MODEL RELEASED. Heart surgery. Image 13 of 24. Surgeons performing heart surgery. The plastic tubes attach the patient to a heart-lung machine, which maintains the patient's circulation while the heart is temporarily stopped. The doctor at left is monitoring the machine's function and the patient's vital statistics. This is heart valve replacement surgery. An artificial (prosthetic) valve will replace a diseased valve. 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.