Bleeding occurs when a blood vessel is dam­aged. If the vessel is internal, blood seeps into surrounding tissue, and a bruise forms. Where delicate blood vessels are near the surface of tissue, as they are in the nose, for example, a very slight injury or irritation may cause bleeding.For most people, minor bleeding causes no harm because the body soon stops it. It does his by means of three main mechanisms that act together. The nearby blood vessels contract, and restrict the flow of blood to the area of the wound. The platelets in the blood gather where the blood vessels are damaged, and stick to the vessel walls and to each other to form a plug. In addition, interlacing strands of a material called fibrin form in the damaged area. Blood cells are then trapped in the fibrin mesh and form a clot that seals the break and stops the bleeding.

In diseases that cause abnormal bleeding, one or more of the mechanisms that halt blood loss does not work right. Bleeding from a cut, which would normally stop within five or ten minutes, may continue for hours, or even days. Minor injuries may cause extensive bruising. There may be internal bleeding, and bleeding in the joints may produce acute pain and eventually cause crippling damage. Two of the most common of such disorders, hemophilia and thrombocytopenia, are discussed here.