The coagulation cascade as described in manuscripts is an over simplification of a very complex phenomenon. Traditionally this cascade has been divided into intrinsic and extrinsic pathways. However, both these pathways are in fact intimately linked.

Steps in Coagulation

• Vessel wall injury.
  
• Platelet aggregation.
  
• Tissue factor (TF) exposure.
  
• Coagulation cascade (intrinsic and extrinsic pathways).

The extrinsic pathway is the most important pathway in vivo!

Steps in the Coagulation Cascade

• The extrinsic pathway is initiated by subendothelial collagen exposure, which in turn stimulates platelet aggregation and TF (thromboplastin) release.
  
• Factor VII (FVII) is then activated (aFVII), which in turn combines with TF and is inhibited by tissue factor pathway inhibitor (TFPI).
  
• aFVII-TF converts factor X (FX) to activated factor X (aFX) and is the point of convergence of the intrinsic and extrinsic pathways.
  
• aFX converts prothrombin to thrombin, which magnifies the coagulation cascade by positive feedback on numerous factors.
  
• Thrombin converts soluble fibrinogen to insoluble fibrin, which forms the thrombus plug (combined with aggregated platelets).
  
• Thrombin also independently activates platelets (PAR-1 and ADP receptors) and releases calcium (necessary for coagulation).