Anatomical backgrounds on gas exchange parameters in the lung

Figure 10 Ventilation-perfusion (V_A/Q) regions detected from gas exchange behaviors of O₂, CO₂, and inert gases. Left ordinate: excretion (E) of inert gas defined as P_A/P_v, where P_A is partial pressure of inert gas in each functional gas-exchange unit, while P_v is that in mixed venous blood. Right ordinate: P_AO2 and P_ACO2 in each functional gas-exchange unit. Abscissa: logarithmic V_A/Q value. E of each inert gas sharply changes in a certain range of V_A/Q depending on its blood-gas partition coefficient (λ). For instance, sharp change in E for SF₆ is found at V_A/Q ranging between 0.001 and 0.01. As such, SF₆ has high sensitivity to detecting gas exchange in regions with very low V_A/Q. On the other hand, E for acetone sharply changes at V_A/Q ranging from 10 to 100, indicating that acetone is susceptible to gas exchange in regions with very high V_A/Q. P_AO2 changes greatly at V_A/Q ranging between 0.1 and 1.0, suggesting that O₂ has sensitivity to detecting gas exchange in regions with moderately low V_A/Q (like cyclopropane). Meanwhile, CO₂ is susceptible to gas exchange in regions with moderately high V_A/Q ranging from 1.0 to 10 (like halothane).