The process of breathing performs a number of important gas exchanges through inhalation and exhalation. When someone inhales, they contract and move their diaphragm, which allows the lungs to expand. This is a part of the process needed to psychically suck in the air. When a person exhales, the diaphragm relaxes and the movement is undone. This allows air to be gradually pushed out of the lungs.
When someone inhales, they bring air into their lungs, which allows it to enter the bronchial tubes. From the bronchial tubes, the air reaches tiny sacs of air known as alveoli. The walls of the alveoli are thin enough for oxygen to pass through to the surrounding blood vessels. The oxygen is then moved throughout the body, as carbon dioxide moves from the capillaries back into the alveoli. This gas exchange is a vital process for the body's functions.
Conversely, exhalation moves the diaphragm up into the chest cavity and reduces the space in it. This forces the air, which is dense with carbon dioxide at that point, out of the lungs and windpipe. It then exits the body either through the nose or mouth. Usually, this requires no physical effort from the body.