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How do the intrinsic and extrinsic pathways of coagulation differ?
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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 intrinsic and extrinsic pathways are two distinct mechanisms for initiating blood coagulation, both of which converge to activate factor X and trigger the common pathway.12
Key Differences
- Initiation: The extrinsic pathway is triggered by tissue factor, a protein found on cells external to blood vessels that is exposed upon injury. In contrast, the intrinsic pathway is initiated by injury within a blood vessel, specifically when blood circulates over damaged internal surfaces, activating factor XII.12
- Components: The extrinsic pathway primarily involves factor VII. The intrinsic pathway involves factors XII, XI, IX, and VIII.1
- Laboratory Assessment: These pathways are evaluated using different clinical tests. The extrinsic pathway is assessed via prothrombin time, while the intrinsic pathway is evaluated using the partial thromboplastin time.13
While these pathways function independently to initiate clotting, they are linked; for instance, factor VIIa from the extrinsic pathway can activate factor IX of the intrinsic pathway to amplify the coagulation process.2