The intrinsic pathway involves the contact activation factors, including XII, prekallikrein, high molecular weight kininogen (HMWK), and XI. These factors are known as contact factors because their activation is initiated by contact with the subendothelial basement membrane that is exposed at the time of a tissue or blood vessel injury. These are all of the necessary components to activate factor X.
Injury to endothelial cells can begin this process. In this pathway, a complex involving factors IX and VIII, in association with ionized calcium and phospholipid on the platelets (PF3), ultimately activates factor X. To accomplish this, factor IX is first activated by the action of factor XIa. In the presence of factor V, factor Xa activates prothrombin (factor II) to thrombin, which converts fibrinogen to fibrin.
Ionized calcium plays an important role in activating certain coagulation factors in the intrinsic pathway. Calcium is not required to activate factor XII, prekallikrein, or factor XI but is necessary for activating factor IX by factor XIa.
Although the complex reactions in the intrinsic pathway take place relatively slowly, they account for most clinical conditions requiring coagulation studies involving the intrinsic coagulation system. A laboratory test that monitors the intrinsic pathway leading to fibrin clot formation is the activated partial thromboplastin time (APTT).
