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thromboplastin:see blood clottingblood clotting,
process by which the blood coagulates to form solid masses, or clots. In minor injuries, small oval bodies called platelets, or thrombocytes, tend to collect and form plugs in blood vessel openings.
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(Factor III), an important coagulation factor in the blood-clotting system; it helps convert prothrombin into thrombin. Thromboplastin is localized in the membranes of formed elements of the blood and in the cells of various organs. A large amount of thromboplastin enters the blood upon injury to tissues. Brain and lung tissues have especially high thrombo-plastic activity, and lung thromboplastin also takes part in the metabolism of the vasoactive polypeptides angiotensin and bradykinin.
Chemically, thromboplastin is a protein-lipid complex. The basis of its activity is its lipid constituent, which contains serine, eth-anolamine, and choline phosphatides. Blood coagulation factors are adsorbed on the phospholipid micelle, creating favorable conditions for the enzymatic reactions involved in blood clotting.
The term “thromboplastin” is sometimes used to designate an active complex that includes, in addition to thromboplastin, a number of protein factors and Ca2+ ions. Such active thromboplastin, which directly converts prothrombin into thrombin, is also called thrombokinase or prothrombinase. Thromboplastin is used in clinical practice to determine the prothrombin level, an indicator of the condition of the blood-clotting system.
I. P. BASKOVA