At vaginal delivery after labor, the fetal lungs clear their luminal fluid rapidly, whereas neonates delivered by elective cesarean section before labor have a higher incidence of transient tachypnea. The principal mechanism of fluid clearance across the alveolar epithelium during labor is:

• A Passive hydrostatic squeezing of fluid through the mouth during passage through the birth canal
• B Catecholamine-driven upregulation of epithelial sodium channel (ENaC) activity with active sodium reabsorption ✓
• C Osmotic water shift into the interstitium caused by sudden fall in fetal aldosterone
• D Rupture of alveolar capillary membranes allowing fluid drainage into pulmonary lymphatics

Explanation

The surge of fetal catecholamines during labor stimulates ENaC and the Na/K-ATPase on alveolar type I cells, driving active sodium transport out of the lumen with osmotic water following, converting the lung from a secretory to an absorptive organ. Birth canal compression contributes little, which is why cesarean-delivered neonates still develop transient tachypnea when this absorptive switch is incomplete.

Reference: Guyton and Hall Textbook of Medical Physiology, 14th ed.

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Written and medically reviewed by the StethoPrep medical team.