-breathing in/inhalation= external intercostal muscles contract (rib cage moves up and down) and diaphragm contracts (flattens), therefore increase in volume in chest and decrease in pressure, so air moves in.
-Breathing out/exhalation= external intercostal muscles relax (rib cage moves down and in) and diaphragm relaxes (back to some shape), therefore decrease in volume in chest and increase in pressure, so air pushed out (aided by elastic recoil in the alveoli)
