Although there are many different types of alveoli in the body, the word alveoli is usually used to talk about small air sacs in the lungs of mammals. These are also called "pulmonary alveoli". These alveoli are located at the ends of the air passageways in the lungs. They have very thin (one cell thick), wet walls and are surrounded with a network of small blood vessels, or capillaries. This allows gases to diffuse, or move across, the surface of the alveolus.