Figure 17-18 Measurement of closing capacity with the use of a tracer gas such as xenon 133 (¹³³ Xe). The bolus of tracer gas is inhaled near residual volume and, because of airway closure in the dependent lung, is distributed only to nondependent alveoli whose air passages are still open (shown as crosshatched in diagram). During expiration, the concentration of tracer gas becomes constant after the dead space is washed out. This plateau (phase 3) gives way to a rising concentration of tracer gas (phase 4) when there is once again closure of the dependent airways because the only contribution made to expired gas is by the nondependent alveoli with a high ¹³³ Xe concentration. CC, closing capacity; FRC, functional residual volume; RV, residual volume; TLC, total lung capacity. (Redrawn with modification from Lumb AB: Respiratory system resistance: Measurement of closing capacity. In Lumb AB [ed]: Nunn's Applied Respiratory Physiology, 5th ed. London, Butterworths, 2000, p 79.)