Cardiopulmonary bypass is a medical procedure used during heart surgery to temporarily take over the function of the heart and lungs. It involves diverting blood away from the heart to a machine that oxygenates the blood and removes carbon dioxide, allowing surgeons to operate on a still and bloodless heart. This technique is essential for complex surgeries, such as coronary artery bypass grafting or heart valve repair. By maintaining blood circulation and oxygen supply to the body, cardiopulmonary bypass helps ensure patient safety and stability during the operation.