Functional residual capacity is best described as which of the following?

• A. Total lung capacity
• B. Tidal volume
• C. Volume of gas in the lungs that is in direct communication with the airways at the end of expiration
• D. Residual volume

Answer: (C)

Functional residual capacity is the amount of air left in the lungs after a normal, passive exhale. This volume remains in the lungs in a way that is still connected to the airways, so it’s the air that can participate in gas exchange between breaths. That’s why the description of a volume of gas in the lungs that is in direct communication with the airways at the end of expiration is the best match. It’s not the total lung capacity (which is the maximum lung volume after a deep breath) or the tidal volume (the amount moved with each normal breath), and while residual volume is the air left after a forced exhale, FRC specifically refers to the volume present after a normal expiration and available for ongoing gas exchange.