Pathway in which coagulation is initiated by components contained within the vascular system (independent of Tissue Factor). In vitro the contact system is exploited for testing purposes (particulate matter used for activation) however its role in vivo is questionable. Physiologically the intrinsic pathway results in the activation of FIX by FXIa in the presence of calcium ions whereas FIX activation by FVIIa requires Tissue Factor. The APTT is affected by all coagulation proteins in the intrinsic pathway (FXII, FXI, FIX, FVIII) and those of the common pathway. The slide show entitled Intrinsic Pathway-APTT shows the components of this pathway.
