Multiple choice The coagulation of blood in vessels is prevented during normal conditions by heparin plasminogin calcium prothrombin Reveal answer Fill a bubble to check yourself A Correct answer Explanation Heparin, produced by mast cells and basophils, acts as a natural anticoagulant by enhancing antithrombin III activity, which inhibits thrombin and factor Xa. This prevents inappropriate clotting in intact blood vessels during normal circulation.