Surfactant reduces surface tension in alveoli. By the Laplace relationship (P = 2T/r), smaller alveoli would have higher pressure and should empty into larger alveoli. The key property of surfactant that prevents this alveolar instability is:
- A Surfactant concentration and surface tension reduction are greater in smaller alveoli because molecules are more closely packed at smaller surface areas, thus equalizing pressure across alveolar sizes ✓
- B Surfactant increases surface tension uniformly across all alveolar sizes
- C Surfactant acts as a rigid structural protein maintaining alveolar architecture
- D Surfactant preferentially coats larger alveoli, preventing over-distension
Correct answer: A. Surfactant concentration and surface tension reduction are greater in smaller alveoli because molecules are more closely packed at smaller surface areas, thus equalizing pressure across alveolar sizes
Explanation
Surfactant (dipalmitoylphosphatidylcholine, DPPC) reduces surface tension in a concentration-dependent manner. As alveolar radius decreases (during expiration), surfactant molecules pack more tightly, increasing surface concentration and reducing surface tension more than in larger alveoli. This decrease in T in the Laplace equation (P = 2T/r) counteracts the increased P that would otherwise result from smaller r. Without this property (e.g., premature infant with insufficient surfactant causing IRDS), smaller alveoli would collapse into larger ones (atelectasis).
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.