Blood is of prime importance in the normal physiologic function of our major organ systems. An important function of blood is to maintain an intact circulatory system following trauma. The process of blood clotting, or coagulation, is a very rapid response to tissue damage where the exposure of tissue factor (also known as thromboplastin) containing cells to the bloodstream through damage to vessel walls initiates a cascade system. 

The goal of the cascade is to form fibrin which will form a mesh within the platelet aggregate to stabilize the clot. The cascade system uses a series of enzymes to work in rapid succession: Once activated, tissue factor binds and activates factor VII which initiates the cascade.

Factor VII is activated through auto-cleavage to factor VIIa which along with co-factor VIIIa converts factor X to Xa . Factor Xa along with co-factor Va converts factor II (prothrombin) to factor IIa (thrombin).  

Finally factor IIa (thrombin) converts fibrinogen to fibrin leading to fibrin deposition and the activation of platelets to form blood clots.

Haemostasis occurs in the following major events:

  1. Vascular constriction limits the flow of blood to the area of injury.  Coagulation begins almost instantly after an injury to the blood vessel has damaged the endothelium.
  2. Platelets become activated by thrombin and aggregate at the site of the injury to form a temporary loose platelet plug. Platelets clump by binding to collagen that becomes exposed following rupture of the endothelial lining of vessels.
  3. To enhance stability of the initially loose platelet plug, a fibrin mesh (also called the clot) forms and entraps the plug.
  4. Finally, the clot must be dissolved in order for normal blood flow to resume following tissue repair. 
Thrombin has a large array of functions. Its primary role is the conversion of fibrinogen to fibrin that is the building block of the haemostatic plug.