blood clotting

The blood clotting cascade. When there is a wound in the skin, the clotting factor prothrombin activator is released by the injured tissues and platelets. This converts the blood protein prothrombin to thrombin. Thrombin splits fibrinogen to form fibrin, which forms a mesh over the wound trapping red blood cells and forming a clot.

Complex series of events (known as the blood clotting cascade) resulting from a series of enzymatic reactions in the blood that prevents excessive bleeding after injury. The result is the formation of a meshwork of protein fibres (fibrin) and trapped blood cells over the cut blood vessels.

When platelets (cell fragments) in the bloodstream come into contact with a damaged blood vessel, they and the vessel wall itself release the enzyme thrombokinase, which brings about the conversion of the inactive enzyme prothrombin into the active thrombin. Thrombin in turn catalyses the conversion of the soluble protein fibrinogen, present in blood plasma, to the insoluble fibrin. This fibrous protein forms a net over the wound that traps red blood cells and seals the wound; the resulting jellylike clot hardens on exposure to air to form a scab. Calcium, vitamin K, and a variety of enzymes called factors are also necessary for efficient blood clotting. Haemophilia is one of several diseases in which the clotting mechanism is impaired.