Substances that prevent blood from clotting are called anticoagulants. Blood contains 50 types of procoagulant and anticoagulant factors. Heparin (mast cells), hirudin (leeches), thrombomodulin, sodium citrate, sodium oxalate etc. are anticoagulants. Heparin, hirudin and thrombomodulin inhibit the generation of thrombin from blood prothrombin. Sodium citrate reacts with calcium ions in the blood to form calcium citrate. Calcium citrate is degraded. Sodium oxalate reacts with calcium ions in the blood to form calcium oxalate. Calcium oxalate is excreted and causes kidney stones.