Cardiopulmonary bypass is commonly used in operations involving the heart. The technique allows the surgical team to oxygenate and circulate the patient's blood, thus allowing the surgeon to operate on the heart. In many operations, such as coronary artery bypass grafting (CABG), the heart is arrested (i.e., stopped) because of the difficulty of operating on the beating heart. Operations requiring the opening of the chambers of the heart, for example, mitral valve repair or replacement, requires the use of CPB to avoid engulfing air systemically and to provide a bloodless field to increase visibility for the surgeon. The machine pumps the blood and, using an oxygenator, allows red blood cells to pick up oxygen, as well as allowing carbon dioxide levels to decrease. This mimics the function of the heart and the lungs, respectively.