A highly sulfated mucopolysaccharide isolated from mammalian (chiefly beef) tissues, with blood anticoagulant activity. Heparin was first found in abundance in the liver, hence the name, but it is present in substantial amounts in the spleen, muscle, and lung as well. Chiefly composed of D-glucosamine and D-glucuronic acid residues, it usually has a molecular weight from 6000 to 20,000. It normally occurs as the sodium or potassium salts and undergoes some degree of desulfation when the free acid is liberated. Full desulfation of heparin yields heparamine, which on treatment with carboxylic acid anhydrides furnishes heparides, that is, amides of the polysaccharide, free of anticoagulant properties but still valuable for treating hyperlipidemia. In the blood of most mammals, heparin is an antagonist to thrombin, prothrombin, and thromboplastin. It lessens the tendency of platelets to agglutinate. When it is injected, its action is almost instantaneous, but it begins to decline rapidly in 2–3 h. Heparin is only effective when injected, and the site of the injection often becomes swollen or inflamed and causes pain. It is used in the treatment of venous thrombosis, embolism, myocardial infarction, and certain types of cerebral thrombosis. Heparin is the single most important anticoagulant used in medicine.