Alveoli increase the respiratory surface area in lungs. This large surface area help in the easy exchange of respiratory gases. Alveoli are surrounded by numerous capillaries. The inner wall of the alveoli is always kept moist. During inspiration, the concentration of oxygen inside the alveoli is higher than that of blood capillaries whereas the concentration of carbon dioxide in blood capillaries is higher than that of alveoli. As a result oxygen from the alveoli diffuses to the capillaries and carbon dioxide from capillaries diffuses to the alveoli
