functional residual capacity

Functional residual capacity is the amount of air left in your lungs at the end of a normal, relaxed breath out — the resting point your chest settles to when you stop and do nothing. It is the lung's neutral idling volume, the place it returns to between every quiet breath.

Physically, it is the balance point between two opposing springs: the lungs always pulling inward to deflate, and the chest wall always springing outward to expand. Where these forces cancel, the system rests — and that resting volume is the FRC.

It equals the residual volume plus the expiratory reserve volume. Because it includes the unexhalable residual volume, FRC cannot be measured by simple spirometry and is obtained by gas dilution or body plethysmography. A typical adult value is around 2.3 to 3 litres.

FRC acts as an oxygen buffer: this standing reservoir keeps supplying oxygen to the blood during the gap of expiration, smoothing what would otherwise be a jerky supply. It rises in hyperinflation (emphysema, severe asthma) and falls when the lung is compressed or stiffened, or in obesity and when lying flat.