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.