Functional residual capacity is the amount of air left in the lungs at the end of a normal, relaxed exhale — the lungs' resting volume between breaths.
FRC is the expiratory reserve volume plus the residual volume.
It's the volume that keeps alveoli open between breaths and buffers oxygen levels. Conditions and positions that reduce FRC — obesity, supine positioning, anesthesia — increase the risk of alveolar collapse.
Roughly 2.4–3 L in an average adult. It's the air remaining at the end of a normal, relaxed exhale.
Lying flat, obesity, pregnancy, general anesthesia, and restrictive lung disease all reduce it, which is why atelectasis is common in these settings.
RV is the air left after a maximal exhale. FRC is the larger volume left after a normal, relaxed exhale, so it includes RV plus the expiratory reserve volume.