MODEL RELEASED. Heart valve surgery. Image 23 of 24. Surgeons performing heart valve replacement surgery. They are replacing a diseased valve with an artificial (prosthetic) heart valve. The plastic blood-filled tubes attach the patient to a heart-lung machine (bottom), which maintains the patient's circulation 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 a valve cannot be repaired it must be replaced. For a sequence showing the operation, see images M560/494 to M560/517.