Effect of local pulmonary blood flow control on gas exchange: Theory

The effect of local pulmonary blood flow control by local alveolar O₂ tension on steady-state pulmonary gas exchange is analyzed with techniques derived from control theory. In a single homogeneous lung unit with normal inspired and mixed venous blood gas composition, the homeostatic effect on local ventilation-perfusion ratios (V̇A/Q̇) regulation occurs over a restricted range of V̇A/Q̇). The homeostatic effect is maximal at a moderately low V̇A/Q̇ (about 0.4) due to the slope of the O₂ dissociation curve. In a multicompartment lung with a lognormal distribution of V̇A/Q̇, regulation of arterial O₂ tension varies with the extent of inhomogeneity. At mild degrees of inhomogeneity where local pulmonary blood flow (Q̇) control acts predominantly on the lower V̇A/Q̇ of the Q̇ distribution, the regulatory effect is best. At severe degrees of inhomogeneity where local Q̇ control acts mainly on the higher VA/Q of the Q distribution, the regulatory effect is worse, and positive-feedback behavior may occur. Local Q̇ control has the potential of reducing the deleterious effects of lung disease on pulmonary gas exchange particularly when it operates in association with other regulatory mechanisms.