Which volume cannot be directly measured but must be computed in lung assessments?

• A. Tidal volume
• B. Residual volume
• C. Inspiratory reserve volume
• D. Expiratory reserve volume

Answer: (B)

Residual volume is the correct choice because it refers to the amount of air that remains in the lungs after a forced expiration. Unlike tidal volume, inspiratory reserve volume, and expiratory reserve volume, which can be measured directly using spirometry, residual volume cannot be determined using standard spirometric techniques alone.

To measure residual volume, specialized techniques such as body plethysmography or gas dilution methods must be used, as it involves accounting for the air that cannot be voluntarily expelled from the lungs. This measurement is essential in assessing lung function and conditions like obstructive or restrictive lung diseases, making it a critical component of comprehensive pulmonary assessments.