Blood- Blood is the main circulating fluid in the human body. Study of blood is called haematology (Gk. Haeme-blood, logos-study). Blood is a fluid connective tissue derived from mesoderm. It is bright red, slightly alkaline (pH 7.4), salty, viscous fluid heavier than water. The average sized adult has about 5 liters of blood which constitutes about 8% of the total body weight.
Composition of blood - Blood is composed of two main components - plasma 55% and blood corpuscles 45%.
Plasma : Plasma is a straw coloured, slightly alkaline, viscous fluid. It contains of 90 to 92% water and 8- 10% of solutes, most of which are proteins (7%). Plasma proteins are serum albumin, serum globulin, heparin, fibrinogen and prothrombin. Other solutes in plasma includes, nutrients such as glucose, amino acids, fatty acids and glycerol; nitrogenous wastes such as urea, uric acid, ammonia and creatinine; gases like oxygen, carbon dioxide and nitrogen; regulatory substances like enzymes and hormones; inorganic substances like bicarbonates, chlorides, phosphates, sulphates of sodium, potassium, calcium, magnesium etc.
Blood corpuscles are suspended in plasma and are three types -
1) erythrocytes (RBCs)
2) leucocytes (WBCs)
3) thrombocytes (platelets)
Blood Coagulation :
Inside an intact blood vessel, blood does not coagulate or clot due to presence of active anticoagulants like heparin and antithrombins, As soon as a blood vessel ruptures, bleeding starts. The conversion of liquid blood into semisolid jelly is called blood coagulation or clotting.
The platelets and injured tissue releases thromboplastin, which initiates the formation of enzyme prothrombinase in the blood. The prothrombinase in presence of Ca ions converts inactive plasma protein called prothrombin into active thrombin. The thrombin converts soluble fibrinogen into insoluble fibrin. The fibrin fibres enmesh platelets, blood cells and plasma to form the clot. The normal clotting time is 2 to 8 minutes.
Blood groups :
Human being exhibit four types of blood groups.
1) A, 2) B, 3)AB and 4)O