The breathing process in human beings is carried out by the diaphragm and the intercostal muscles. These structures are responsible for creating the movements of the thoracic cavity which allow air to move in and out of the lungs. During inspiration, the diaphragm contracts and moves downward while the external intercostal muscles contract to lift the ribs upward and outward, thereby increasing the volume of the thoracic cavity and reducing the pressure inside, so that air enters the lungs. During expiration, the diaphragm and intercostal muscles relax, reducing the thoracic cavity volume and forcing air out. Thus, the actual breathing movements are controlled by the intercostal muscles and diaphragm, not by the trachea, alveoli, larynx, or bronchi.

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