Prothrombin activator extrinsic, formation

Both intrinsic and extrinsic pathways generate activated factor X. This protease, in turn, catalyses the proteolytic conversion of prothrombin (factor II) into thrombin (Ha). Thrombin, in turn, catalyses the proteolytic conversion of fibrinogen (I) into fibrin (la). Individual fibrin molecules aggregate to form a soft clot. Factor XHIa catalyses the formation of covalent crosslinks between individual fibrin molecules, forming a hard clot (Figures 12.3 and 12.4). 

The standard assay measures the time taken for the formation of a fihrin clot in citrated plasma after the addition of calcium ions and thromhoplastin to activate the extrinsic clotting system - the prothrombin time. The normal prothrombin time is 11 to 13 seconds greater than 25 seconds is associated with severe bleeding. 

The extrinsic pathway is based on the availability of tissue thromboplastin, a phospholipoprotein procoagulant found in most tissue cells, as well as in erythrocytes and leukocytes. It is made available whenever such cells are injured by a variety of means. Small amounts of tissue thromboplastin are capable of activating prothrombin by the extrinsic pathway within seconds. As a consequence, its intravascular application may lead to immediate thrombin formation and vascular occlusion. 

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