Hemostasis is a physiologic mechanism that maintains blood in a fluid state within the circulation. The coagulation of blood is mediated by cellular components and soluble plasma proteins. In response to vascular injury, circulating platelets adhere, aggregate, and provide cell-surface phospholipid for the assembly of blood-clotting enzyme complexes. The extrinsic pathway of blood coagulation is initiated when blood is exposed to non-vascular-cell–bound tissue factor in the subendothelial space (Figure 1). Tissue factor binds to activated factor VII, and the resulting enzyme complex activates factors IX and X of the intrinsic and common coagulation pathways, respectively. Factor IX activated by the . . .