Lungs serve the purpose of gas exchange. we inhale air, drawing in oxygen (o2) and exhale air, expelling carbon dioxide (co2). this is done across thin-walled pockets of cells called alveoli. because body cells consume o2 and generate co2, concentrations of co2 and o2 are higher and lower, respectively, at alveoli surfaces than they are in adjacent capillaries. both of these molecules are small, and freely pass through membrane bilayers. based on this information, the movement of o2 from the alveoli of the lungs into the bloodstream is an example of