Which factor initiates in vivo coagulation by activating factor VII?

The initiation of in vivo coagulation is primarily triggered by tissue factor, which is also known as thromboplastin. Tissue factor is a glycoprotein that is expressed on the surface of certain cells, especially those of the vascular endothelium and subendothelial tissues. When tissue factor comes into contact with circulating factor VII in the bloodstream, it binds to factor VII and catalyzes its activation to factor VIIa. The activated factor VIIa then plays a crucial role in the coagulation cascade, particularly in activating factor X, which leads to the conversion of prothrombin to thrombin and ultimately results in the formation of a fibrin clot.

This process is critical to hemostasis, especially in instances of vascular injury, where rapid clot formation is necessary to prevent excessive blood loss. Other factors mentioned, such as protein C, plasmin activator, and thrombomodulin, play important roles in the coagulation and fibrinolysis systems but do not initiate coagulation by activating factor VII. Protein C is involved in the regulation of coagulation, while thrombomodulin serves to activate protein C, and plasmin activator is involved in the breakdown of clots through fibrinolysis. Therefore, tissue factor's role as the