Procoagulant Subsystem and Activation of Factor VII

The first complex to form is almost certainly the extrinsic factor X activation complex. Factor Vila binds to tissue factor, an integral membrane protein that is exposed upon injury of the blood vessel. The phospholipid bilayer surface is provided by the damaged cell membranes. In vivo, Ca + is always present in the blood.  

Two hypotheses exist for the origin of factor Vila. In one, factor VII is considered to have some proteolytic activity prior to a proteolytic cleavage. In the other view, low concentrations of thrombin and/or factor Xa are considered present at all times in the circulating blood. Thrombin, factor Xa, and factor Vila can activate factor VII to 

In the absence of tissue factor, factor Vila cleaves the Arg peptide bond in factor X at a rate so low that it is not readily detectable in vitro. In the presence of tissue factor and membrane surface phospholipids, factor Vila rapidly activates factor X. The estimates for the magnitude of the increase in activation rates in the presence of tissue factor 

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