The residual volume is that part of the air in the lung which cannot be exhaled; it represents the remaining volume of lung when forced exhalation of the lung has occured.
It is calculated by subtracting expiratory reserve volume from functional residual capacity. The residual volume is usually 20-25% of total lung capacity.
The residual volume increases in diffuse airway obstruction. In this situation there is also an increase in the ratio of residual volume to total lung capacity.
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