Surgery to replace a damaged mitral heart valve, showing surgeons handling the numerous sutures (stitches) that locate the replacement valve in the heart. The mitral valve divides the left atrium and ventricle of the heart, and consists of two cusps (flaps). It allows blood to pass from atrium to ventricle, but prevents backflow. Damaged valves do not close properly and a reflux (backflow) of blood occurs, with potentially fatal results. Many types of artificial valves exist, some made from metal and fabrics. In this operation the replacement valve was made from natural tissue, obtained from porcine pericardium (the tough membrane that surrounds a pig's heart).