Final answer:
Expiratory reserve volume (ERV) is the additional air that can be forcefully exhaled after a normal exhalation, which is an essential component of respiratory volumes used to evaluate lung function.
Step-by-step explanation:
Expiratory reserve volume (ERV) is the extra amount of air that can be exhaled actively from the lungs after a normal, quiet exhalation. This is not the volume of air left in the lungs after maximal expiration, but rather the additional volume that can be forced out after a standard breath. In men, this can be up to 1200 milliliters. ERV is one of the four major respiratory volumes, alongside tidal volume (TV), inspiratory reserve volume (IRV), and residual volume (RV), used to assess lung function. Measuring these volumes provides important information about a person's respiratory health.