Blood coagulation is a host defense system that assists in maintaining the integrity of the closed, high-pressure mammalian circulatory system after blood vessel injury. After initiation of clotting, the sequential activation of certain plasma proenzymes to their enzyme forms proceeds through either the intrinsic or extrinsic pathway of blood coagulation(Figure 1A)(Davie and Fiatnoff, 1964; Mac-Farlane, 1964). These plasma glycoproteins, including factor XII, factor XI, factor IX, factor X, factor VII, and prothrombin, are zymogens of serine proteases. As a family, these proteins bear marked structural and functional homology to the digestive proteases trypsin and chymotrypsin. They are each converted from an inactive form to an active enzyme by limited proteolysis of one or two peptide bonds. However, most of the blood clotting enzymes are effective on a physiologic time scale only when assembled in complexes on membrane surfaces with protein cofactors such as factor VIII and factor V. Calcium ions have a critical role in that many of the component reactions in blood coagulation are either Ca2+-dependent or require Caz+ for the interaction of proteins with membrane surfaces.