Structure of the blood

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:

  • the blood cells or corpuscles which are solid.
  • the plasma which is liquid.


Blood Cells or Corpuscles

There are three types of blood cells or corpuscles. These are:

  • Red 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 ½ million red blood cells. Their normal life span is about 120 days (4 months) before they are destroyed by the liver. The red blood cells are mainly produced by the bone marrow. The red colour of the cells is due to the presence of iron compound called haemoglobin.


Adaptation of Red Blood Cells in Performing its functions

  1. Possession of Biconcave Shape: The possession of biconcave shape or disc- like shape increases its surface area for oxygen uptake.
  2. Possession of Elastic Membrane: This feature enables the Red blood cell to flow easily through the blood vessels.
  3. Presence of Haemoglobin: This feature enables it to take up oxygen.
  4. Presence of thin membrane: Thin membrane is needed for easy diffusion of oxygen or carbon dioxide.



  1. 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.
  2. They transport small quantities of carbon dioxide.
  3. They take part in blood clotting.


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. They are larger than the red blood cells in diameter and fewer in number than the red blood cells, i.e ., about 5,000 – 10,000 of them exist in a cubic mililitre of blood. They are made in the red bone marrow, the lymph nodes or the spleen. They can live for many months.

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



  1. The white blood cells help to defend the body against diseases by ingesting the bacteria and virus that cause diseases.
  2. They produce antibodies or immunity for the body.


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, i.e ., 250,000 – 400,000 per mm³ of human blood.

Function: Platelets aid 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% water). Many substances are dissolved in it including plasma proteins, antibodies, hormones, enzymes, gases, digested food, salts and other waste products.



  1. Plasma transport the end products of digestion or digested food especially glucose and amino acids.
  2. It transports waste products, hormones, heat from very actively respiring tissues, carbon dioxide, urea or waste materials, antibodies, amino acids and mineral salts.
  3. It participates in blood clotting.
  4. It transports oxygen.



The lymph is a colourless liquid associated with the lymphatic system. It is a fluid, similar in composition to the tissue fluid but contains 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 muscles. Lymph moves through lymph vessels, e.g. lacteal. Some swellings do exist in groups along the lymph vessels especially in the neck, groin and armpit called lymph nodes. The 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 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 the lacteal is associated with the absorption of fatty acids and glycerol.


Functions of Blood

The mammalian blood performs a number of functions which include:

  1. Transport of oxygen: The blood transports oxygen with the aid of a pigment in the red blood cells called haemoglobin. The oxygen is combined with the haemoglobin to form oxyhaemoglobin. When haemoglobin is dull red, oxyhaemoglobin is said to be oxygenated and it is dark red. As oxygenated blood circulates round the body and reaches the cells and tissues, oxyhaemoglobin dissociates, releases oxygen and reverts to haemoglobin which is carried back to the lung to be re-oxygenated.
  2. Temperature regulation: Heat produced in the liver and the muscles are distributed throughout the body thereby 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 villi of 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 and urea 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.
  7. Production of antibodies: Antibodies are produced by white blood cells and are transported by the blood round the body. They help in the defence of the body by destroying pathogenic organisms or their harmful products.
  8. Blood clotting: The platelets in the blood are able to initiate the process of blood clotting when an injury is sustained.
  9. Transport of water: Water which forms about 90% of the blood is transported by the blood to the various cells for various metabolic activities. The level of water in the body is always maintained by the blood.
  10. Transportation of mineral salts: Mineral salts such as sodium are transported from one part of the body to another through the blood.


Body Defence Functions of the Blood

The blood is able to defend the body against disease-causing organisms such as bacteria and viruses 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. Neutralisation: In this process, the white blood cells also produce antitoxins which neutralise 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 prevent germs or diseases causing organisms from entering the body.


Process of Blood Clotting

One of the methods by which the blood defends the body is by clotting at the site of a wound, thus preventing further loss of blood and the entry of pathogenic organisms at the point of the wound or injury. When the body sustains an injury in which a blood vessel is cut, the process of clotting takes place as follows:

  1. Platelets plugs are formed due to exposure of damaged blood vessels to air.
  2. The platelets release a chemical which causes involuntary muscles in the walls of the damaged blood vessels to constrict and so reducing blood flow.
  3. In case the damage is so great to be sealed by a plug, the platelets release an enzyme called thrombokinase or thromboplastin.
  4. The thrombokinase converts an inactive prothrombin into an active enzyme called thrombin.
  5. The thrombin in the presence of calcium salts (Ca++ions) converts fibrinogen into fibrin.
  6. The fibrin forms a mesh of fine threads which traps blood cells forming a clot that covers the wound.
  7. The mesh of fibrin blocks the cut in the blood vessel and prevents further bleeding while the white blood cells entangled in the fibrin mesh prevent the entry of pathogenic organisms like bacteria, virus and fungi into the body through the wound or injury.

The clot dries to form a scab which drops off after a new skin tissue is formed in the injured area. Some people possess blood which is unable to clot. These people suffer from a disease called haemophilia. In a haemophilia patient, a small cut may cause the loss of much blood as there is no clot formation to prevent bleeding.



The white blood cells are known to destroy bacteria and other germs by either ingesting them or by producing chemicals capable of destroying them or their harmful products. Such chemicals are called antibodies. The white blood cells that ingest bacteria are called phagocytes while those that secrete antibodies are called lymphocytes.


Different types of lymphocytes that produce different corresponding antibodies exist. These types include:

  1. Antitoxins: Antitoxins are antibodies which can neutralise the poisons (toxins) produced by bacteria or other pathogens.
  2. Agglutinins: Agglutinins are antibodies which can cause the bacteria or other pathogens to stick together so as to reduce their effectiveness.
  3. Lysins: Lysins are antibodies capable of dissolving the outer coats of the bacteria or other pathogens and make them to disintegrate.
  4. Precipitins: Precipitins are antibodies which can precipitate toxins produced by bacteria or other pathogens.

A particular type of bacteria or pathogen stimulates the formation of a specific antibody. Antibodies are usually produced in excess of the amount needed to destroy the infecting agent or its toxins. The excess antibodies remain and circulate in the blood for sometime (the length of such time varies from one antibody to another) and confers immunity against infection by the particular pathogen. Antibodies are specific in their action as the antibody formed against one pathogen will not confer immunity against a different pathogen. Immunity created in the body by itself is called natural immunization.

People can protect themselves from certain serious diseases artificially by causing lymphocytes in the body to recognise these antigens. This is done by injecting small dose of the weakened or dead micro-organisms into the body. Such protective methods are known as artificial immunisation or vaccination.

The major effect of Aquired Immune Deficiency Syndrome (AIDS) is that when the virus causing AIDS attacks and destroys certain white blood cells in the immune system, the infected person is left without a defence when disease causing micro-organisms invade the body. As a result, he suffers from all kinds of microbial infections which eventually cause the death of the individual.