The cross section of a vein showing the action of Heparin, an anticoagulant drug. As seen in this exhibit, a deep vein thrombosis (DVT) within a deep vein of the leg shows reduced blood flow as a result of the clot. Heparin is then used to prevent the growth of the thrombus, causing the body to then respond by breaking down the clot over time until the clot is eventually cleared completely.