MODEL RELEASED. Heart surgery. Surgeon (right) and nurse performing coronary bypass surgery. Blood is being transfused into the patient to replace blood lost during the surgery. The operation involves grafting blood vessels onto the heart to bypass blocked or narrowed coronary arteries, the arteries that supply oxygen to the heart muscle. The grafted blood vessels are usually taken from the patient's leg. Blocked coronary arteries cause angina pectoris (chest pain) and heart attacks (myocardial infarctions).