Heart valve replacement. Image 6 of 6. Artificial (prosthetic) heart valve being stitched into place. The plastic tubes (top right) are attached to a heart-lung machine, which maintains the patient's vital functions while the heart is temporarily stopped. The dark green threads are sutures, used to stitch the valve in 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. For a sequence showing heart valve replacement see images M560/445 to M560/450.