The natural response of the body to stop bleeding (haemostasis) consists of blood vessel constriction, platelet plug formation and blood clotting (coagulation).
Coagulation of blood involves proteins called clotting factors.
A total of 13 clotting factors – designated by Roman numerals I to XIII – are found in the blood. Injury to blood vessel triggers the release of these clotting factors in a certain order. They activate a complex series of events to form a clot which consists of a fibrin mesh around the platelet plug. Clotting is initiated by two processes or pathways – a slower but larger intrinsic pathway (contact activation pathway) or a very rapid extrinsic pathway (tissue factor pathway) – which lead to fibrin formation.
Clotting factor disorders can result in improper clot formation or haemorrhage (bleeding), or thrombosis (obstructive clotting). In haemophilia there is impaired blood clotting due to lower or impaired clotting factors. Haemophilia A is due to factor VIII deficiency, haemophilia B is due to factor IX deficiency and haemophilia C is due to factor XI deficiency. In thrombophilia there is an increased tendency to clot mostly due to an abnormality of a blood-clotting factor which leads to an increase in their function.