Coagulation and fibrinolysis

The two final phases in the haemostatic process, plasma coagulation with the formation of a fibrin clot, and fibrinolysis leading to the dissolution of fibrin clots, are reviewed. Coagulation may be initiated either by reactions occurring between components of the blood alone, the intrinsic pathway, or by reactions which also involve tissue components, termed the extrinsic pathway. In the diagnosis of coagulation disorders, it is convenient to divide the intrinsic pathway into three phases. In phase 1, resulting in the activation of factor (f) X, are involved f XII, XI, VIII and IX, platelet phospholipids, and calcium. In phase 2, prothrombin is converted to thrombin by f Xa in conjunction with f V, phospholipids, calcium. In phase 3, thrombin converts fibrinogen to fibrin, which is then stabilized by f XIII. Antithrombin III is the most important inhibitor. The key component in fibrinolysis is plasminogen, which under the influence of various activators is converted to plasmin. Plasmin is a serine protease and its main in vivo target is fibrin. Alpha 2-antiplasmin and a fast-acting inhibitor of tissue plasminogen activator are the most important inhibitors.