The substances which prevent blood coagulation are known as anticoagulants. The natural anticoagulants found in blood are heparin, antithrombin and plasmin.
- Heparin is the most powerful anticoagulant secreted by liver cells and mast cells. It neutralizes the conversion of prothrombin to thrombin. Antithrombin inhibits any thrombin formed accidently.
- Plasmin is an inactive proteolytic enzyme, which is activated by certain substances found in the uterine wall. Hence normal menstrual blood generally does not clot.
- Another naturally occurring anticoagulant is hirudin found in the saliva of Leech(Hirudinaria granulose).
- The anticoagulant used in the blood bank to prevent coagulation are oxalates of Sodium and potassium, sodium citrate, EDTA (Ethylene Diamine Tetra Acetic acid) etc. Chilling of blood delays blood coagulation since low temperature depresses the action of clotting promoting factors.