Closing Capacity — FRCA Primary MCQ

Correct answer: A — In the supine position, its population-average value exceeds functional residual capacity from approximately 44 years of age

Explanation lettering: C = shown as A · A = shown as B · D = shown as C · E = shown as D · B = shown as E

C is correct. Closing capacity is the lung volume during expiration at which dependent small airways begin to close; it equals closing volume plus residual volume. It increases with age. Supine positioning reduces FRC, mainly through a reduction in expiratory reserve volume, so the population-average closing capacity exceeds FRC from about 44 years of age. Airway closure may then occur during normal tidal breathing, reducing dependent ventilation and causing ventilation–perfusion mismatch. A describes airway reopening during inspiration rather than the expiratory closing point. B reverses the age relationship and mechanism: loss of elastic recoil contributes to earlier closure. Smoking can increase closing volume, so D is false. E reverses the correct relationship: closing capacity is closing volume plus, not minus, residual volume.

Reference: Ruff F. Effects of age and posture on closing volume. Scandinavian Journal of Respiratory Diseases Supplement. 1974;85:190–200. https://pubmed.ncbi.nlm.nih.gov/4534910/