Arteries are the blood vessels which carry blood away from the heart. In comparison to veins, arteries have narrow lumen and more muscular thick walls to bear the pressure of pumping action of heart. Angina is constricting chest pain caused due to reduced blood supply to the heart wall. Persons with blood group-AB are universal acceptor, while those with blood group-O are universal donor. Calcium ions are required during blood clotting for the conversion of prothrombin into thrombin, and fibrinogen into fibrin.