Blood consists of cells suspended in plasma. It circulates through the blood vessels, carrying oxygen and nutrients to the organs and removing carbon dioxide and other waste products for excretion. In addition, it is the vehicle by which hormones and other humoral transmitters reach their sites of action.
The cells are red cells or corpuscles (ERYTHROCYTES), white cells (LEUCOCYTES and LYMPHOCYTES), and platelets.
The red cells are biconcave discs with a diameter of 7.5 µm. They contain haemoglobin - an iron-containing porphyrin compound, which takes up oxygen in the lungs and releases it to the TISSUES OF THE BODY.
The white cells are of various types, named according to their appearance. They can leave the circulation to wander through the tissues. They are involved in combating infection, wound healing, and rejection of invading foreign bodies such as bacteria. Pus consists of the bodies of dead white cells.
Platelets are the smallest cellular components and play an important role in blood clotting (see COAGULATION).
Erythrocytes are produced by the bone marrow in adults and have a life span of about 120 days. White cells are produced by the bone marrow and lymphoid tissue. Plasma consists of water, ELECTROLYTES and plasma proteins; it comprises 48-58 per cent of blood volume. Plasma proteins are produced mainly by the liver and by certain types of white cells. Blood volume and electrolyte composition are closely regulated by complex mechanisms involving the KIDNEYS, ADRENAL GLANDS and HYPOTHALAMUS.
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