Describe the mechanics of breathing at rest

During inhalation, the diaphragm contracts to flatten and the internal intercostal muscles contract, moving the rib cage up and out. This causes an increase in thoracic cavity volume, causing the pressure in the thoracic cavity to drop below that of the surrounding environment. Therefore, air rushes into the thoracic cavity. In exhalation, the diaphragm relaxes and domes up, whilst the external intercostals relax bringing the rib cage back down and in. This creates a lower volume in the thoracic cavity and therefore increase the pressure in the thorax. This causes air to be forced out of the thoracic cavity and back into the surrounding air.