Explain the mechanism of clotting of blood.

Explanation

When a blood vessel is injured a number of physiological mechanisms are activated that promote hemostasis, and stops bleeding. Blood clots within 3-6 minutes after damage of a blood vessel.
Mechanism of blood clotting : Blood clotting takes place in three essential steps.
i) Formation of prothrombin activatio : It is formed by two pathways.
a) Intrinsic pathway : It occurs when the blood is exposed to collagen of injured wall of blood vessel. This activates factor XII, all in turn it activates another clotting factor, which activates yet another reaction, which results in the formation of prothrombin activator.
b) Extrinsic pathway : It occurs when the damaged vascular wall or extra vascular tissue comes into contact with blood. This activates the release of tissue thromboplastin, from the damaged tissue. It activates the factor VII. As a result of these cascade reactions, the final product formed is the parothrombin activator.
ii) Activation of prothrombin : The prothrombin activator, in the presence of sufficient amount of `Ca^(2+)`, causes the convertion of inactive prothrombin to active thrombin.
iii) Convertion of soluble fibrinogen into fibrin : Thrombin converts the soluble protein fibrinogen into soluble, fibrin monomers, which are held together by weak hydrogen bonds. The factor XIII replaces hydrogen bonds with covalent bonds and cross links the fibers to from a meshwork and prevent the blood bleeding.