Vicious Circle of Clot Formation
Once a blood clot has started to develop, it normally extends within minutes into the surrounding blood.
That is, the clot itself initiates a vicious circle (positive feedback) to promote more clotting. One of the most important causes of this is the fact that the proteolytic action of thrombin allows it to act on many of the other blood-clotting factors in addition to fibrinogen. For instance, thrombin has a direct proteolytic effect on prothrombin itself, tending to convert this into still more thrombin, and it acts on some of the blood-clot-ting factors responsible for formation of prothrombin activator. (These effects, discussed in subsequent para-graphs, include acceleration of the actions of Factors VIII, IX, X, XI, and XII and aggregation of platelets.) Once a critical amount of thrombin is formed, a vicious circle develops that causes still more blood clotting and more and more thrombin to be formed; thus, the blood clot continues to grow until blood leakage ceases.