What is the definition of tidal volume in respiratory physiology?

Tidal volume refers specifically to the volume of air that is inhaled or exhaled during a normal, unforced breath. This value represents the typical, relaxed breathing pattern that occurs during situations such as resting or sleeping and is a fundamental concept in respiratory physiology. By measuring tidal volume, one can assess the lung function and determine how much air is moving in and out of the lungs with each breath cycle.

The other options describe different respiratory values but do not accurately define tidal volume. The residual volume, for example, pertains to the amount of air remaining in the lungs after a person has exhaled as much air as possible, while total lung capacity encompasses all the air in the lungs, both in a fully inflated state and the residual volume. The concept of air exchanged only in inspiration does not reflect the dynamic nature of tidal breath, which includes both inspiratory and expiratory phases.