HEPARIN IS A POWERFUL AND INSTANTANEOUSLY ACTING ANTICOAGULANT, EFFECTIVE BOTH IN VITO AND IN VITRO. IT ACTS INDIRECTLY BY ACTIVTING PLASMA ANTITHROMBIN III AND MAY BE OTHER SIMILAR COFACTORS. THE COMPLEX THEN BINDS TO CLOTTING FACTOR XA, IIA, IXA, XIIA, AND XIIIA AND INACTIVATES THEM. AT LOW CONCENTRATION OF HEPARIN, FACTOR XA MEDIATED CONVERSION OF PROTHROMBIN TO THROMBIN IS SELECTIVELY AFFECTED. HEPARIN DOES NOT HAVE FIBRINOLYTIC ACTIVITY; THEREFORE, IT WILL NOT LYSE EXISTING CLOTS.THROMBIN IIA MEDIATED CONVERSION OF FIBRINOGEN TO FIBRIN IS ALSO QUITE SENSITIVE.