1. The process of converting the liquid blood into a semisolid form is called blood clotting or coagulation.

2. The process of clotting may be initiated by contact of blood with any foreign surface (intrinsic process) or with damaged tissue (extrinsic process). 

3. Intrinsic and extrinsic processes involve interaction of various substances called clotting factors by a step wise or cascade mechanism. 

4. There are in all twelve clotting factors numbered as I to XII (factor VI is not in active use). 

5. Interaction of these factors in a cascade manner leads to formation of enzyme, Thromboplastin which helps in the formation of enzyme prothrombinase. 

6. Prothrombinase inactivates heparin and also converts inactive prothrombin into active thrombin. 

7. Thrombin converts soluble blood protein- fibrinogen into insoluble fibrin. Fibrin forms a mesh in which platelets and other blood cells are trapped to form the clot.

8. These reactions occur in 2 to 8 minutes. Therefore, clotting time is said to be 2 to 8 minutes.