Heparin blocks which clotting pathways?

Heparin is an anticoagulant that primarily functions by inhibiting the intrinsic pathway of the coagulation cascade. This pathway involves several factors, including factor XII, XI, IX, and VIII, which are all critical for the amplification of the clotting process. Heparin enhances the activity of antithrombin III, a protein that inactivates several enzymes involved in the clotting process, particularly thrombin (factor IIa) and factor Xa.

By targeting the intrinsic pathway, heparin effectively slows down the formation of fibrin clots, providing key therapeutic benefits, especially in the management of various thromboembolic disorders. The intrinsic pathway is crucial in the development of thrombus, making heparin an effective choice in preventing and treating blood clots.

While heparin influences the common pathway to some extent since factor Xa and thrombin are involved in both the intrinsic and common pathways, its primary mechanism is through the intrinsic pathway. The extrinsic pathway is primarily initiated by tissue factor and is less affected by heparin. Additionally, heparin does not directly inhibit platelet aggregation; instead, its anticoagulant effects are largely due to its impact on specific clotting factors.