The process of breathing (respiration) is divided into two distinct phases: inspiration (inhalation) and expiration (exhalation), the relaxation of the diaphragm occurs at the completion of the air intake
During inspiration, the diaphragm contracts and pulls downward while the muscles between the ribs contract and pull upward which increases the size of the thoracic cavity and decreases the pressure inside, as a result air rushes in and fills the lungs
But opposite happens during expiration, 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
