Final answer:
Heparin is an anticoagulant that works by inhibiting the conversion of prothrombin to thrombin. It is stored in mast cells and is released when tissues are injured.
Step-by-step explanation:
Heparin is an anticoagulant substance that opposes coagulation. It works by inhibiting the conversion of prothrombin (factor II) into thrombin in the common pathway of blood clotting. Heparin is also found on the surfaces of cells lining the blood vessels. It is stored in mast cells and released when tissues are injured.