Extrinsic system (blood coagulation) is triggered by the release of: September 2008

Ans. B: Thromboplastin Factor X can be activated by reactions in either of two systems, an intrinsic and an extrinsic system. The extrinsic system is triggered by the release of tissue thromboplastin, a protein-phospholipid mixture that activates factor VII. The tissue thromboplastin and factor VII activate factors IX and X. In the presence of PL, Ca2+, and factor V, activated factor X catalyzes the conversion of prothrombin to thrombin. The extrinsic pathway is inhibited by a tissue factor pathway inhibitor that forms a quaternary structure with TPL, factor VIIa, and factor Xa.