The first breath of a newborn requires a very high inspiratory pressure (~50–70 cmH2O) to expand the lungs. The primary cause of this high resistance is:

• A High viscosity of amniotic fluid that must be cleared from airways
• B Underdevelopment of intercostal muscles requiring high diaphragmatic effort
• C Narrowed glottis from fetal breathing movements persisting postnatally
• D High surface tension of the air-liquid interface due to absent/deficient surfactant ✓

Explanation

Before birth, fetal lungs are filled with liquid. The first breath must overcome the Laplace pressure generated at the alveolar air-liquid interface: P = 2T/r. With no surfactant and small alveolar radius, the surface tension (T) is very high, requiring 50–70 cmH2O of negative intrathoracic pressure. Once lung expansion begins and surfactant spreads, compliance improves dramatically. In premature infants, surfactant deficiency (IRDS) means every breath requires near-maximal effort.

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

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Written and medically reviewed by the StethoPrep medical team.