Final answer:
The residual volume is the amount of air left in the lung after exhaling the expiratory reserve volume. It cannot be measured directly but can be calculated. Residual volume is important for preventing fluctuations in respiratory gases and for maintaining lung function.
Step-by-step explanation:
The volume in the lung can be divided into four units: tidal volume, expiratory reserve volume, inspiratory reserve volume, and residual volume. Tidal volume (TV) measures the amount of air that is inspired and expired during a normal breath. The expiratory reserve volume (ERV) is the additional amount of air that can be exhaled after a normal exhalation. The inspiratory reserve volume (IRV) is the additional amount of air that can be inhaled after a normal inhalation. The residual volume (RV) is the amount of air that is left after expiratory reserve volume is exhaled.
The residual volume is the only lung volume that cannot be measured directly because it is impossible to completely empty the lung of air. This volume can only be calculated rather than measured. Residual volume is important for preventing large fluctuations in respiratory gases and for maintaining lung function.