SOLUTE PERMEABILITY OF THE ALVEOLAR EPITHELIUM

Abstract

Investigations were made of the permeability of the in vivo mammalian alveolar epithelium to determine if restriction of solute diffusion could explain the ability of the alveolar lining cells to maintain the separation of the gas phase of the alveolus from the liquid phase of interstitial and vascular spaces of the lung. Using a split lung preparation, trace amounts of non-electrolyte, lipid insoluble solutes dissolved in normal saline were infused into one lung which was then inflated. Transfer out of the alveolar space was determined by serial samplings of the alveolar liquid and transfer constants were calculated from the change in concentration of each substance with time. The rates of transfer of the various sized substances were analyzed in terms of pore theory to determine an equivalent pore radius for the alveolar epithelium. Individual animals had an equivalent pore radius across the alveolar barrier of 20-40 A with an average of 26 A. The total area of the solute movement was estimated to be only 2×10−8th of the total alveolar surface area, pointing to the intercellular junctions as the anatomic site. Pores of this size impose virtually complete restriction to protein movement, significantly restrict NaCl diffusion, and produce an osmotic gradient favoring free water absorption from the alveolus.