Breathing: Fetal Lung Liquid

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Fetal lung liquid is secreted across the pulmonary epithelium and enters the future airspaces due to an osmotic gradient created by the transepithelial flux of ions into the lung lumen. This liquid leaves the lungs by flowing out of the trachea, whereby it is either swallowed or contributed to amniotic fluid volume. The high resistance to liquid movement through the fetal upper airway promotes the retention of lung liquid within the future airways, which provides a small (1 or 2 . mmHg) internal distending pressure on the lungs. This acts as an internal hydrostatic splint that maintains the fetal lungs in a distended state and provides a stretch stimulus that is essential for normal fetal lung growth. If the distending influence of fetal lung liquid is absent, the lungs fail to grow, which is the primary mechanism for fetal lung hypoplasia in humans. At birth, the airways are cleared of liquid to allow the entry of air and the onset of air breathing, due to a reversal of the transepithelial ionic gradient that promotes reabsorption into the lung parenchyma; this is stimulated by the release of stress-related hormones during labor. Increases in transpulmonary pressure, due to changes in fetal posture, are likely to also contribute to the clearance of fetal lung liquid at birth.

Original languageEnglish
Title of host publicationEncyclopedia of Respiratory Medicine, Four-Volume Set
PublisherAcademic Press
Number of pages5
ISBN (Print)9780123708793
Publication statusPublished - 1 Jan 2006


  • Epithelial ion transport
  • Fetal breathing movements
  • Fetal lung liquid
  • Fetus
  • Lung development
  • Lung liquid clearance

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