32.3 Common pathway:

The paths, the intrinsic as well as the extrinsic, can be seen to merge at factor Xa as shown in the schematic representation below. This activated Xa begins the catalysis and converts prothrombin to thrombin.

Once thrombin is formed it is believed to be involved in three main functions. (a) Thrombin catalyzes the formation of fibrin, it also contributes to the activation of factors XI and VIII in the intrinsic pathway and factor V, with factor Va then serving as a cofactor for factor Xa. (b) It is involved in the activation of factor XIII. (c) It also controls the plasma enzyme activation cascade by positive feedback mechanism.

Thrombin also activates platelets. For the extrinsic pathway tissue factor is the initiator of clot formation in the body. For intrinsic pathway factor XII is the initiator of blood clotting in the body. Generally the production or formation of, thrombin takes place exclusively by the extrinsic pathway. The amount of thrombin obtained is very less to cause enough stabilized coagulation, still it is sufficient to kick start thrombin's positive-feedback effects on the intrinsic pathway by the activation of factors XI and VIII and platelets. By this way the intrinsic pathway can be triggered independently of factor XII, and these pathway further result in amplification of thrombin required for adequate coagulation. Thus, the extrinsic pathway is capable of plugging into the intrinsic pathway. Though, the extrinsic pathway synthesizes less amount of thrombin yet it bundles with the intrinsic pathway which has more potential and amplifies the production of thrombin. By coupling with the intrinsic pathway the extrinsic pathway eliminates the requirement of factor XII as the initiator of clotting mechanism in intrinsic pathway. Indirectly initial thrombin production by extrinsic pathway is capable of eliminating the need for factor XII. Thrombin after recruiting the intrinsic pathway in the mechanism of blood clotting, catalyses the prothrombin to thrombin conversion step on its own by activating factor V and thrombocytes. Indirectly liver is an important player involved in clotting and persons suffering from liver disease mostly have serious bleeding problems. Liver is the organ involved in the production of many of the plasma clotting factors and bile salts which are very important for the absorption of vitamin K in the intestine. The liver requires this vitamin to produce prothrombin and several other clotting factors in order to absorb the lipid-soluble substance vitamin K. The liver requires this vitamin to produce prothrombin and several other clotting factors.