Which two items combine to form functional residual capacity?

• A. Vital capacity and residual volume
• B. Expiratory reserve volume and residual volume
• C. Inspiratory reserve volume and tidal volume
• D. Inspiratory reserve volume and expiratory reserve volume

Answer: (B)

Functional residual capacity is the amount of air left in the lungs after a normal exhale. It equals the expiratory reserve volume plus the residual volume. After a regular breath, you still have air you could potentially exhale further (ERV), and there is air that can’t be exhaled at all (RV); together these volumes make up the resting end-expiratory volume, or FRC.

So, FRC = ERV + RV. The other pairs don’t describe what remains after a normal exhale: vital capacity plus residual volume would sum to total lung capacity, and pairs involving inspiratory reserve or tidal volume describe inspiratory or total capacities rather than the end-expiratory resting volume.