Blood clots are formed when platelets adhere to each other and form a loose meshwork. Clotting starts with activation of factor XII, which initiates coagulation. The initial product of coagulation is thrombin, which cleaves fibrinogen into fibrin, a highly cross-linked protein. Fibronectin, von Willebrand factor and other proteins are added and stabilize the clot. Thrombin also activates the intrinsic coagulation pathway. After coagulation is initiated, activated factor V converts Fibrino Gens prothrombin into thrombin, and the activated form of factor VII (factor VIIa) triggers the conversion of factor X to factor Xa. Factor V acts as the regulatory protein in the intrinsic coagulation pathway and factor X is the active enzyme that completes the conversion of prothrombin into thrombin. The intrinsic pathway is also called contact pathway since factors V, VIII and XI are present in the activated form. The activation is inhibited by anticoagulant drugs such as heparin and warfarin.
The final stage of coagulation is the formation of a fibrin clot. Fibrinogen first polymerizes and then self-polymerizes.