Contact activation pathway in blood coagulation begins with which factor?

The contact activation pathway in blood coagulation begins with Factor XII. This pathway is crucial for initiating the intrinsic pathway of coagulation, which is triggered when blood comes into contact with negatively charged surfaces, often exposed by vascular injury.

When Factor XII is activated, it goes on to activate Factor XI, which then activates Factor IX, leading to a cascade that ultimately results in the generation of thrombin and the formation of a clot. Factor XII is sensitive to contact with surfaces and is not dependent on tissue factor, which differentiates it from the extrinsic pathway that is initiated by Factor VII when it binds to tissue factor.

In summary, Factor XII serves as the critical initiator of the contact activation pathway, setting off a series of activations that contribute to the overall coagulation process, underscoring its central role in hemostasis.