Common pathway of blood coagulation

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:

• removal of small fibrinopeptides from the large fibrinogen precursor; this favours the polymerization of fibrinogen into strands of fibrin
• activation of factor XIII to XIIIa; XIIIa is the fibrin-stabilizing factor, which in the presence of calcium ions, interlinks fibrin strands
• activation of platelets
• activation of protein C, an antithrombotic plasma enzyme

Therefore, a network of insoluble fibrin is formed which is localised to the site of injury and traps oncoming blood cells.