Heart bypass graft. Illustration of a heart that has had a blocked coronary artery treated by coronary artery bypass graft (CABG) surgery. Coronary arteries are small blood vessels on the outer surface of the heart. They supply oxygenated blood to keep the heart pumping. A blockage can cause a fatal heart attack. With CABG an artery or vein is harvested from elsewhere in the body and used to bypass the blockage.