Bruising is caused by bleeding from vessels into the tissues, usually following trauma. The bruise changes colour from blue to green to yellow as a result of the haemoglobin degrading and being removed. Bruising may be excessive in a number of conditions affecting clotting Normal blood clotting mechanisms involve both platelets, which stick together when activated by the damage to the vessel wall, and the clotting factors which act together in a cascade to produce the strands of fibrin which, by sticking on the mass of platelets, build the blood clot.

Conditions which cause a reduction in the number of platelets - thrombocytopaenia - produce bruising and bleeding. Deficiency of clotting factors (haemophilia, liver disease such as occurs in alcoholism, vitamin K deficiency) can cause prolonged bleeding following trauma or during surgery.