Final answer:
The expiratory reserve volume (ERV) is the additional air that can be forcefully exhaled after a normal exhalation, contributing to the total lung capacity.
Step-by-step explanation:
The additional volume of air that can be forcefully expelled from the lungs after a normal exhalation is known as the expiratory reserve volume (ERV). After a typical exhalation, we still have a reserve amount of air that can be exhaled forcefully, which is not usually expelled during normal breathing. This ERV is part of our lung capacities and is essential for providing flexibility in the respiratory system. The total lung capacity is the maximum amount of air the lungs can hold, and it includes the tidal volume, ERV, inspiratory reserve volume (IRV), and the residual volume which is the air left in the lungs after maximal exhalation. These units together determine the maximum amount of air that can be moved in or out of the lungs in a single respiratory cycle.