QS Study

Mechanism of Clot Formation through Intrinsic Pathway

Blood coagulation refers to the process of forming a clot to stop bleeding. The intrinsic pathway is activated by trauma inside the vascular system and is activated by platelets, exposed endothelium, chemicals, or collagen. This pathway is slower than the extrinsic pathway but more important. It involves factors XII, XI, IX, VIII. The intrinsic mechanism for blood clotting begins with trauma to the blood itself and continues through the following series of cascading reactions –

  • Blood trauma causes activation of factor XII and release of platelet phospholipid.
  • Activated factor XII acts on factor XI to activate it. HMW kinogen, prekallikrein stimulates this step.
  • Activated XI acts on IX in presence of Ca2+ to activate it.
  • Activated IX acts on X in presence of Factor VIII, Ca2+, platelet phospholipid to activate it.
  • Activated factor X combines with factor V and tissue phospholipid to form prothrombin activator.
  • Within few seconds, this prothrombin activator splits prothrombin to thrombin with the help of platelet phospholipid.

Fig: Clot Formation through Intrinsic Pathway

The intrinsic pathway is activated when blood comes into contact with sub-endothelial connective tissues or with a negatively charged surface that is exposed as a result of tissue damage.