Normal exhalation during quiet breathing occurs due to the

Normal exhalation during quiet breathing occurs due to the relaxation of diaphragm and external intercostal muscles. Relaxation of the diaphragm and the inter-costal 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 intra-pulmonary pressure to slightly above the atmospheric pressure causing the expulsion of air from the lungs, i.e., expiration.