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How does tissue factor start the extrinsic pathway of blood coagulation?

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Fibrin in blood clotting — bleeding and blood clotting Red blood cells (erythrocytes) trapped in a mesh of fibrin threads. Fibrin, a tough, insoluble protein formed after injury to the blood vessels, is an essential component of blood clots.

The extrinsic pathway of blood coagulation is initiated when a blood vessel is injured, exposing blood to tissue factor—a protein normally found on cells external to the blood vessels.12

Once exposed, tissue factor functions as a cofactor with factor VII to facilitate the activation of factor X. This process can occur through two primary mechanisms:1
* Tissue factor and factor VII directly activate factor X.1
* Alternatively, factor VII activates factor IX, which subsequently activates factor X.1

Once factor X is activated, it enters the common pathway, where it converts prothrombin to thrombin. Thrombin then catalyzes the conversion of fibrinogen into insoluble fibrin threads, which form the structural mesh of a blood clot. This extrinsic pathway is considered the physiologically significant mechanism for rapidly initiating coagulation in response to tissue injury.12