What volume remains in the lungs after a maximal expiration?

The volume that remains in the lungs after a maximal expiration is known as the residual volume. This is the amount of air that cannot be voluntarily expelled from the lungs, ensuring that some air remains in the airways and alveoli to keep them open and prevent lung collapse. Residual volume plays a critical role in maintaining gas exchange continuity by preventing the complete collapse of alveoli and aiding in the mixing of fresh and stale air during subsequent breaths.

In contrast, functional residual capacity includes both the residual volume and the expiratory reserve volume, referring to the total volume of air remaining in the lungs after a normal exhalation. Total lung capacity is the sum of all lung volumes, which represents the maximum amount of air the lungs can hold, while expiratory reserve volume specifically represents the additional air that can be forcibly exhaled after the end of a normal expiration. Each of these other terms describes different aspects of lung volume, but only residual volume directly answers the question regarding the air left after maximum expiration.