The common pathway of coagulation starts with the conversion of factor X to Xa; in the intrinsic pathway this is caused by factor IXa, in the extrinsic pathway this is caused by tissue factor. Co-factors are required for both limbs.
Factor Xa requires its own co-factors for activity. These include calcium ions, circulating factor V and the negatively-charged platelet surface for localisation. It is then able to cleave prothrombin (factor II) to thrombin (factor IIa).
Thrombin is an enzyme with four key functions:
Therefore, a network of insoluble fibrin is formed which is localised to the site of injury and traps oncoming blood cells.
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