Blood Clotting (Coagulation)

The coagulation system is activated in response to vascular damage. The initial step involves
the exposure of tissue factor at the injured site for factor VIIa binding .  The association of
these two proteins leads to the activation of factor IX and factor X to become factor IXa and Xa
which bind anionic phospholipid surfaces such as activated platelets.  Factor Xa then
activates prothrombin to thrombin.  With the generation of thrombin, soluble fibrinogen
molecules are converted into an insoluble fibrin mesh at the injured site to hinder bleeding .   
At the same time, thrombin activates factor VIII and factor V.  The resulting factor VIIIa and
factor Va bind to anionic phospholipid, and form complexes with factor IXa and factor Xa,
respectively.  The VIIIaIXa and VaXa complexes are known as tenase and prothrombinase and
are potent enzymatic activators for factor X and prothrombin, respectively.  The generation of
these two complexes leads to an amplification of thrombin generation for further fibrin
production to arrest blood loss.  Therefore, the VIIIa- and Va-containing tenase and
prothrombinase complex play a central role in the amplification of thrombin generation and
fibrin deposition at the site of injury.
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Basic Knowledge