Fig. 6.
Diagram showing how PaO2 depends on the integrated effects of extrapulmonary factors (ventilation, blood flow, Pv̄O2) and intrapulmonary factors [V̇a/Q̇ inequality (which includes shunting) and diffusion limitation]. For each factor, the sign indicates the direction in which PaO2 will move as the indicated factor increases in value (i.e., the positive or negative effect on PaO2 of an increase in the factor). Actual PaO2 thus represents the integrated effects of the individual factors. An increase in blood flow raises PaO2 by increasing Pv̄O2, but may at the same time act to lower it by 1) shortening red cell transit time in the lung, worsening diffusion limitation, and 2) causing more V̇a/Q̇ inequality.