Ventilation/Perfusion Matching: Of Myths, Mice, and Men

Abstract

Despite a huge range in lung size between species, there is little measured difference in the ability of the lung to provide a well-matched air flow (ventilation) to blood flow (perfusion) at the gas exchange tissue. Here, we consider the remarkable similarities in ventilation/perfusion matching between species through a biophysical lens and consider evidence that matching in large animals is dominated by gravity but in small animals by structure.

Keywords: computational models; lungs; perfusion; ventilation.

FIGURE 1.

Human lung model used for simulation of V̇_A/Q̇ Transverse sections are shown for the model with 1) structure, hydrostatics, and gravitational deformation; 2) no tissue deformation; 3) only tissue deformation; and 4) zero gravity. The table indicates logSDV and logSDQ calculated for each simulation, as well as the correlation (ρ) between V̇_A and Q̇. Some material is from Ref. , and used with permission from Journal of Applied Physiology.

FIGURE 2.

Perfusion distributions in dog and mouse lungs Perfusion (Q̇) distributions in dog (top) and mouse (bottom) lungs, as simulated using the model of Clark et al. (12) with no gravitational effects (A); including hydrostatic effects and a linear gradient in elastic recoil pressure (B); same as in B but replacing the linear gradient in elastic recoil with deformation of the lung tissue under gravity and a distribution of elastic recoil pressures (C) predicted by the model of Tawhai et al. (68). The Q̇ distribution in dog responds considerably to the inclusion of gravitational factors, however the mouse Q̇ distribution is unchanged. D: summary of the dominating mechanisms.

FIGURE 3.

Ventilation distributions in dog and mouse lung models Ventilation (V̇) distributions in dog (top) and mouse (bottom) lung models as simulated using the model of Swan et al. (66) with no gravitational effects (A; a constant elastic recoil pressure of 5 cmH₂O acting throughout the lung and a uniform distribution of acinar volumes/compliance at functional residual capacity); including a linear gradient in acinar volume at FRC, equivalent to a gradient in elastic recoil pressure of 0.25 cmH₂O/cm lung height (B); and including a deformation of lung tissue under gravity and a distribution of elastic recoil pressures predicted by the model of Tawhai et al. (68) (C). The V̇ distribution in dog responds considerably to the inclusion of gravitational factors, including a strong contribution from local tissue compliance. However, as with the Q̇ model results, the mouse V̇_A distribution is dependent predominantly on structure. D: summary of the dominating mechanisms.

FIGURE 4.

V̇_A/Q̇ distributions in dog and mouse lung models V̇_A/Q̇ distributions in dog (A) and mouse (C) lung models. Although the mechanisms driving V̇_A/Q̇ matching do not have the same relative influence between the species, there is remarkable similarity in the distribution of V̇_A/Q̇ through the lung models, and so the relative distribution of each is not significantly different. These distributions can be formulated in the form of a standard MIGET log plot for dog (B) and mouse (C), and show distributions typical of those obtained experimentally.