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STRUCTURE AND FUNCTION OF BLOOD
The mammalian blood comprises four main components;
- Plasma
- Red corpuscles
- White corpuscles
- platelet
Plasma: The plasma is a pale yellow liquid made of mainly of water (about 90%), with many substances dissolved in it. These include digested food, mineral salts, vitamins, hormones, dissolved oxygen and excretory products such as urea and carbon (iv) oxide. The plasma also contains large molecules, the plasma proteins, such as fibrinogen, which assist in the clothing of blood in damaged tissues.
Red corpuscles or erythrocytes: They are tiny, biconcave, disc-like cell without any nucleus in adult mammals. They contain a red pigment-haemoglobin, a protein that contains iron. This enables the red blood corpuscles to readily combine with oxygen in area of high oxygen concentration (i.e., the alveoli of lungs) to form oxyhaemoglobin. This is the form in which oxygen is carried to all body tissues. They also readily give oxygen in places where the oxygen concentration is low (e.g., all the tissues except those near the alveoli). Erythrocytes are synthesised in the red bone marrows of sternum, ribs and vertebrae. There are about 51/2 million of them in a cubic centimetre of blood. They live for about 120 days and are destroyed in the liver or spleen.
Erythrocytes
Erythrocytes, or red blood cells, are the primary carriers of oxygen to the cells and tissues of the body. The biconcave shape of the erythrocyte is an adaptation for maximizing the surface area across which oxygen is exchanged for carbon dioxide. Its shape and flexible plasma membrane allow the erythrocyte to penetrate the smallest of capillaries.
White corpuscles or leucocytes:
There are many types of white corpuscles all of which have nuclei. They are made in the red bone marrow, the lymph node or the spleen. They live for many months. Those that are irregular in shape, i.e., the phagocytes, are commonest. They are large with lobed nuclei. Like the Amoeba, they have pseudopodia and are able to pass through the walls of the capillaries into the tissue fluid. In the lymphatic system, they ingest bacteria, virus and dead cells, and help in preventing diseases. The ingestion of materials is called phagocytosis and hence such white corpuscles are called phagocytes. Those that produce antibodies are called lymphocytes and are produced in the lymph glands. They produce chemicals called antibodies, which stick to the surface of germs and kill them. White corpuscles are fewer than the red corpuscles. There are about 5000 of them in a cubic millimetre of blood.
Lymphocyte
Scanning electron micrograph of a normal T lymphocyte. Lymphocytes are specialized white blood cells whose function is to identify and destroy invading organisms such as bacteria and viruses. Some T lymphocytes directly destroy invading organisms, whereas other T lymphocytes regulate the immune system by directing immune responses.
Platelets: These are tiny, irregularly-shaped particles formed in the red bone marrow. They lack nucleus. In damaged tissues, they break down and librate an enzyme, which catalyses the first of a series of reactions, fibrinogen, a blood protein, is converted to threads of fibrin, which form a mesh that plugs the wound. This stops the bleeding.
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