Which is the anti-coagulant substance in blood?

The primary anticoagulant substance in blood is heparin, which is produced by mast cells and basophils. Heparin works by enhancing the activity of antithrombin III, a protein that inhibits thrombin and other clotting factors, thus preventing excessive blood clotting. Additionally, other anticoagulants like protein C and protein S also play crucial roles in regulating coagulation.