Heart valve surgery. Image 19 of 24. Artificial (prosthetic) heart valve (not seen) being attached. The sutures (white thread), attached to the valve and heart, help to position and hold the valve in 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. 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.