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Bleeding is the loss of blood from the human body. Children are put more in danger by bleeding as they have less blood to lose. The average adult can lose approximately 2 quarts of blood without lasting harm but will be in medical danger after 4 quarts and could die of hypovolemic shock if more blood is lost.

A major technique of first aid is to control bleeding through direct pressure or the application of a bandage over the wound.

Blood lost by bleeding can be replaced by blood transfusion.

In extreme cases of an injured limb a tourniquet may be used; the injured limb must generally be amputated afterwards, just below the level the tourniquet is applied. This is "losing a limb to save a life". However, most first aid instruction no longer teaches the use of the tourniquet because the risk may be greater than the benefit.

Bleeding, or bloodletting, is one of the oldest medical practices, practiced among diverse peoples, including the Greeks, the Egyptians and the Mesopotamians.

Hippocrates mentions bloodletting but in general relied on dietary techniques. Erastistratus, however, theorized that many diseases were caused by plethoras, or overabundances, in the blood, and advised that these plethoras be treated, initially, by exercise, sweating, reduced food intake, and vomiting. Herophilus advocated bloodletting. Archagathus, one of the first Greek physicians to practice in Rome, practiced phlebotomy extensively and gained a most sanguinary reputation. Galen, too, advocated bloodletting, of a specific nature determined by the disease: arterial or venous, and distant or close to the area of the body affected. He linked different vessels with different organs, according to their suppoed drainage: the vein in the right hand would be let for liver problems: the vein in the left hand, for problems with the spleen, and the more severe the disease, the more blood would be let. Fevers required copious amounts of bloodletting.

The Talmud recommended specific day of the week and days of the month for bloodletting, and similar rules, though less codified, can be found among Christian writings advising which saint's days were favourable for bloodletting. Islamic authors too advised boodletting, particularly in fevers, and when Islamic theories became known in the Latin-speaking countries of Europe, bloodletting became more widespread.

The venesection itself was often recommended by physicians but carried out by barbers, a division of labours that led to the distinction between physicians and surgeons. Bloodletting was practiced prophylactically as well as therapeutically.

The practice continued throughout the Middle Ages but began to be questioned in the sixteenth century, particularly in northern Europe and the Netherlands. In France, the court and university physicians advocated frequent phlebotomy. In England, the efficacy of bloodletting was hotly debated, declining throughout the eighteenth century, and briefly revived for treating tropical fevers in the nineteenth century. Benjamin Rush was notorious in the United States for the copious bloodletting he practiced.

Today the inefficacy of bloodletting for most diseases is well established. It is practiced scientifically in only a few instances where an excess of blood is actually involved in causing disease, such as polycythemia vera.