And once again, inhalation is an active process. It needs ATP because our muscles need ATP to actually contract. Now, once the lungs are filled with air, the individual alveoli of the lungs essentially exchange the carbon dioxide and oxygen. We take an oxygen into the capillaries of our body and we expel our carbon dioxide from the capillaries and to our lungs, eventually expelling them to the outside. Now, once we exchange the oxygen for carbon dioxide, how exactly does exhalation actually take place? Well, once we're at this stage, what happens is the diaphragm and these intercoastal, external intercostal muscles begin to relax.