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Terms in this set (109)

Activation of thrombin and formation of fibrin.

INTRINSIC PATHWAY: Begins with factor XII and through a cascade of enzymatic reactions activates factors XI, IX, and VII in sequence. All the components that ultimately lead to fibrin clot formation are intrinsic to the circulating plasma and no surface is required to initiate the process.

EXTRINSIC PATHWAY: Requires exposure of tissue factor on the surface of the injured vessel wall to initiate the arm of the cascade beginning with factor VII.

*Activation of the extrinsic or intrinsic clotting pathway results in production of activated factor X (the two pathways CONVERGE), and activation proceeds in sequence of factors II (prothrombin) and I (fibrinogen). Clot formation occurs after proteolytic conversion of fibrinogen to fibrin.

Propagation of the clotting reaction ensues with a sequence of 4 enzymatic reactions, each of which involves a proteolytic enzyme that generates the next enzyme in the cascade by cleavage of a proenzyme and a phospholipid surface, such as a platelet membrane. Factor VIIa combines with factor IXA to form the intrinsic factor complex, which is responsible for the bulk of conversion of factor X to Xa. Factor Xa combines with factor Va, on the activated platelet membrane surface, to form the PROTHROMBINASE COMPLEX which is responsible for converting prothrombin to thrombin. Thrombin has multiple factors in the clotting process, such as conversion of fibrinogen to fibrin and activations of many factors (including XII which stabilizes the fibrin clot), as well as activation of platelets. Once formed, thrombin leaves the membrane surface and converts fibrinogen by two cleavage steps into FIBRIN (the clot) and two small peptides termed fibrinopeptides A and B.

*Calcium ion is needed for these cascades