Air is moved into the lungs when the diaphragm and intercostal muscles contract to enlarge the chest cavity. This expansion causes air pressure within the chest cavity to decrease, allowing higher air pressure from outside the body to be drawn in through the nose and mouth, filling the lungs.
Respiration begins with inhalation where air is drawn into the lungs. Once air has been drawn through the bronchial tubes of the lungs, it enters the alveoli. The alveoli are tiny air sacks with thin walls that oxygen is able to pass through in order to enter the bloodstream where a red blood cell protein known as hemoglobin is able to carry it to the rest of the body. At the same time that oxygen is entering into the bloodstream, a waste gas called carbon dioxide passes through the capillaries of the alveoli.
Exhalation occurs when the diaphragm and intercostal muscles are relaxed, shrinking the size of the chest cavity and expelling air that is rich in carbon dioxide from the body. While inhalation requires muscle contraction, exhalation requires no effort except in cases where lung disease has caused physical damage to the lungs. Contraction of the abdominal muscles may assist exhalation during periods of physical exertion.