How does Heparin prevent blood from clotting? binds calcium activates calcium inhibits thrombin heparin does not prevent clotting

Heparin prevents blood clotting by **inhibiting thrombin**. While it does interact with antithrombin III, which in turn affects Factor Xa, the primary mechanism of action is the potentiation of antithrombin III's ability to inactivate thrombin. This prevents the conversion of fibrinogen to fibrin, thus disrupting the clotting cascade.