What is residual volume?

• A. Largest amount of air that can be forcibly exhaled.
• B. The amount of air that can be forcibly inhaled after normal inspiration.
• C. The amount of air that cannot be forcibly exhaled.
• D. Air remaining after residual volume.

Answer: (C)

Residual volume is the amount of air left in the lungs after a maximal exhalation. Even with a forceful exhale, some air remains in the airways and alveoli to keep the lungs partially inflated and to maintain ongoing gas exchange between breaths. This trapped air cannot be expelled, which is why it defines residual volume. In healthy adults, RV is part of the functional residual capacity (FRC), since FRC equals RV plus the expiratory reserve volume. RV cannot be measured by standard spirometry and is typically assessed with body plethysmography or gas-dilution techniques.