What defines expiratory reserve volume (ERV)?

Expiratory reserve volume (ERV) specifically measures the amount of air that can be forcibly exhaled after the completion of a normal exhalation. It represents the additional air that can be removed from the lungs beyond the typical tidal volume—this is the air that remains in the lungs after a normal breath is completed. Understanding ERV is essential in assessing pulmonary function and lung capacity. In contrast, the other options focus on different aspects of lung volumes or capacities, such as air remaining in the lungs after exhalation or the total air capacity of the lungs, which do not specifically pertain to the additional air that can be exhaled beyond the normal breath.