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 they are activated by contact with the exposed subendothelial basement membrane during tissue or blood vessel injury. These are all necessary components to activate factor X.
Injury to endothelial cells can begin this process. In this pathway, a complex involving factors IX and VIII, associated with ionized calcium and phospholipid on the platelets (PF3), ultimately activates factor X. To accomplish this, factor IX is first activated by factor XIa. In the presence of factor V, factor Xa activates prothrombin (factor II) to thrombin, which converts fibrinogen to fibrin.
Ionized calcium is vital 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 occur 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).
