Heart valve surgery. Image 17 of 24. Artificial (prosthetic) heart valve being stitched into place. The plastic tubes attach the patient to a heart-lung machine, which maintains the patient's circulation while the heart is temporarily stopped. The retractor (metal bars) keeps the chest open. The dark blue threads are sutures used to stitch the valve 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 a valve cannot be repaired it must be replaced. For a sequence showing the operation, see images M560/494 to M560/517.