The role of heparin in intrinsic blood coagulation

A number of inhibitors of intrinsic blood coagulation have been examined. Particular emphasis was placed on heparin, heparin analogues, and hirudin. These inhibitors interfere primarily with two clotting factors in the intrinsic clotting scheme. These include factor IX a and thrombin. Our data suggest that factor IX a is the heparin‐and hirudin‐sensitive factor in plasma when coagulation is initiated via factors XII or XI, and that subsequent clotting factors remain in inactive forms. Studies with heparin and hirudin also clarified the role of thrombin in the interaction of factors IX a and VIII. It was concluded from these experiments that factor VIII readily interacts with factor IX a in the presence of calcium ions and phospholipid independent of thrombin.