Which statement correctly defines functional residual capacity (FRC)?

Which statement correctly defines functional residual capacity (FRC)?

• A. TV + IRV
• B. RV + ERV
• C. RV + VC
• D. RV + ERV + TV

Answer: (B)

Functional residual capacity is the volume of air left in the lungs after you exhale normally. It reflects the resting position where the elastic recoil forces of the lungs and chest wall balance. This volume is the sum of residual volume, the air that remains after a maximal exhale, and expiratory reserve volume, the extra amount you can exhale beyond a normal breath. So, FRC = ERV + RV. Other ways of combining volumes describe different lung capacities. For example, tidal volume plus inspiratory reserve volume equals inspiratory capacity (the maximum you can inhale after a normal exhale). Residual volume plus vital capacity equals total lung capacity (the maximum amount the lungs can hold). A combination like RV plus ERV plus TV doesn’t correspond to a standard lung volume.

Functional residual capacity is the volume of air left in the lungs after you exhale normally. It reflects the resting position where the elastic recoil forces of the lungs and chest wall balance. This volume is the sum of residual volume, the air that remains after a maximal exhale, and expiratory reserve volume, the extra amount you can exhale beyond a normal breath. So, FRC = ERV + RV.

Other ways of combining volumes describe different lung capacities. For example, tidal volume plus inspiratory reserve volume equals inspiratory capacity (the maximum you can inhale after a normal exhale). Residual volume plus vital capacity equals total lung capacity (the maximum amount the lungs can hold). A combination like RV plus ERV plus TV doesn’t correspond to a standard lung volume.