Loss of blood from the circulatory system, which may be caused by damage to the blood vessels or by a bleeding disorder. Bleeding may be visible (external) or concealed (internal). Rapid loss of more than 10 per cent of the blood volume can cause symptoms of shock, with pallor, sweating and fainting.
The speed with which blood flows from a cut depends on the type of vessel is injured: blood usually oozes from a capillary, flows from a vein and spurts from an artery. The blood can collect around the damaged blood vessels just beneath the skin and cause a bruise; this is if an injury does not break open the skin.
Any lost blood that gets mixed with other body fluids such as urine or sputum (phlegm) will usually be noticed quite readily: bleeding from the lining of the digestive tract may make faeces or vomit appear darker than usual. Until severe anaemia develops internal bleeding may not be discovered.