When you inhale - the intercostal muscles contract, expanding the rib cage -the diaphragm contracts, pulling downwards to increase volume of the chest -pressure inside the chest is lowered and air is sucked into the lungs When you exhale - the intercostal muscles relax, ribcage drops inwards and outwards -the diaphragm relaxes, moving back upwards to decrease volume of the chest -pressure inside the chest increases and air is forced out.
