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Figure 1 The coagulation cascade. Interaction of the intrinsic pathway (initiated by the activation of Factor XI) and the extrinsic pathway (activated by vascular or tissue injury) results in a cascade of reactions that generate thrombin (Factor IIa). Thrombin is produced from prothrombin Factor II by the action of activated Factor X (Xa). Thrombin then acts on fibrinogen to generate fibrin, which undergoes crosslinking, resulting in a thrombin–fibrin clot.
