What are the adaptations of alveoli to increase gas exchange?

Alveoli are folded to increase their surface area to volume ratio, so more volume of air can diffuse in from the blood and and in from the bronchioles. The lining of the alveoli walls are also moist so the gases can easily dissolve. The walls are thin so the diffusion distance is short. They have a rich blood supply as the blood capillaries are close and have thin walls, so there is a steep concentration gradient. The concentration gradient steepness is increased more as the blood is highly concentrated with carbon dioxide while the alveoli have a high concentration of oxygen, favouring diffusion of carbon dioxide into the alveoli and oxygen into the capillaries.