Video narration
Watching a patient breathe quietly, why don't the lungs collapse during the pause after normal expiration? That pause is not an empty endpoint; it is a resting volume. Functional residual capacity, or FRC, is the gas left after passive expiration: ERV plus RV-not residual volume alone. Here, inward lung recoil balances outward chest-wall recoil. So the lungs retain an oxygen reservoir and buffer against closure between breaths. They don't collapse: that balance leaves FRC behind after each expiration.