The anatomy of the aveoli is shown. Blood vessels are shown in red in the walls of the lumen. Protective mucus is secreted by the smaller cells in the walls. The architecture of alveolar walls is preserved by elastic and collagen fibers, which make up a complete network from the conducting airways up to the pleura.
Oxygen must transverse multiple cell membranes in order to reach the pulmonary vasculature.
Extra info: The alveolar vasculature is typically behind only a single layer of alveolar cells.