1. A substance present in tissues, platelets, and leukocytes necessary for the coagulation of blood; in the presence of calcium ions thromboplastin is necessary for the conversion of prothrombin to thrombin, an important step in coagulation of blood. It is now generally believed that thromboplastin activity may be developed through blood (intrinsic) or tissue (extrinsic) systems. Tissue thromboplastin (factor III) interacts with factor VII and calcium to activate factor X; active factor X combines with factor V in the presence of calcium and phospholipid to produce thromboplastin activity (also commonly called thromboplastin).
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