Mechanism of Blood Coagulation
Basic Theory. More than 50 important substances thatcause or affect blood coagulation have been found in the blood and in the tissues—some that promote coag-ulation, called procoagulants, and others that inhibit coagulation, called anticoagulants. Whether blood will coagulate depends on the balance between these two groups of substances. In the blood stream, the anti-coagulants normally predominate, so that the blood does not coagulate while it is circulating in the blood vessels. But when a vessel is ruptured, procoagulants from the area of tissue damage become “activated” and override the anticoagulants, and then a clot does develop.
General Mechanism. All research workers in the field ofblood coagulation agree that clotting takes place in three essential steps: (1) In response to rupture of the vessel or damage to the blood itself, a complex cascade of chemical reactions occurs in the blood involving more than a dozen blood coagulation factors. The net result is formation of a complex of activated sub-stances collectively calledprothrombin activator. (2) The prothrombin activator catalyzes conversion of prothrombin into thrombin. (3) The thrombin acts asan enzyme to convert fibrinogen into fibrin fibers that enmesh platelets, blood cells, and plasma to form the clot.
Let us discuss first the mechanism by which the blood clot itself is formed, beginning with conversion of prothrombin to thrombin; then we will come back to the initiating stages in the clotting process by which prothrombin activator is formed.