Define tidal volume.

• A. The maximum amount of air that can be inspired after a normal inhalation
• B. The amount of air remaining after maximal exhalation
• C. The total amount of air the lungs can hold
• D. The amount of air moved in or out of the lungs during a single respiratory cycle

Answer: (D)

Tidal volume is the amount of air moved in or out of the lungs during a single respiratory cycle, that is, a normal breath during quiet breathing. In a healthy adult it’s typically about 500 mL, though it varies with body size, age, sex, and activity level. This volume is different from inspiratory reserve volume (the extra air you can inhale after a normal inhalation), expiratory reserve volume (the extra air you can exhale after a normal exhalation), residual volume (air left after maximal exhalation), and total lung capacity (the maximum air the lungs can hold).