What is the volume of air remaining in the lungs after a forceful expiration?

The volume of air remaining in the lungs after a forceful expiration is known as residual volume. This component of lung volume is crucial because it represents the air that cannot be voluntarily expelled, even during a maximal effort to exhale. Residual volume is essential for maintaining gas exchange and preventing the alveoli from collapsing completely, which would impair oxygen uptake.

In contrast, vital capacity refers to the maximum amount of air that can be exhaled after a maximum inhalation, emphasizing the total quantity of usable air. Total lung capacity is the sum of all lung volumes, including inspiratory and expiratory reserves, tidal volume, and residual volume, thus incorporating much more than just the air left after forceful expiration. Tidal volume is the amount of air inhaled or exhaled during normal breathing at rest, so it does not represent the leftover air post-expiration.

Thus, residual volume specifically addresses the air volume left after a forceful breath out, making this the correct response.