The extrinsic pathway is initiated by the entry of tissue factor (factor III) from the injured wall of a blood vessel. When this substance enters the vascular system, factor VII is activated to VIIa, a potent enzyme capable of activating factor X to Xa in the presence of ionized calcium.
Factor VII participates only in the extrinsic pathway. Only activated factor VII is needed in the extrinsic pathway to activate factor X to its activated form, Xa. In addition to quickly providing small amounts of thrombin, which leads to fibrin formation, the thrombin generated in the extrinsic pathway can enhance the activity of factors V and VIII in the intrinsic pathway. To monitor the extrinsic pathway leading to fibrin clot formation in the laboratory, the prothrombin time (PT) test is performed.
