Newer Concepts of Blood Coagulation
THE fundamental concepts of blood coagulation formulated almost 50 years ago by Morawitz1 have not changed significantly. The theory is simple but fails to explain three important features. These are the mode of action of the various substances involved in the clotting process, the origin of thrombo-plastin, and the function of platelets. During the past decade knowledge pertaining to the mechanism of blood coagulation has become increasingly complex.
Theory of Morawitz
Calcium + thromboplastin + prothrombin → thrombin
Thrombin + fibrinogen → fibrin
It now appears that prothrombin is converted to thrombin with the aid of conversion and accelerator factors from plasma and platelets, and that the formation of fibrin is enhanced by an accelerator from platelets.
Origin of Thromboplastin and the Function of Platelets
Morawitz’s theory presumes the liberation of free thromboplastin as a preliminary step in clotting. Quick2,3 states that thromboplastin still remains an undefined entity. Free thromboplastin is present in tissues. In plasma it exists as a precursor which is activated by an enzyme derived from platelets. This plasma precursor, thromboplastinogen, may be identical with the plasma fraction known as antihemophilic globulin which is lacking in hemophilia. Following the lysis of platelets, an enzyme called thromboplastinogenase is liberated, which activates thromboplastinogen in the plasma to thromboplastin. Thromboplastin then acts stoichiometrically with prothrombin complex to form thrombin. Most investigators agree that thromboplastin is of at least two varieties. One is a slower acting thermostable, fat soluble phospholipid and the other a thermolabile lipoprotein. Both have special. . .