- Blood platelets: Also called thrombocytes, blood platelets are non-nucleated, round or oval, biconvex disc-like bodies. They are 2-3 micrometers in diameter and their number normally varies from 0.15 to 0.45 million per microlitre of blood. They bud off from the cytoplasm of very large Mega-karyocyte cells of bone marrow. Their normal life-span is about a week. When a blood vessel is injured, platelets get clumped at the injured spot and release certain chemicals called Platelet Factors. These promote blood coagulation. Thrombocytopenia, decrease in platelet count. Purpura, is a group of bleeding disease due to thrombocytopenia Blood coagulation: When blood oozes out of a cut, it sets into gel within a few minutes. This is called coagulation. Coagulation is brought about by hydrolysis of soluble fibrinogen of plasma to insoluble fibrin. This is catalysed by an enzyme called thrombin. Fibrin precipitates as a network of fibers. This network traps many blood cells, particularly RBCs, to form a red solid mass called the Blood Clot. The clot seals the wound in the vessel to stop the bleeding. The straw coloured fluid left after clotting of blood is called Serum. The serum cannot be coagulated as it lacks fibrinogen.
Thrombin occurs in normal blood as an inactive globulin called Prothrombin. It must be activated to thrombin before blood coagulation can occur. In case of injury to a blood vessel, coagulation promoting substances called Thromboplastins are released into the blood from clumped platelets and damaged tissues. Thromboplastins help in the formation of the enzyme Prothrombinase. This enzyme hydrolyses prothrombin to thrombin to initiate coagulation. Ca2+ ions are essential for both the activation and action of thrombin.
Blood normally contains an anticoagulant; Heparin which prevents activation of prothrombin, Heparin is released from mast-cell granules. Blood also contains Antithrombin which inhibits any thrombin formed accidentally.
Blood drawn from a blood vessel can be kept un-coagulated by adding a pinch of oxalate (sodium or potassium oxalate) to it. Oxalate precipitates Ca2+ and consequently prevents coagulation. Chilling of blood also delays coagulation because cold depresses the action of coagulation promoting enzymes.