The process of blood clotting starts when prothrombinase catalyzes the conversion of prothrombin (an inactive protein in the blood plasma) into thrombin (active protein) in the presence of Ca2+ ion (blood clotting factor IV). Thrombin converts fibrinogen into fibrin that makes fibrin threads on polymerization with blood corpuscles and forms blood clots. Therefore, if Ca2+ is removed from the blood it will prevent clotting. It may also be done by adding some anticoagulants, e.g. sodium or potassium oxalates, EDTA.