During expiration,

During expiration, the diaphragm and the external intercostal muscles relax. As it relaxes, the diaphragm becomes dome-shaped. As the external intercostals relax, the ribs and the sternum are depressed. These movements reduce the thoracic volume and thereby the pulmonary volume. This reads to an increase in intrapulmonary pressure to slightly above the atmospheric pressure  causing the expulsion of air from the lungs.