Similar to the case for pulmonary blood flow, height and gravity cannot explain all aspects of a nonuniform distribution of ventilation. Ventilation is more uniform in the supine lung than in the upright lung because the vertical height is reduced. However, considerable differences in ventilation persist between the apex and base of the supine lung, and these are called intraregional differences in ventilation. Such intraregional differences in ventilation are caused by local differences in resistance (R) and compliance (C) of terminal airways, which determines the airways time constant (r):

Time constants are a measure of the time it takes to fill a given volume, so terminal airways with greater resistances or compliance are ventilated more slowly than airways with lower resistance or compliance. Lung disease can alter resistance and/or compliance and increase time constants fivefold. Because lung disease is frequently patchy in the lungs, this leads to large intraregional differences in ventilation. The harmful effects of such ventilatory inequality on gas exchange are explained in Chapter 21.

The single breath nitrogen test can be used to assess the degree of ventilatory inequality in humans. A single breath of 100% oxygen (O2) is inspired, and the N2 concentration is plotted as a function of expired volume on the following expiration; this is the same procedure used to measure dead space by Fowler's method (see Chapter 18, Fig. 10). During the inspiration, O2 will dilute the resident N2 more in better ventilated lung regions. During a slow expiration, different lung regions will empty at different rates and the alveolar plateau of N2 concentration will slope upward (unlike the flat plateau shown for an ideal lung in Chapter 18, Fig. 10). Frequently, a large increase in nitrogen concentration will occur at the end of the slow expiration, as airway closure collapses airways in the bottom of the lung, and the more poorly ventilated apical regions of the lung contribute more to the expired gas.

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