residual volume
Residual volume is the air that stays in the lungs no matter how hard you breathe out. Try it: blow out everything you can, and there is still air inside — the lungs never fully deflate, like a sponge that stays a little puffed even when squeezed.
This leftover air is useful, not wasted. By keeping the alveoli partly inflated between breaths, it prevents the tiny air sacs from collapsing shut and keeps gas exchange going continuously, so blood passing through never sees fully empty lungs.
In an average adult the residual volume is about 1.2 litres. It cannot be measured by blowing into a spirometer (you cannot exhale it), so it is found indirectly — by gas-dilution techniques or by body plethysmography, which measures all the gas in the chest.
Residual volume rises when air is trapped behind narrowed airways, as in emphysema, where loss of elastic recoil lets the airways collapse on exhalation. A high residual volume means the lungs sit chronically over-inflated, which flattens the diaphragm and makes each breath harder.
In severe emphysema the residual volume can balloon to two or three times normal, so much of the lung is stale trapped air that fresh breaths barely move.
Residual volume swells with air trapping.
Because residual volume cannot be exhaled, spirometry alone cannot reveal it. Diagnosing air trapping and hyperinflation requires full lung-volume measurement, typically body plethysmography.