Which of the following is true about Residual volume?

The volume of air remaining in the lungs after a maximal effo to exhale all the air possible (ie, after your best effo to "empty" your lungs) is the residual volume. This volume cannot be measured directly by a spirometer, which measures only change in volume. Since you cannot voluntarily change your lung volume below the residual volume, the spirometer cannot measure it. Other methods (eg, body plethysmography, ine gas dilution) must be used to measure residual volume.