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bloodletting,also called bleeding, practice of drawing blood from the body in the treatment of disease. General bloodletting consists of the abstraction of blood by incision into an artery (arteriotomy) or vein (venesection, or phlebotomy). Local bloodletting is the abstraction of blood from smaller vessels by watercupping or by leeching. From antiquity through the 18th cent. bloodletting was widely practiced in western medicine. A broad assortment of ailments were believed to result from the impurity or superabundance of blood in the system; periodic bloodletting was felt to assure the patient of good health. In modern times the medicinal leech (Hirudo medicinalis) is still used in some areas of the world for the removal of blood from bruises and black eyes. Venesection is employed to treat erythremia, an abnormal condition characterized by the overproduction of red blood cells, and to relieve the congestion of blood resulting from acute heart failure.
the drawing of blood from the blood vessels (most often from a vein) for therapeutic purposes.
In modern medicine the indications for bloodletting are strictly limited. It is effective when there is acute cardiac insufficiency and it is necessary to decrease the flow of blood to the heart, thus lightening its work; when there is edema of the lungs, in order to decrease the quantity of blood in the pulmonary vessels; in serious hypertensive crises to rapidly lower the blood pressure; in some blood diseases; and in some types of poisoning, such as carbonmonoxide or illuminating gas (which contains carbon monoxide) poisoning. In bloodletting 200–400 milliliters of blood are usually drawn by puncturing or incising the vein and sometimes by using medicinal leeches.