Heparin inactivates all of the following clotting factors except -

Ans. is 'd' i.e., Vila Anticoagulant action of heparin* Anticoagulant Heparin is a powerful and instantaneously acting anticoagulant, effective bothin vivo and in vitro.* It acts indirectly by activating plasma antithrombin III (AT III, a serine proteinase inhibitor).* The heparin-AT III complex then binds to clotting factors of the intrinsic and common pathways (Xa, Ha, IXa, XIa, XIIa and XIIIa) and inactivates them but not factor Vila operative inthe extrinsic pathway.* At low concentrations of heparin, factor Xa mediated conversion of prothrombin to thrombin is selectively affected.* The anticoagulant action is exerted mainly by inhibition of factor Xa as well as thrombin (IIa)mediated conversion of fibrinogen to fibrin.* Low concentrations of heparin prolong aPTT without significantly prolonging PT. High concentrations prolong both. Thus, low concentrations interfere selectively with the intrinsic pathway,affecting amplification and continuation of clotting, while high concentrations affect the common pathway as well.