Carbon dioxide plays essential and life?sustaining roles in the human body. It helps maintain the acid?base balance of blood and tissues, ensures the normal function of enzymes and cells, and stimulates the respiratory center in the brain to control breathing rate and depth. It also promotes the release of oxygen from hemoglobin to tissues, supports proper vascular tone, and participates in many metabolic processes; although it is a waste product of cellular respiration, appropriate levels of CO? are necessary for normal physiological activities and overall homeostasis.