Heart valve replacement. Image 2 of 6. Surgeon stitching a heart in preparation for the attachment of an artificial (prosthetic) heart valve. The sutures will be used to position and hold the valve in place when it is inserted into the patient's heart. The plastic tubes (centre top and bottom) are connected to a heart- lung machine, which maintains the patient's vital functions 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 the valve cannot be repaired then it must be replaced. See images M560/445 to M560/450.