Alveoli are sac-like evaginations of the respiratory bronchioles, alveolar ducts, and alveolar sacs where the exchange of gases between the inhaled air and the blood in the alveolar capillaries occurs. The numerous tiny alveoli produce a tremendous surface area for gas exchange that is compactly arranged in the lungs. The wall or septum of each alveolus is normally very thin in order to permit gas exchange. It consists of epithelial cells on the surface of the lumen (pneumocytes), the endothelial cells of the capillaries, and basement membranes of the pneumocytes and endothelial cells that are typically fused. The pneumocytes are divided into two types. (i) Type 1 pneumocytes are thin, flat cells that make up 90% of the surface of the alveolar lumen. They have a very thin cytoplasm across which gas exchange occurs. (ii) Type 2 pneumocytes are cuboidal cells that produce and secrete surfactant, give rise to type 1 pneumocytes, and are involved in alveolar repair. An occasional myofibroblast may be encountered in the alveolar septum. The orifices of alveoli along the alveolar ducts and alveolar sacs are composed of elastic and collagen bundles that are a continuation of the bronchial and bronchiolar elastic bundles. All of the elastic fibers, including the alveolar meshwork, are interconnected in all directions, forming an integrated elastic network essential to the uniform expansion and contraction of the lung during respiration.