Residual volume is the amount of air remaining in the lungs after a maximal exhalation and cannot be expelled. Spirometry measures only the air that can be actively inhaled or exhaled, such as tidal volume, inspiratory capacity, and vital capacity. Because residual volume is trapped and does not move in or out during normal respiratory maneuvers, it cannot be directly measured by spirometry. Instead, it requires other techniques like gas dilution or body plethysmography for accurate assessment. This is crucial in evaluating lung function and diagnosing restrictive or obstructive lung diseases.

Reference: Pulmonary Physiology, Michael G. Levitzky, 8th Edition.