When an injury is caused to a blood vessel bleeding starts which is stopped by a process called blood clotting or blood coagulation. This process can be described under three major steps. 

(i) First step : 

(a) At the site of an injury, the blood platelets disintegrate and releases platelet factor-3 (= Platelet thromboplastin). 

(b) Injured tissues also release thromboplastin. 

(c) These two factors combine with calcium ions (Ca⁺⁺) and certain proteins of the blood plasma to form an enzyme called prothrombinase. 

(ii) Second step : 

(a) The prothrombinase inactivates heparin (or antiprothrombin-anticoagulant) in the presence of calcium. 

(b) Prothrombinase catalyzes breakdown of prothrombin (inactive plasma protein) into an active protein thrombin. 

(iii) Third step : 

(a) Fibrins are formed by the conversion of inactive fibrinogens in the plasma by the enzyme thrombin. 

(b) The fibres of fibrin form a dense network upon the wound and trap blood corpuscles to form a clot. 

(c) The clot seals the wound and stops bleeding.