Effect of lung inflation on alveolar permeability to solutes.

The alveolar epithelium separates gas and liquid phases in the lung. Osmotic forces available for separating these phases will be determined by the solute permeability of the epithelium. The relative rates of diffusion of several simultaneously studied solutes of known molecular size have been used for measuring the equivalent pore radius across the alveolar epithelium in vivo in the perinatal period and in adult animals. In perinatal sheep, the equivalent pore radius increases from 0.5 nm in the fetal state to 1.5-4.5 nm during the first minutes of spontaneous ventilation. Postnatal animals, 12-60 hours of age, have pore radii of 0.7-1.4 nm. Static inflation of the lungs, in fetal lambs, produced pressure-dependent increases in pore radii. In adult sheep and rabbits the measured radius of equivalent pores across the epithelium in vivo was positively correlated with the degree of inflation of the lungs (P less than 0.005). Measurements varied from 0.5 nm at low levels of inflation to large leaks at high levels. These experiments indicate that solute permeability across the alveolar epithelium is a dynamic function of the inflation of the lung, rather than a static feature.