Haemostasis is a natural response of the body to stop bleeding and loss of blood. Coagulation or blood clotting, often referred to as secondary haemostasis, is the final step in the process of haemostasis. It causes the blood to form solid masses (clots). When a blood vessel is damaged it constricts to allow less blood to be lost. Platelets adhere to the walls of the damaged blood vessel to form a platelet plug which temporarily seals the break in the vessel. At the same time clotting factors are released due to trigger by the injured blood vessel and the platelets. They activate a complex series of events to form a clot which consists of a fibrin mesh around the platelet plug. The clot reinforces the platelet plug. Problems with coagulation can result in haemorrhage (bleeding) or thrombosis (obstructive clotting). In haemophilia there is impaired blood clotting due to lower or impaired clotting factors.