The popularity of bloodletting in the Renaissance followed the ideas of Galen. There were two key concepts. The first was that blood was created and then used up, it did not circulate and so it could 'stagnate' in the extremities. The second was that humoural balance was the basis of illness or health. Galen believed that blood was the dominant humour and the one in most need of control. In order to balance the humours, a physician would either remove 'excess' blood (plethora) from the patient or give them an emetic (to induce vomiting) or diuretic (to induce urination). Even after the humoral system fell into disuse, the practice was continued by surgeons and barber-surgeons. Though the bloodletting was often recommended by physicians, it was carried out by barbers. This division of labour led to the distinction between physicians and surgeons. The barbershop's red-and-whitestriped pole, still in use today, is derived from this practice: the red represents the blood being drawn, the white represents the tourniquet used, and the pole itself represents the stick squeezed in the patient's hand to dilate the veins. Bloodletting was used to 'treat' a wide range of diseases, becoming a standard treatment for almost every ailment, and was practiced prophylactically as well as therapeutically.

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