heparin---Mechanism Heparin works by inhibiting the three major clotting factors (thrombin,thromboplastin,and prothrombin). It slows the process of thromboplastin synthesis,decelerates the conversion of prothrombin to thrombin,and inhibits the effects of thrombin on fibrinogen,blocking its conversion to fibrin. heparin heparin AT Ill thrombin* AT II <thrombin* heparin heparin. AT III thrombin AT Ill thrombin* can also be Factor Xaheparin AT III thrombin* AT III heparin thrombin* thrombin* heparin AT III AT III thrombin* heparin Fig. 50-3 * can also be Factor Xa heparin---Mechanism Heparin works by inhibiting the three major clotting factors (thrombin, thromboplastin, and prothrombin). It slows the process of thromboplastin synthesis, decelerates the conversion of prothrombin to thrombin, and inhibits the effects of thrombin on fibrinogen, blocking its conversion to fibrin