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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.¹ Together with platelets, the fibrin strands form a stable blood clot.²

Currently, there are 2 models of the coagulation cascade. These are the intrinsic/extrinsic model, and the cell-based model.

Intrinsic and Extrinsic Model

This model divides the initiation of coagulation into distinct parts: the extrinsic pathway and the intrinsic pathway.² The extrinsic pathway is the primary initiator of coagulation, while the intrinsic pathway leads to the successive activation of Factors IX and X.

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.² 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.^2,3 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 facilitates thrombin production on the platelet surface.⁴