blood clotting, the conversion of blood from a free-flowing liquid to a semisolid gel. Although clotting can occur within an intact blood vessel, the process usually starts with tissue damage. Within seconds of injury to the vessel wall, platelets adhere to the site. If normal amounts of calcium, platelets, and tissue factors are present, prothrombin is converted to thrombin. Thrombin acts as a catalyst for the conversion of fibrinogen to a mesh of insoluble fibrin, in which all the formed elements are immobilized. Also called blood coagulation. Compare hemostasis. See also anticoagulant, coagulation.
