Fig. 4.

On the O₂–CO₂ diagram for alveolar air, the thin parallel lines are isopleths for a lung respiratory quotient of − 1.0, which intercept the x-axis at different inspired air points. As the inspired partial pressure of oxygen ($P_I{O}_2$) decreases, the isopleth is displaced to the left. The black dots correspond to the incurring alveolar gas composition at each $P_I{O}_2$. The line connecting all these points (thick black curve) is the alveolar air curve. $P_A{\mathit{CO}}_2$ and $P_A{O}_2$ are the alveolar partial pressures of carbon dioxide and oxygen, respectively. ${\dot{V}}_A/{\dot{V}}_R{\mathit{CO}}_2$ alveolar ventilation to lung carbon dioxide flow ratio

[Modified after Ferretti (2015)]