What is the “respiratory membrane” that oxygen must diffuse through to reach the blood stream?
Oxygen will move by passive diffusion from inside the space of an alveolus through the layer of simple squamous epithelium that forms the alveolus & its “basement membrane” (not a true membrane, but an area containing elastic & collagenous fibers), across the space between the alveolus & the capillary surrounding it, then through the basement membrane of the capillary & the simple squamous tissue of the capillary itself (endothelium). Anything that causes thickening of these membranes, such as emphysema or lung cancer, will make breathing more difficult. The diffusion of carbon dioxide OUT of the capillary & into the alveolus for exhalation will follow this same pathway in reverse. Figure 16.19 illustrates the relationship of the cells of an an alveolus and a capillary. (Diffusion distance is also increased by the surfactant that sits on the inside of the alveolus. Conditions such as pneumonia, asthma or cystic fibrosis which add mucus to the respiratory passageways will complicate the
