Surfactant reduces alveolar surface tension and prevents alveolar collapse. According to the Law of Laplace, smaller alveoli tend to collapse into larger ones. Surfactant prevents this because:

• A Surfactant forms a rigid film that mechanically prevents alveolar wall deformation
• B Surfactant concentration increases as alveolar radius decreases, lowering surface tension more in smaller alveoli ✓
• C Surfactant increases lung compliance uniformly regardless of alveolar size
• D Surfactant activates stretch receptors to reflexively dilate smaller alveoli

Explanation

By the Laplace relationship P = 2T/r, smaller alveoli generate higher inward pressure. Surfactant (DPPC + surfactant proteins B and C) works by increasing its surface concentration as alveoli shrink during expiration, because the same amount of surfactant covers a smaller area. This concentration-dependent reduction in surface tension is greater in smaller alveoli, effectively equalizing the collapsing pressure across alveoli of different sizes and preventing the smaller ones from emptying into larger ones (alveolar instability).

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

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Written and medically reviewed by the StethoPrep medical team.