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Alveolus Micro Anatomy

The average alveolar diameter is between 0.05 to 0.33 mm, with the largest alveoli at the pulmonary apex and the smallest at the base when in the upright position. The walls of the alveoli are asymmetrically arranged. Gas exchange occurs on the thin side of the alveolar epithelium. The thick side of the alveoli is where fluid and solute exchange occurs, and also provides structural support for the alveolus. Gas exchange occurs between the alveolar epithelium and capillary epithelium which are separated only by their respective cellular and basement membrane. On the thick side, the alveolar and capillary endothelium are separated by the pulmonary interstitial space which is made up for elastin, collagen and possibly nerve fibers.

The pulmonary epithelium contains two main cell types. Type I pneumocytes, which are flat, form tight junctions with one another that prevent the passage of large oncotically active molecules into the alveolus (i.e. albumin). Type II pneumocytes are much more numerous than Type I pneumocytes, and are round cells that contain cytoplasmic inclusions called lamellar bodies, which contain surfactant. Unique to type II pneumocytes is the ability to divide and produce type I pneumocyte if need be. Additionally, they are resistant to O2 toxicity.