heparin, anticoagulant produced by cells in many animals. A polysaccharide, heparin is found in the human body and occurs in greatest concentration in the tissues surrounding the capillaries of the lungs and the liver. The substance, extracted from animal tissues, is used clinically to delay blood clotting.

Naturally occurring mixture of organic compounds used as a short-term anticoagulant to prevent thrombosis during and after surgery and for initial treatment of various heart, lung, and circulatory disorders in which there is increased risk of blood clotting. Comprising complex carbohydrate molecules called mucopolysaccharides, it normally is present in the human body in liver and lung tissues. It was discovered in 1922 and originally used to prevent clotting in blood taken for laboratory tests.

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