Figure 1. Schematic of the Extrinsic Coagulation Cascade.

(A) Upstream coagulation factors are activated by substances exposed by vessel injury; chief among these factors is TF. Activated upstream coagulation factors initiate a cascade of events that culminate in the activation of platelets and the key protease FIIa. Thrombin forms an amplification loop by activating itself and other coagulations factors as well as platelets.

(B) Activated platelets then aggregate to form platelet plugs, which serve as scaffolds for fibrin clots.