects the body against excessive loss of blood and entry of pathogens through the injured part. Blood clotting is the
process by which blood stops oozing out of a cut or wound. It is important because of the following reasons.

  1. It prevents excessive loss of blood from the body.
  2. It is a step towards healing of cuts and wounds.
  3. The blood clot creates a barrier to prevent entry of bacteria and other pathogens in the body.

The Process of Blood Clotting:
When blood is exposed to air as a result of a cut or wound, the platelets in the blood at the damaged tissue stimulate the release of a chemical called thromboplastin (thrombokinase). In the presence of calcium ions and vitamin K, thromboplastin stimulates the conversion of prothrombin to thrombin enzyme. Thrombin then catalyzes the conversion of soluble blood protein fibrinogen to the insoluble form fibrin. Fibrin forms fibers, which form a mesh and trap blood cells and proteins. This mesh dries to form a scab, which is called the blood clot.

Summary of blood clotting

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