When you inhale-the intercostal muscles contract, expanding the ribcage- the diaphragm contracts, pulling downwards to increase volume of the chest- pressure inside the chest is lowered 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.
