who voluntarily breathe oxygen close to residual volume?

Airway closure as a result of the reduced FRC may lead to atelectasis. In the supine position, the expiratory reserve volume has a mean value of approximately 1 L in males and 600 mL in females. Therefore the reduction in FRC following the induction of anaesthesia will bring the lung volume close to residual volume. This will tend to reduce the end-expiratory lung volume below the closing capacity (CC), particularly in older patients (see Fig. 2.11), resulting in airway closure and collapse of lung. Pulmonary atelectasis can easily be demonstrated in conscious subjects who voluntarily breathe oxygen close to residual volume, and Figure 21.10 shows the effect on arterial Po2 of simulating the reduction in FRC that occurs during anaesthesia. Even if lung collapse does not occur, the airway narrowing caused by reduced lung volume creates areas with low V˙/Q˙ ratios that contribute to impaired gas exchange.

FRC降低导致的气道闭合可能引起肺不张。仰卧位时，补呼气量平均值约为1 L（男性）和600 mL（女性）。因此，麻醉诱导后FRC的降低将使肺容量接近残气量，倾向于将呼气末肺容量降低至闭合容量(CC)以下，特别在老年患者中（见图.2.11），导致气道闭合、肺萎陷。在接近残气量吸氧的清醒自主呼吸的受试者中，很容易证实肺不张，图21.10显示了模拟麻醉期间的FRC降低对PaO₂的影响。即使未发生肺萎陷，肺容量减少引起的气道狭窄也会产生低V/Q比值的区域，从而导致气体交换受损²⁸。

who voluntarily breathe oxygen close to residual volume?啥意思？