This chapter discusses the development of fluid transport across the pulmonary epithelia. In the fetus, active secretion of Cl across both airway and alveolar epithelia drives water into the lung, and the resulting elevation of intraluminal hydrostatic pressure is essential important for normal lung development. However, at the time of birth, the primary type of active solute transport across pulmonary epithelia switches, within minutes, from active secretion of Cl<sup>-</sup> to active absorption of Na<sup>+</sup>. The transepithelial osmotic gradients are reversed and luminal liquid is absorbed as the lungs fill with air. This switch is affected by the interplay of a number of hormones of which the ultimate is usually epinephrine. Changes in arterial O<inf>2</inf> tension and in the extracellular matrix help maintain liquid absorption. The perinatal increase in Na absorption is initially due to insertion of ENaC into the apical membrane, followed later by an increase in the numbers of Na-K-ATPase on the basolateral membrane. The perinatal decrease in active Cl secretion is probably mainly due to a change in apical membrane potential difference consequent on increased Na conductance.
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