Anticoagulants are substances that inhibit coagulation, or clotting, of the blood. They are used to keep blood for transfusions from clotting; to treat conditions involving dangerous blood clotting, such as cerebral thrombosis and coronary heart disease; and in situations where there is a risk of dangerous clotting, such as during some surgical operations, in particular the installation of artificial heart valves.

Donated blood tends to clot before it is absorbed into the recipient 's circulatory system. Sodium phosphate was introduced as an anticoagulant to overcome this problem in 1869. In 1914 sodium citrate was shown to be an effective and harmless anticoagulant for donated blood; its use was of great value during World War I and had become standard by 1917.

For centuries, medical practitioners had used leeches to suck blood from patients. In 1884 J. B. Haycraft showed that blood flowed freely during this procedure because the leeches secreted an anticoagulant...