Heparin. Molecular graphic of heparin, an anticoagulant produced by the human body and used as a drug to treat thrombosis. Heparin is a large, complex polysaccharide with a linear shape, seen here end-on. The regularly spaced sulphate groups are emphasized by representing sulphur atoms as large yellow spheres. The rest of the molecule is represented by red sticks. It is produced by basophil white blood cells and by mast cells in the liver, and acts by preventing conversion of prothrombin to thrombin during the blood clotting process.