Which term describes the volume of air that participates in gas exchange?

• A. Alveolar ventilation
• B. Inspiratory capacity
• C. Tidal volume
• D. Vital capacity

Answer: (A)

The key idea is the portion of air that actually reaches the gas-exchange surfaces in the lungs. That volume is called alveolar ventilation. It matters because not all inspired air is useful for gas exchange—air in the conducting airways (dead space) doesn’t participate in exchanging oxygen and carbon dioxide. So, per breath, alveolar ventilation is the tidal volume minus the dead-space air, and per minute it’s that value multiplied by the breathing rate. For example, with a typical tidal volume around 500 mL and a physiologic dead space around 150 mL, about 350 mL reaches the alveoli each breath; at 12 breaths per minute, that’s roughly 4.2 L of alveolar ventilation per minute. This distinguishes it from inspiratory capacity (how much you can inhale after a normal exhale), tidal volume (air moved in and out in a normal breath, which includes dead-space air), and vital capacity (the maximum air you can exhale after a full inhalation).