Which statement accurately reflects the LaPlace equation and the role of surfactant?

• A. Alveolar surface tension is increased with the presence of surfactant
• B. Alveolar surface tension is decreased with the presence of surfactant
• C. Alveolar surface tension is not affected by the presence of surfactant
• D. Alveolar surface tension is only affected in large alveoli

Answer: (B)

Surfactant lowers surface tension at the air–liquid interface of the alveoli, which is crucial for keeping alveoli stable across a range of sizes. The LaPlace relationship for a spherical alveolus is P = 2γ/r, meaning the pressure needed to hold an alveolus open rises with surface tension and falls as the radius increases. In small alveoli, high surface tension would make them prone to collapse, but surfactant reduces γ, especially when the alveolus is smaller, so the pressure required to keep small and large alveoli open is decreased. This stabilizes alveoli, reduces the work of breathing, and prevents collapse. Therefore, alveolar surface tension is decreased with the presence of surfactant. The other statements misstate the effect: surfactant does not increase surface tension, it does not have no effect, and it does not affect only large alveoli.