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.