The amount of air remaining in the alveolus after expiration is referred to as:

The amount of air remaining in the alveolus after expiration is known as residual volume. This is the air that cannot be expelled from the lungs and remains in the airways and alveoli even after a forced exhalation. This volume is crucial because it prevents the alveoli from collapsing completely and helps maintain a consistent exchange of gases during the breathing cycle, ensuring that there are always some oxygen and carbon dioxide available for exchange during inhalation and exhalation.

Tidal volume refers to the amount of air breathed in or out during a normal breath, which does not account for residual air. Vital capacity measures the maximum amount of air that can be exhaled after a maximum inhalation, encompassing tidal volume, inspiratory reserve volume, and expiratory reserve volume but does not include the residual volume. Inspiratory reserve volume refers to the amount of additional air that can be inhaled after a normal inhalation. Thus, these other terms pertain to different aspects of lung capacity and airflow, highlighting the specific role of residual volume in lung function and gas exchange.