surface tension
Summary
Surface tension is the cohesive force between liquid molecules at the air-liquid interface that creates resistance to surface expansion. In the lungs, pulmonary surfactant (primarily dipalmitoylphosphatidylcholine) reduces alveolar surface tension according to Laplace's law, preventing alveolar collapse and reducing work of breathing.
Detail
Surface tension arises from hydrogen bonding between water molecules at the air-liquid interface in alveoli. Without surfactant, high surface tension would cause smaller alveoli to collapse into larger ones (following Laplace's law: P = 2T/r, where pressure is inversely related to radius). Pulmonary surfactant, produced by type II pneumocytes starting around 24-28 weeks gestation, contains 90% phospholipids (mainly DPPC) and 10% proteins (SP-A, SP-B, SP-C, SP-D). Surfactant reduces surface tension from ~70 to ~25 dynes/cm, with greater reduction at lower lung volumes, providing stability across different alveolar sizes. Surfactant deficiency causes neonatal respiratory distress syndrome (NRDS), characterized by atelectasis, hyaline membrane formation, and respiratory failure. The lecithin/sphingomyelin (L/S) ratio >2.0 indicates fetal lung maturity. Surfactant replacement therapy and antenatal corticosteroids are key treatments for preventing/managing NRDS.
Sources
- Costanzo Physiology 6th Edition
- First Aid for USMLE Step 1
- West's Respiratory Physiology
- Guyton and Hall Textbook of Medical Physiology
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