Alveolar Epithelial Fluid Transport in Lung Injury

Abstract

This chapter discusses the regulation of lung fluid transport by lung epithelial active ion transport mechanisms. Ion transport occurs across both the alveolar and the distal airway epithelium. Both experimental models of pulmonary edema and clinical studies are considered to demonstrate how active Na+ and active Cl- transport participate and regulate alveolar edema resolution. Some of the material in this chapter has been discussed in recent reviews. For decades, it was believed that Starling forces, i.e. differences in hydrostatic and protein osmotic pressures, accounted for removal of excess air space fluid. This misconception remained partly because experiments measuring solute fluxes across the lung epithelial and endothelial barriers were done at room temperature and the animal species used was the dog, a species that later was demonstrated to have a very low rate of active ion and fluid transport. Also, until the late 1970s and early 1980s, there were few good animal models to study the resolution of pulmonary edema. Furthermore, isolation and culture of alveolar epithelial type II cells was just evolving to become a useful experimental technique. Although removal of interstitial pulmonary edema by lung lymphatics and lung microcirculation had been discussed by Staub in his review of pulmonary edema, there was still no information on how pulmonary edema was removed from the distal air spaces of the mature lung.