Coagulation cascade
Also known as: clotting cascade, secondary hemostasis
The coagulation cascade is the sequence of enzyme activations that converts circulating clotting factors into a stable fibrin clot. Its intrinsic and extrinsic pathways converge on a common pathway that activates thrombin and forms fibrin.
The coagulation cascade is the second stage of hemostasis. After a platelet plug forms at an injured vessel (primary hemostasis), the cascade reinforces it: a chain of serine proteases, each activating the next, amplifies a small trigger into a burst of thrombin, which converts soluble fibrinogen into insoluble fibrin strands that mesh the plug into a stable clot.
The cascade is traditionally drawn as three pathways. The extrinsic pathway starts when tissue injury exposes tissue factor, which activates factor VII — this is the physiologic trigger and is measured by prothrombin time (PT). The intrinsic pathway involves factors XII, XI, IX, and VIII and is measured by activated partial thromboplastin time (aPTT). Both converge on the common pathway: factor X activation, conversion of prothrombin (factor II) to thrombin, and fibrin formation, with factor XIII cross-linking the clot. Most factors are made in the liver, and several (II, VII, IX, X) require vitamin K.
The cascade explains major bleeding and clotting disorders and their lab patterns: hemophilia A (factor VIII deficiency) and hemophilia B (factor IX) prolong aPTT, warfarin blocks the vitamin K-dependent factors and prolongs PT/INR, and heparin potentiates antithrombin to inhibit thrombin and factor Xa.
USMLE Step 1 tests the cascade in hematology — matching factor deficiencies and anticoagulants to PT or aPTT changes is a recurring question type — while the MBLEx covers clotting at a conceptual level within its cardiovascular and blood physiology content.
Key takeaways
- The cascade amplifies a vascular injury signal into thrombin, which converts fibrinogen to fibrin and stabilizes the platelet plug.
- The extrinsic pathway (tissue factor, factor VII) is measured by PT; the intrinsic pathway (XII, XI, IX, VIII) by aPTT.
- Both pathways converge on the common pathway: factor X, thrombin, fibrin, and factor XIII cross-linking.
- Factors II, VII, IX, and X are vitamin K-dependent and are inhibited by warfarin; heparin acts through antithrombin.
- USMLE Step 1 tests factor deficiencies and anticoagulant effects on PT and aPTT; the MBLEx covers clotting within blood physiology.