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Blood heparin research. Researcher uses a pipette in diluting samples of the blood anticoagulant heparin. This is part of an assay to determine the potency of low molecular weight heparin. It varies in molecular weight according to the methods used for its isolation. Heparin is produced in liver cells, some white blood cells, and certain other sites. It acts by inhibiting the action of the enzyme thrombin in the final stage of blood coagulation. An extracted purified form of heparin is used for the prevention of blood coagulation in patients with thrombosis and similar conditions. (KEYSTONE/SCIENCE PHOTO LIBRARY/ROBERT LONGUEHAYE, NIBSC)