How does Unfractionated heparin exert its anticoagulant effect?

1. activation of antithrombin-iii activity-unfractionated heparin (ufh), as well as the other indirect anticoagulants, exert their anticoagulant activity mainly through activation of antithrombin-iii (at), an endogenous inhibitor of various clotting factors. heparin binds to at-iii, which accelerates the natural binding of at to clotting factors. this binding is irreversible and leads to clotting factor inhibition. once at is bound to the clotting factor, the heparin molecule dissociates from the complex and repeats this function.

clotting factors affected by ufh-at complex include thrombin (iia) and xa, xia, ixa, and xiia. low-molecular weight heparin (lmwh) is too small to bridge at to thrombin, but is able to bridge at to factor xa (hence the need for factor xa monitoring when using lmwh in select patients).