Relaxation of the diaphragm and the external intercostal muscles returns the diaphragm and sternum to their normal positions and reduce the thoracic volume and thereby the pulmonary volume. This leads to an increase in intrapulmonary pressure to slightly above the atmospheric pressure causing the expulsion of air from the lungs (expiration). Contraction of diaphragm and raising of ribs and sternum cause and increase in the thoracic volume leading to inspiration.