The heart and lungs work together to keep the body's cells supplied with oxygen. During circulation, the heart pumps oxygen-depleted blood to the lungs, and then receives oxygenated blood from the lungs for distribution to the rest of the body.

Often times, the heart can become damaged from heart disease or trauma. Open-heart surgery, where the chest is opened and the heart is exposed, may be necessary to repair the damage. During some open-heart procedures, it may be necessary to stop the heart in order to repair the heart's muscle, valves, or other structures. A heart-lung machine allows the surgeon to carefully stop the heart while still maintaining blood circulation.

The machine consists of a pump, which functions as the heart, and an oxygenator, which replaces the function of the lungs.

During a heart-lung bypass, oxygen-poor blood is first diverted from the upper chambers of the heart and is directed to a reservoir in the heart-lung machine. The blood is then transferred to the oxygenator, which infuses the blood with oxygen. Next, a pump returns the blood to the patient's arterial system, where the body can resume blood circulation on its own. Following repair of the heart, the heart is restarted and the heart-lung machine is removed.