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Surgery in progress to replace a mitral heart valve, showing the heart exposed through the large incision in the patient's chest wall. The tubes carrying blood are connected to a heart-lung machine (not in view), used to maintain the patient's vital functions during the operation. The mitral valve allows a one-way flow of blood from left atrium to left ventricle, and prevents backflow. Damaged valves do not fully close, and a reflux of blood occurs, with potentially fatal results. Artificial valves may be made of metal, fabrics and, as in this case, of natural tissue obtained from porcine pericardium (the membrane that surrounds a pig's heart). (KEYSTONE/SCIENCE PHOTO LIBRARY/MARTIN DOHRN.)