The fundamental reaction in the clotting of blood is conversion of the soluble plasma protein fibrinogen to insoluble fibrin. Blood clotting is achieved in several steps-
(i) Blood platelets disintegrate on coming in contact with air and release platelet factor- 3 or platelet thromboplastin.
(ii) The traumatised cells at the place of injury also release tissue thromboplastin.
(iii) Platelet thromboplastin and tissue thromboplastin combine to form an enzyme prothrombinase in the presence of Ca++.
(iv) Then prothrombinase inactivates heparin and catalyzes prothromin into thrombin.
(v) Thrombin acts on fibrinogen and converts it into insoluble form, the fibrin.
(vi) Fibrin molecules undergo polymerisation and form fibrin threads.
(vii) Fibrin threads form a network on the wound in which RBCs entangled constituting a clot and block the passing out RBC.
(viii) The pale yellow coloured fluid oozing out from the clot is serum.
(ix) Time taken for normal blood clotting varies from 4-10 minutes.
(x) Vitamin K is essential for normal blood clotting. Vitamin K is given to a new born child immediately after delivery to facilitate clotting of blood of the umbilical cord. A newborn baby especially a premature baby has low levels of prothrombin and clotting factors due to low capacity to synthesize clotting factors compounded by deficiency of vitamin K.
To occur these changes i.e. the explained extrinsic system which is triggered by the release of tissue thromboplastin (factor III), various factors are also needed which are collectively known as the intrinsic system because it occurs inside blood vessels.
Hence, the entire mechanism of clotting is referred to as a cascade mechanism since it involves a sequential arrangement of a series of reactions each involving specific enzymes and substrates and other molecules. If any of these is missing or any step is blocked the whole mechanisms fails.