Development of Salt and Water Transport across Airway and Alveolar Epithelia

Continuous secretion of liquid into the fetal mammalian lung is important for its normal development. The main driving force for this secretion is local osmotic gradients created across alveolar and airway epithelia by active secretion of Cl – . At the time of birth, the primary type of active solute transport across pulmonary epithelia switches, within minutes, from active secretion of Cl – to active absorption of Na + , a change that promotes absorption of liquid. Recent research, however, suggests that transepithelial hydrostatic pressure gradients generated during inspiration may be of more importance than solute transport in reabsorption of fetal lung liquid. Nevertheless, active Na + absorption across pulmonary epithelia persists in the air-filled adult lung as a guard against alveolar flooding in pneumonia and other inflammatory pulmonary diseases. This chapter reviews the solute transport processes of airway epithelium, their developmental changes, the cell types responsible, and the mechanisms by which lung liquid is absorbed at birth.