At that point, we have this difference in pressure known as the pressure difference or the negative pressure difference. And that creates that pressure gradient that is needed to actually allow the movement of the air from the outside environment and into our lungs. At this point, air rushes into the lungs and inhalation actually takes place. 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.