well, if you bulletpoint all the main points and then select the most relevant 3, that should help.
stuff like: co2 diffuses from the atmosphere to the capillaries across the alveolar epithelium (one cell thick) whilst o2 diffuses back out of the alveolus to the atmosphere. you could also mention shizzle like haemoglobin releasing the o2 molecules, and state how the alveolus is adapted to it's function (one cell thick, moist, copious blood supply, large surface area - to speed up diffusion rate).
hope that helps, but knowing the question in it's entirety would have made it easier to address you more specifically.