HEPARIN

anticoagulant drug that is used to prevent dangerous blood clots from forming during and after surgery and in various heart, lung, and circulatory disorders in which there is an increased risk of blood-clot formation. Heparin is a naturally occurring mixture of mucopolysaccharides and was discovered by W.H. Howell in 1922. It is present in the human body in tissues of the liver and lungs. Heparin was originally used to prevent the clotting of blood taken for laboratory tests. Its use as a therapy for patients who already have a blood clot in a vein (venous thrombosis) began in the 1940s; low-dose heparin treatment to prevent blood clots from forming in patients who are at high risk for pulmonary embolisms and other clotting disorders was introduced in the early 1970s. Heparin's biological activity depends on the presence of natural antithrombin, a substance in blood plasma that binds and deactivates serum clotting factors. Heparin accelerates the binding rate of antithrombin by a factor of 1,000, thus greatly reducing the blood's ability to clot. Heparin is poorly absorbed by the intestine and so must be given by injection. Most commercial heparin is obtained from cow lungs or pig intestines. Because of its anticlotting effect, the drug creates a significant risk of excessive bleeding; the latter is usually controlled by monitoring the dosage of heparin given and decreasing it if necessary.