GENERAL MECHANISM OF BLOOD COAGULATION -

All researchers agree that blood clotting is a cascade mechanism and involves three essential steps - (a) Formation of prothrombin activator. It is formed in two ways :

Extrinsic pathway -

Intrinsic pathway

(b) Conversion of prothrombin into thrombin -

(c) Conversion of fibrinogen into fibrin -

                            Thrombin

Fibrinogen                                                         Fibrin Threads (insoluble) 

(soluble)

Fibrin threads form a meshwork which traps blood cells and platelets to form a clot so the process is called blood clotting. Within a few minutes, the clot begins to contract and a pale coloured fluid called serum is discharged. This is called clot-retraction.