The intrinsic pathway name is derived from the fact that all the clotting factors required for blood clotting is present “within” the plasma. Let us understand the steps involved in this pathway.

• •  The first plasma protein involved in the intrinsic pathway is called Hageman factor or factor XII. Factor XII is initially in inactive state but on activation becomes factor XIIa. Factor XII converts to factor XIIa only when it comes in contact with rough surface like the collagen fibres underlying damaged endothelium.

•  This process of activation is called contact activation and is a complex process requiring the participation of several other plasma proteins.

•  Factor XIIa then catalyzes the activation of factor XI to factor XIa which then causes activation of factor IX to factor IXa.

•  Factor IXa then activates factor X to factor Xa which is the enzyme responsible for the conversion of prothrombin to thrombin.

•  Another plasma protein factor VIIIa serves as a cofactor (not an enzyme) in the factor IXa mediated activation of factor X.