Which type of lung volume cannot be measured directly by spirometry?

Residual volume refers to the amount of air that remains in the lungs after a person has exhaled as much air as possible. This volume is significant because it helps prevent the lungs from collapsing and ensures that gas exchange can continue in the alveoli. However, residual volume cannot be directly measured using spirometry, which is a technique that typically captures volumes of air that can be inhaled or exhaled during breathing.

Spirometry measures volumes such as tidal volume, which is the amount of air inhaled or exhaled in a normal breath, as well as forced vital capacity— the total amount of air that can be forcibly exhaled after a full inhalation. Inspiratory reserve volume, defined as the amount of air that can be inhaled after a normal tidal inhalation, can also be measured directly with spirometry. In contrast, because residual volume cannot be directly exhaled or inhaled after maximum effort, clinicians often use indirect methods, such as gas dilution techniques or body plethysmography, to estimate this volume.

Thus, the ability to measure residual volume indirectly versus direct measurement of other lung volumes highlights its unique and essential role in pulmonary function testing.