Fluid balance in the air filled alveolar space.

Studies in vitro and in vivo of the alveolar epithelial membrane, which show it can normally be characterized as having a "pore radius" of 0.5 to 1.5 nm, have been presented frequently over 20 years in a number of species (2, 3, 6, 8, 12). The alveolar epithelium appears to share these permeability properties with other epithelia. The changes that can be produced in the solute permeability of the alveolar epithelium by inflation are profound and point to its fragility (table; see text) in experimental situations that are not closely controlled. It is likely that experimental results unable to demonstrate molecular sieving bear the burden of proving that this property has not been altered by the experimental procedure. The inflation changes in epithelial permeability appear restricted to situations in which only a fraction of the lung expands in response to high pressures, such as in the clinical situation of respiratory failure where much of the air space is fluid filled or atelectatic, and large distending pressures are used to ventilate a small fraction of the total alveoli. Inflation-induced permeability changes, particularly to protein, may be important in alveolar water balance or in protein-induced dysfunctions of lung surfactant in such clinical situations.