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A course on composition and structure of the blood


A course on composition and structure of the blood

Author : Kingsley Mayor Omenaiko

The blood is a tissue in a fluid form. An adult man has about 5-6 litres of blood. Blood is made up of two major components:

Blood components

1) The blood cells or corpuscles which are solid

2) The plasma which is liquid

Blood cells or corpuscles

There are three types of blood cells or corpuscles

  • Reb blood cells (Erythrocytes)
  • White blood cells (Leucocytes)
  • Blood platelets (Thrombocytes)
Red blood cells (Erythrocytes)

The red blood cells also called erythrocytes are small, round and biconcave or disc like in shape. They have no nucleus. One cubic litre of blood has about 5.5 million red blood cells.

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Their normal life span is about 120 days ( 4 Months ) before they are destroyed by the liver. They red blood cells are mainly produced by the bone marrow.

They red colour of the cells is due to the presence of iron compound called Haemoglobin.

Functions of the Red blood cells

The pigment, haemoglobin in the red blood cells helps to transport oxygen from the lungs to the body cells. Haemoglobin combines readily with oxygen to form Oxyhaemoglobin in the lungs.

White blood cells (Leucocytes)

The white blood cells which are also called leucocytes are irregular and amoeboid in shape. They are large, colourless and contain nucleus.

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They are larger than the red blood cells in diameter and fewer in number than the red blood cells.

About 5,000 to 10,000 of them exists in a cubic millimeter of blood. They are made in the red bone marrow, the lymph nodes or the spleen. They can live for many months.

Types of white blood cells

There are two types of white blood cells:

1) Phagocytes

The Phagocytes are found in the lymphatic system where they ingest bacteria, viruses and dead cells and help in preventing the diseases.

The ingestion of materials is called phagocytosis , and hence such white corpuscles are called Phagocytes.

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2) Lymphocytes

The lymphocytes produce antibodies and are made in the lymph glands. They produce chemicals called antibodies which stick to the surface of germs and kill them.

Function of the white blood cells

The white blood cell helps to defend the body against diseases by ingesting the bacteria and virus that cause diseases.

Blood platelets (Thrombocytes)

The blood platelets also called thrombocytes are tiny irregular cell fragments without nucleus. They are produced in the red bone marrow. Platelets are fewer in number and smaller in size than the red blood cells.

About 250,000-400,000 per mm3 of human blood contains platelets.

Function of the blood platelets

Platelets help in the clotting of blood.


Plasma is the liquid component of the blood. It is a pale yellow liquid made up mainly of water ( about 90% ). Many substances are dissolved in it including plasma, proteins, antibodies, hormones, enzymes, gases, digested food, salts and water and other waste products.

Function of the blood plasma

It helps to transport the substances that are dissolved in it as well as the cells that float in it.


The lymph is a colourless liquid associated with the lymphatic system. It is a fluid similar in composition to the tissue fluid but contain extra lymphocytes.

It has no red cells. The lymph returns fluid to the main vein through openings in the subclavian ( left jugular ) vein below the neck. Movement of the lymph is enhanced by the action of the muscles.

Lymph moves through the lymph vessels, e.g lacteal. Some swellings do exists in groups along the lymph vessels especially in the neck, groin and armpit called lymph nodes.

They lymph nodes are where lymph passes through to be made pure before entering into the blood stream. The lymphatic system is second to the circulatory system, but unlike the circulatory system it ends blindly.

Functions of the lymph
1) Body defence

The lymph aids body defence. The lymph node produces white blood cells. Disease causing micro-organisms , foreign particles, etc found in the lymph are filtered out in the lymph nodes and engulfed by phagocytes.

2) Absorption of fatty acids and glycerol

The lymphatic system such as lacteal is associated with the absorption of fatty acids and glycerol.

Functions of the Blood

The mammalian blood performs a number of functions which includes :

1) Transport of Oxygen

The blood transports oxygen with the aid of a pigment in the red blood cells called haemoglobin.

2) Temperature regulation

Heat produced in the liver and the muscles are distributed through out the body hereby keeping the body temperature almost uniform.

3) Transportation of digested food

Digested food substances like amino acids, glucose, fatty acids and glycerol are transported from the villli of the small intestine through the blood to all the cells and tissues within the body either for use or storage.

4) Transportation of excretory products

Excretory products such as Carbon dioxide, water, urea, etc are transported by blood from the cells that produce them to the various excretory organs like lungs, skin, liver, and kidney for elimination.

5) Transportation of hormones

Hormones are transported through the blood from the area of production to the target organs where they act.

6) Defence against infection

The white blood cells transported by the blood are able to defend the body against infection by microbes or pathogens.

Body Defence functions of the blood

The blood is able to defend the body against disease causing organisms such as bacteria, virus, etc in four major ways :

1) Clumping

In this process, the lymphocytes, a type of white blood cells produce chemical substances or antibodies which react with the surface of bacteria or the antigen which cause the bacteria or antigen to clump with antibody.

2) Neutralization

In this process, the white blood cells also produce antitoxins which neutralize the toxins produced by pathogenic agents thereby making them harmless.

3) Engulfing

Phagocytes, another type of white blood cells engulf the disease causing organisms or pathogens and digest them.

4) Clotting

The clotting of blood through the aid of the blood platelets prevents germs or disease causing organisms from entering the body.

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