Blood clotting is a mechanism to prevent excessive loss of blood from the body. A dark reddish brown scum formed at the site of a cut. It is a clot formed mainly of a network of threads called fibrins in which dead and damaged formed elements of blood are trapped.

Fibrins are formed by the conversion of inactive fibrinogens in the plasma by the enzyme thrombin. Thrombin, in turn are formed from another inactive substance called prothrombin. An enzyme complex, thrombokinase is required for the above reaction. Heparin decrease the clotting ability of the blood and prevent clotting in blood vessels.