During inhalation, the diaphragm contracts and pulls downward, while the muscles between the ribs contract and pull upward. This increases the size of the thoracic cavity and decreases the pressure inside it. As a result, air rushes in to fill the lungs.

During exhalation, the diaphragm relaxes, and the volume of the thoracic cavity decreases, while the pressure within it increases. As a result, the lungs contract and air is forced out.

​Your lungs are protected by pleural membranes and the rib cage.