What is the reason for having residual volume in the lung?

The residual volume is the volume of air in the lungs at the end of maximal expiration (the volume of air which cannot voluntarily exhale from lungs) Residual air cannot be exhaled. Volume can be estimated through gas dilution techniques and the use of helium in inspired air.

• The lungs are never completely empty. There is always some air left in the lungs after a maximal exhalation.
• If this residual volume did not exist and the lungs emptied completely, the lung tissues would stick together.
• The energy necessary to re-inflate the lungs would be extremely large. Therefore, there is always some air remaining in the lungs.
• Residual volume is also important for preventing large fluctuations in respiratory gases (O2 and CO2).
• The residual volume is the only lung volume that cannot be measured directly because it is impossible to completely empty the lung of air. This volume can only be calculated rather than measured.