closing volume


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closing volume

 (CV)
the difference between the closing capacity and the residual volume; the volume of gas in the lungs in excess of the residual volume at the time when small airways in the dependent portions of the lungs close during maximal exhalation, as measured by the single breath nitrogen washout test. Closing volume normally increases with age, is also increased in obstructive airways disease, and can be used to detect small airways disease before symptoms appear. From residual volume, the patient inhales pure oxygen to total lung capacity, and then exhales slowly and evenly while the nitrogen concentration of the exhaled gas is recorded. Because the lower portions of the lung expand more during inspiration, the nitrogen remaining in the alveoli is mixed with more oxygen in the lower portions than in the upper portions. Thus, when the closing volume is reached there is a sharp rise in the nitrogen concentration, because most of the gas is coming from upper air spaces. The closing capacity is equal to closing volume plus residual volume.

clos·ing vol·ume (CV),

the lung volume at which the flow from the lower parts of the lungs becomes severely reduced or stops during expiration, presumably because of airway closure; measured by the sharp rise in expiratory concentration of a tracer gas that had been inspired at the beginning of a breath that started from residual volume.

clos·ing vol·ume

(klōz'ing vol'yūm)
The lung volume at which the flow from the lower parts of the lungs becomes severely reduced or stops during expiration, presumably because of airway closure; measured by the sharp rise in expiratory concentration of a tracer gas that had been inspired at the beginning of a breath that started from residual volume.