Introduction Components of the Hemostatic System

Following endothelial injury, vessel-wall response involves vasoconstriction, platelet plug formation, coagulation, and fibrinolysis regulation. In normal circumstances, platelets circulate in the blood in an inactive form. After injury, platelets undergo activation, which consists of (a) adhesion to the subendothelium, (b) secretion of granules containing chemical mediators (e.g., adenosine diphosphate, thromboxane A2, thrombin, etc.), and (c) aggregation. Chemical factors released from the injured tissue and platelets stimulate the coagulation cascade and thrombin formation. In turn, thrombin catalyzes the conversion of fibrinogen to fibrin and its subsequent incorporation into plug.

The coagulation system consists of intrinsic and extrinsic pathways. Both pathways are composed of a series of enzymatic reactions that ultimately produce thrombin, fibrin, and a stable clot. In parallel with the coagulation, the fibrinolytic system is activated locally. Plasminogen is converted to plasmin, which dissolves the fibrin mesh (Fig. 67-1)1



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