Perfusionists construct extracorporeal circuits to allow the cardiothoracic surgeon to connect the heart-lung machine to an open heart surgery patient. Whether the patient is older or newborn, whether receiving coronary artery bypass grafting, cardiac valve replacement or surgical correction of a cardiac birth defect, the perfusionist operates the heart-lung machine to bypass the patient's heart and lungs.

The heart-lung machine replaces the function of the heart and lungs to provide a bloodless, motionless surgical field for the surgeon. The perfusionist ventilates the patient's blood to control the level of oxygen and carbon dioxide. The perfusionist propels the patient's blood back into the arterial system to provide nutrient blood flow to all the patient's vital organs and tissues during heart surgery.