Aerospace Physiology 25-07 Practice Test

Gas exchange happens by diffusion across very thin barriers in the lungs, primarily in the walls of the alveoli. The alveolar walls are a single layer of cells (the alveolar epithelium) right next to a network of pulmonary capillaries, separated by a tiny shared basement membrane. This creates a respiratory membrane with a large surface area and an extremely short diffusion distance, so oxygen can move from the air in the alveoli into the blood and carbon dioxide can move from the blood into the alveolar air efficiently. In contrast, the trachea and bronchi are thick-walled, cartilage-supported conduits whose job is to conduct and condition air, not to exchange gases; their walls are far too thick and lack the close association with blood vessels needed for diffusion. The diaphragm is a muscle that drives breathing by changing thoracic volume, not a site for gas exchange.