Molecular models of the anticoagulant heparin. Heparin is a polysaccharide (sugar) molecule found naturally in the human body. It is in greatest concentration in tissues surrounding the capillaries of the liver and lungs, where it inhibits the formation of blood clots. Heparin is made synthetically as a drug to prevent thrombosis and embolism in patients with vascular diseases. It is also used during open-heart surgery, heart bypass surgery and kidney dialysis, to prevent clotting.