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In the extrinsic pathway of blood coagulation, a tissue factor is produced after injury. Likely sources include endothelial cells and the surface cells of atheromatous plaques. The factor is held within the plasma membrane.
The tissue factor interacts with factor VIIa, calcium ions and phospholipid to activate factor X to Xa. Factor Xa then acts on prothrombin according to the common pathway of coagulation.
Factor XIIa indirectly facilitates the extrinsic pathway by converting factor VII to VIIa.