Which statement about the distribution of O2 and CO2 in upright lungs is true?

Which statement about the distribution of O2 and CO2 in upright lungs is true?

• A. PaO2 is greater at the apex than at the base
• B. PaCO2 is greater at the apex than at the base
• C. Both PaO2 and PaCO2 are greater at the apex than at the base
• D. Both PaO2 and PaCO2 are greater at the base than at the apex

Answer: (A)

In upright lungs, gravity creates a gradient in both blood flow and ventilation. Blood flow is greater at the base, while the apex tends to have relatively more ventilation relative to perfusion, giving a higher V/Q ratio at the apex and a lower V/Q ratio at the base. A higher V/Q ratio means more oxygen is delivered to the blood per unit of blood flow and CO2 is removed more efficiently at that region. Thus PaO2 is higher in blood exiting the apical regions than in blood exiting the basal regions, reflecting better oxygen uptake there. PaCO2, on the other hand, is lower at the apex because the increased ventilation relative to perfusion enhances CO2 elimination. Therefore the statement that PaO2 is greater at the apex than at the base is true, while PaCO2 is not greater at the apex.

In upright lungs, gravity creates a gradient in both blood flow and ventilation. Blood flow is greater at the base, while the apex tends to have relatively more ventilation relative to perfusion, giving a higher V/Q ratio at the apex and a lower V/Q ratio at the base. A higher V/Q ratio means more oxygen is delivered to the blood per unit of blood flow and CO2 is removed more efficiently at that region.

Thus PaO2 is higher in blood exiting the apical regions than in blood exiting the basal regions, reflecting better oxygen uptake there. PaCO2, on the other hand, is lower at the apex because the increased ventilation relative to perfusion enhances CO2 elimination. Therefore the statement that PaO2 is greater at the apex than at the base is true, while PaCO2 is not greater at the apex.