Respiratory Lining of Human Lung
In Figure you can see the thin lining of alveolar capillary membrane which is 0.2 to 0.6 thick. Gas exchange actually takes place here. Wherever there is exchange of gases, the barrier must be thin and it is so in the alveolar membrane. This membrane separates vascular fluids and alveolar space. The inner space of the alveoli which comes in contact with the inspired air is covered with alveolar epithelium. This epithelial surface is lined by a lipoprotein mixture called lung surfactant. This surface lining reduces the surface tension at inter phase, across which opposed diffusion gradients of oxygen and carbon dioxide are developed. Another function of the surfactant is to help in the first breath taken by an infant or by a new born animal. Before birth the alveoli are collapsed and the thin layer of moisture between them makes it difficult for them-to be opened (try opening a plastic bag with a thin film of water between it). Surfactant reduces the muscular effort to open the lungs.
Figure: Microscopic structure of respiratory lining of human lung. a) Note the presence of smooth muscles that can haven marked effect on the dimension of the air ways. b) Higher magnification of alveolo - capillary membrane which separate alveolar space from capillary blood.