Alveoli are the specialised gas exchange surface in the lungs.They are thin, blindly ending air sacs 100-300 μm in diameter. There are about 200 million per lung.They are thin and delicate and easily damaged so are located inside the body for protection.Each alveolus is made of epithelial cells 0.05 to 0.3μm thick.The alveoli are able to stretch when filled with air and recoil to expel air.Each alveolus is surrounded by a network of capillaries. The capillary walls are only one cell thick and just wide enough for a red blood cell to squeeze through.Collectively they have a large surface area for diffusion of oxygen into the body and diffusion of carbon dioxide out of the body.