A: Blood clotting takes place in three phases:
1. Formation of prothrombin activator (occurs in two ways)
a. Extrinsic pathway: When blood in shed, the damaged wall of blood vessel or extra vascular tissue comes in contact with blood, thromboplastin is released from the tissue
cells. It activates factor VIII. As a result of these cascade reactions, prothrombin activator is formed.
b. Intrinsic pathway: When blood is exposed to collagen of damaged wall of blood vessel, it activates factor XII. After the cascade of reactions prothrombin activator is formed.
2. Activation of prothrombin: Prothrombin activator activates the prothrombin into
thrombin in presence of Ca++ ions.
3. Formation of fibrin threads: Thrombin converts the soluble fibrinogen
into soluble fibrin. Factor XIII stabilises the soluble fibrin into insoluble
fibrin threads. They form a net on the damaged part of blood vessel. In the meshes of fibrin net, blood cells entangle and thus blood clot in formed. Within few minutes after clot is formed, it begins to contract (clot reaction) and thus fluid is expelled out. It is called serum (i.e., plasma without fibrinogen).
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