Explanation Process of clotting of blood At the site of injury, the blood vessels and the blood platelets release a substance called thrombokinase or thromboplastin. Thromboplastin in the presence of calcium ions converts prothrombin present in the plasma to thrombin. Prothrombin is a plasma protein produced in the liver when Vitamin K is present. Thrombin, acts as an enzyme, and reacts with the soluble protein fibrinogen present in the plasma and converts it into insoluble threads called fibrin. Fibrin forms sticky, microscopic threads that form a network on the wound. Corpuscles are trapped in the network. The network then shrinks. When the network of fibrin shrinks, it squeezes out the rest of the plasma, which is the serum. Serum is blood plasma without the protein fibrinogen and blood corpuscles. The solid mass that is left behind is called the clot or the thrombus. The final result of clotting is a hard lump, which can plug the damaged blood vessels. This prevents further loss of blood and also the entry of micro-organisms from outside. Haemophilia Haemophilia is a genetic disorder. The disease is characterised by deficient blood clotting that results in severe bleeding internally or externally as the blood lacks fibrin. This disease is controlled by two closely linked genes present on the X- chromosomes. Individuals suffering from Haemophilia A lack antihaemophilic globulin or AHG and individuals suffering from Haemophilia B lack plasma thromboplastin. Haemophilia A is more common and is treated by antihaemophilic globulin or AHG transfusions.