The alveoli are covered by a rich blood supply of capillaries that gives a diffusion gradient for O2 to move into the blood and CO2 to move into the lungs. The alveoli are highly folded and there is a high surface area: volume ratio for gas exchange. It has the presence of moist walls that makes gases dissolve in the moisture and helps tham pass across the exchange surface. The wall is also permeable which allow gases to pass through and it contains extensive blood supply that ensures O2 rich blood is taken away from the lungs and CO2 rich blood is taken to the lungs.