On a single-breath nitrogen washout tracing, a distinct upward step in alveolar nitrogen concentration appears at the very end of expiration (phase IV). The lung volume at which this step begins is called the closing volume. It is characteristically increased in:

• A Healthy young adults
• B Pulmonary embolism
• C Smokers with early small airway disease ✓
• D Anemia

Explanation

Closing volume marks the point where dependent small airways begin to close during expiration, trapping gas in the bases while nitrogen-rich apical gas dominates the last exhaled portion. Smoking and aging cause premature small airway closure through loss of elastic recoil and airway inflammation, raising closing volume above functional residual capacity. Pulmonary embolism and anemia affect perfusion and oxygen carriage respectively and do not alter airway closure dynamics.

Reference: West's Respiratory Physiology: The Essentials, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.