What is the role of Factor VIIa in normal hemostasis?
  Normal hemostasis requires activation of both Factor X and Factor IX. Factor VIIa/tissue factor (TF)-activated Factor Xa and Factor IXa play distinct roles in coagulation. Factor Xa cannot move to the platelet surface because of the presence of normal plasma inhibitors, but instead remains on the TF-bearing cell and activates a small amount of thrombin. This thrombin is not sufficient for fibrinogen cleavage but is critical for hemostasis since it can activate platelets, activate and release Factor VIII from von Willebrand factor (vWF), activate platelet and plasma Factor V, and activate Factor XI. Factor IXa moves to the platelet surface, where it forms a complex with Factor VIIIa and activates Factor X on the platelet surface. This platelet surface Factor Xa is relatively protected from normal plasma inhibitors and can complex with platelet surface Factor Va, where it activates thrombin in quantities sufficient to provide for fibrinogen cleavage.
 
 
     
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