What does tidal volume refer to in respiratory terms?

Tidal volume is defined as the amount of air that is inhaled or exhaled during a normal breath. It reflects the depth of breathing; specifically, it indicates how much air is being moved in and out of the lungs with each breath cycle at rest.

When considering the other choices, the rate of breathing pertains to the frequency of breaths taken in a minute, which is distinct from the volume of each breath. Total lung capacity encompasses the maximum volume of air the lungs can hold, combining several components including tidal volume, but does not solely define what tidal volume is. The description of air expelled during expiration only is not applicable, as tidal volume includes both inhalation and exhalation.

Thus, understanding tidal volume as a measure of the depth of breathing is fundamental in respiratory physiology, particularly in assessing lung function and respiratory health in clinical scenarios.