Intrapleural pressure (cm H2O)
FIGURE 11 Regional differences in ventilation at FRC are caused by a gradient of intrapleural pressure down the lung as explained in the text. The base of the lung operates on a more compliant (steeper) part of the pressure-volume curve, so volume changes more at the bottom than at the top of the lung for the same intrapleural pressure change. (After West, Ventilation/blood flow and gas exchange, New York: Blackwell Scientific, 1990.)
Figure 11 shows how this difference in Ppl between the top and bottom of the lung and the nonlinear compliance curve increases regional ventilation at the bottom of the lung at FRC. If the whole-lung static compliance curve is assumed to apply to any region of the lung (a reasonable assumption for this discussion), then the top of the lung is relatively more expanded and less compliant than the bottom of the lung. The bottom regions of the lung operate on a steeper portion of the compliance curve, so an inspiratory effort is more effective at increasing volume in the bottom of the lung. The decrease in Ppl with inspiratory muscle contraction is the same at the top and bottom of the lung.
At lower lung volumes, the situation may change. At RV, for example, Ppl may be positive at the bottom, but still negative at the top of the lung. The effects on ventilation at RV can be estimated from the compliance curve in Fig. 11 assuming a Ppl of +3.5 cm H2O at the bottom and -4.0 cm H2O at the top of the lung at RV. At RV, the compliance curve is flat for the bottom of the lung, so no volume change would occur, and all the volume change would occur in higher parts of the lung with greater compliance. Such airway closure traps gas in alveoli behind small collapsed airways at the base of the lungs and only occurs at very low lung volumes in healthy young individuals. However, as lung compliance decreases with age or pulmonary disease, reduced elastic recoil tends to increase Ppl. This makes the lungs more susceptible to airway closure, and ventilation to the base of the lungs may be intermittent.
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