Your lungs always have around 1200ml of air in them that can’t be voluntarily expelled. The amount of exchangeable amount of air in each lung is around 480 ml and only about 350ml actually reaches the aviolies for gas exchange into the blood cells.
CO2 is acidic (lowering the pH), Oxygen + release of CO2 is alkalizing (bringing our pH back up to a healthy 7.35-7.45) hence why we can breathe ourselves into better health (among many many other reasons).
