A pressure-volume curve of the lung shows that, at any given lung volume above residual volume, the transpulmonary pressure required is lower during deflation than during inflation. This hysteresis of the lung pressure-volume curve is best explained by:

• A Greater alveolar surface tension during expiration than inspiration
• B Recruitment of previously collapsed alveoli and changing surfactant film area ✓
• C Active contraction of bronchial smooth muscle during inflation
• D Progressive stiffening of collagen fibers only during expiration

Explanation

Hysteresis arises because many alveoli that were closed at low volumes must be popped open during inflation against high surface tension, requiring greater pressure. Once open, surfactant spreads over a larger film area and surface tension falls, so deflation occurs at lower pressures. The inspiratory and expiratory limbs differ mainly because of recruitment-derecruitment and surfactant behavior, not because surface tension is intrinsically different in one direction.

Reference: West's Respiratory Physiology: The Essentials, 10th ed.

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Written and medically reviewed by the StethoPrep medical team.