Anticoagulants

The processes that underly the clotting of blood are still far from adequately understood, in spite of the many facts uncovered by generations of patient investigators. The classical scheme associated with the name of Morawitz 1 , although neither complete nor universally accepted, is still useful as a summary of the main events. According to this scheme, the plasma contains three essential ingredients of the complete clotting system: these are ionised calcium and the proteins prothrombin and fibrinogen. The addition of a fourth ingredient, thrombokinase (or better thromboplastin, since its enzymic nature is still in doubt) completes the system, and initiates clotting. Thromboplastin is present in the tissues generally and also in the blood platelets. When blood makes contact with tissue fluid, or the platelets are injured by contact with a hydrophilic surface, prothrombin reacts with thromboplastin and with calcium ions to form an enzyme, thrombin: this is the first stage of clotting. In the second stage of clotting thrombin acts upon fibrinogen, changing it into the insoluble protein fibrin, whose threads constitute the matrix of the clot.