Coagulation of blood (or clotting) occurs in a series of steps as follows:
- The injured tissue cells and the platelets which disintegrate at the site of the wound release a substance thrombokinase (also called thromboplastin).
- The thrombokinase acts as an enzyme and with the help of calcium ions present in the plasma, it converts a substance prothombin (inactive) of the plasma, into thrombin (active). Vitamin K, a fat-soluble vitamin is essential for the production of prothombin.
- Thrombin in the presence of calcium ions reacts with the soluble fibrinogen of the plasma to convert it into insoluble fibrin. Fibrin is a solid substance that forms threads. These microscopic threads of fibrin are sticky and form a mesh or network at the site of wound.
- Blood cells are trapped in the network of the fibrin; the network then shrinks and squeezes out the rest of the plasma which is in the form of a clear liquid, the serum. The solid mass which is left behind is called clot (or thrombus).