Final answer:
During moderate aerobic exercise, tidal volume increases the most, as it allows for greater air exchange to meet the body's demands for oxygen and carbon dioxide removal.
Step-by-step explanation:
During moderate aerobic exercise, the respiratory variable that increases the most is tidal volume (TV). Tidal volume refers to the amount of air that enters the lungs during quiet breathing. During exercise, increased oxygen demand and carbon dioxide production lead to an increase in tidal volume to facilitate greater air exchange.
The inspiratory reserve volume (IRV) and expiratory reserve volume (ERV) may also increase, but not as significantly as tidal volume. The residual volume (RV) typically does not change with exercise since it represents air that remains in the lungs after maximal exhalation, to prevent alveolar collapse and ensure consistent gas exchange.
Residual volume also prevents large fluctuations in respiratory gases (O₂ and CO₂). However, during a moderate aerobic workout, it is the tidal volume that sees the greatest increase to meet the body's demands for gas exchange.