COAGULATION CASCADE
Understanding the coagulation cascade
Coagulation
Coagulation involves a complex set of protease reactions involving different proteins. The final result of these reactions is to convert fibrinogen, a soluble protein, to insoluble strands of fibrin.14 Together with platelets, the fibrin strands form a stable blood clot.
Currently, there are 2 models of the coagulation cascade. These are the intrinsic/extrinsic model, and the cell-based model.14
Intrinsic and Extrinsic Model
This model divides the initiation of coagulation into distinct parts: the extrinsic pathway and the intrinsic pathway.15,16 The extrinsic pathway is the primary initiator of coagulation, while the intrinsic pathway leads to the successive activation of Factors IX and X. Activated Factor X (Factor Xa) plays a central role in the coagulation cascade, as it occupies a point where the intrinsic and extrinsic pathways
converge.16,17
Cell-Based Model
The cell-based model identifies the membranes of tissue factor–bearing cells and platelets as the sites where activation of specific coagulation factors occurs.16 This model comprises a 3-phase process—initiation, amplification, and propagation.
Initiation occurs at the point of vascular injury, where tissue factor–bearing cells bind to and activate Factor VII.16 A small amount of thrombin is then produced, activating platelets and cofactors; this is the amplification phase. During the third phase of the clot formation—propagation—the prothrombinase complex (comprising Factor Xa and cofactors bound to activated platelets) initiates a burst of thrombin production on the platelet surface, which leads to production of a stable fibrin clot.
