Breathing is a physiological process that involves the inhalation and exhalation of air. Physically, breathing results in the expansion and contraction of the lungs as air moves in and out. This physical change alters the volume and pressure of the chest cavity and lungs. Chemically, breathing involves the exchange of gases between the air and the lungs. During inhalation, oxygen is taken up from the air and transported to the cells throughout the body, while carbon dioxide, a waste product of cellular respiration, is released from the lungs during exhalation. Therefore, breathing is a combination of physical changes (expansion and contraction of the lungs) and chemical changes (gas exchange).