Heart valve replacement. Image 3 of 6. Artificial (prosthetic) heart valve (upper centre) being lowered into place. The plastic tubes (bottom) are attached to a heart- lung machine, which maintains the patients vital functions while the heart is temporarily stopped. The metal bars are part of the retractor, which keeps the chest open. Sutures (dark green thread), attached to the valve and heart, help the valve to be lowered into place. There are four valves in the human heart, all of which prevent the backflow of blood. Damage to the valves can lead to malfunctioning of the heart and death. If the valve cannot be repaired then it must be replaced. M560/445 to 450