intrinsic pathway
Summary
The intrinsic pathway is one of two initial pathways of the coagulation cascade, activated by exposure of blood to subendothelial collagen or negatively charged surfaces (e.g., in vitro contact activation). It involves factors XII, XI, IX, and VIII, converging with the extrinsic pathway at factor X activation. Its function is measured clinically by the partial thromboplastin time (PTT).
Detail
The intrinsic pathway begins when factor XII (Hageman factor) is activated by contact with exposed collagen, damaged endothelium, or artificial negatively charged surfaces (e.g., glass tubes in lab testing). Activated XII (XIIa) converts prekallikrein to kallikrein and activates factor XI to XIa. Factor XIa activates factor IX to IXa, which then combines with its cofactor factor VIIIa, calcium, and phospholipid surface (tenase complex) to activate factor X to Xa. This converges with the extrinsic pathway at the common pathway, where Xa combines with Va to convert prothrombin (II) to thrombin (IIa), which cleaves fibrinogen to fibrin.
Clinical significance: The intrinsic pathway is assessed by the partial thromboplastin time (PTT), which is prolonged in deficiencies of factors XII, XI, IX, VIII, or with use of unfractionated heparin (which potentiates antithrombin III, inhibiting thrombin and factor Xa). Hemophilia A (factor VIII deficiency) and Hemophilia B (factor IX deficiency, Christmas disease) both prolong PTT and cause deep tissue/joint bleeding. Factor XII deficiency prolongs PTT but does NOT cause a bleeding disorder clinically (paradoxically can predispose to thrombosis) - a classic board question. Von Willebrand disease can also prolong PTT due to reduced factor VIII stability (vWF carries factor VIII).
The intrinsic and extrinsic (tissue factor/factor VIIa driven, measured by PT) pathways are largely a laboratory construct; in vivo, the tissue factor-factor VIIa complex is the primary physiologic initiator of coagulation, with the intrinsic pathway (particularly factor XI) serving to amplify thrombin generation and stabilize the clot. This explains why factor XII deficiency doesn't cause bleeding despite prolonging PTT, while factor VIII/IX deficiencies do cause bleeding despite intact extrinsic pathway - amplification via intrinsic pathway is needed for adequate thrombin burst.
Sources
- First Aid for the USMLE Step 1
- Robbins and Cotran Pathologic Basis of Disease
- Goldman-Cecil Medicine
- Guyton and Hall Textbook of Medical Physiology
Reviewed by AnkiBoss editorial — medical student review. Information here is for study reference only and is not medical advice. Spotted an error? Let us know.