Heparin is the most widely used antithrombotic agent. It act by activating: (PGI June 2009)

Ans: B. (Antithrombotic III) Heparin inhibit Factor Xa & IIaQ (thrombin)( i.e., not activate)Heparin# The anticoagufant action is exerted mainly by inhibition of factor Xa as well as thrombin (IIa) mediated conversion of fibrinogen to fibrin# It acts indirectly by activating plasma antithrombin IIIQ The heparin - AT III complex then binds to clotting factors of the intrinsic & common pathways (Xa, IIa(Thrombin), IXa, Xlla & XIIIa)& inactivate them but not factor Vila, operative in the extrinsic pathway.# Low concentration of heparin prolong aPTT without significantly prolonging PTQAdverse Effects1. Bleeding due to overdose is the most serious complication of heparin therapy. Haematuria is generally the first sign.Thrombocytopen ia is a noth er common problemTransient & reversible alopecia is infrequentHypersensitivity reactions are rare- urticaria, rigor, fever & anaphylaxis.Osteoporosis may develop on long term use