how many marks is it? or how much detail have u gotta give?
air is inhaled into the lungs, so there is plenty of oxygen in the lungs. gas exchange happens in the alveoli (tiny sacs in lungs). As there is a higher concentration of oxygen in the lungs than in the surounding blood vessels (capillaries), most of the oxygen molecules will passively diffuse into the blood. There is a higher concentration of carbon dioxide in the blood than in the alveoli, so CO2 passively diffuses into the alveoli, so it can be exhaled.