Heart valve replacement. Image 4 of 6. Artificial (prosthetic) heart valve (upper left) being lowered into place. The sutures (dark green thread), attached to the valve and heart, help to position and hold the valve in place. The plastic tubes (top right) are connected to a heart-lung machine, which maintains the patient's vital functions while the heart is temporarily stopped. 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. For a sequence showing heart valve replacement see M560/445 to M560/450.