When your body temperature drops or the body is deprived of oxygen, the body starts to protect itself. It starts with your fingers and toes, diverting blood away from them so that more important parts of the body can still function. This goes on with the limbs and organs until the last vital organs (brain, heart, lungs, liver, kidneys) are still getting blood and oxygen. The temperature slows down body processes so that less oxygen is used by the remaining organs. The brain is the last organ to shut down. It runs on the very minimum amount of oxygen and glucose possible while in the cold state. There are also autoperfusion machines that pump blood to the lungs and throughout the body during some operations. A ventilator makes the lungs work. This means the mechanical heart and lungs keep the body alive while the real heart and lungs are inactive. BTW, it s called bypass surgery because they are bypassing the Coronary Artery, not because of the perfusion machine.