BRS Physiology error?

[318038]

I had a question regarding page 138 in the BRS Physio book. It states that at the apex (high V/Q) that there is more gas exchange even though the P02 is high and the PCO2 is low. I thought at the base of the lung, where P02 is lower and and PCO2 is high, there is the most gas exchange. Am I completely off on this? I am confused by point #4 under figure 4-11. Thanks for the help

 

[turkeyjerky]

nobody's gonna ask you what part of the lung has the most gas exchange. the point is that, at the apex, there is more high O2 air relative to low O2 blood (because of the increase in ventilation to perfusion ratio), hence the PO2 is highest at the apex of the lungs. per unit blood, a greater amount of gas exchange occurs at the apices. we know this because blood in zone 1 will go from a mixed venous PO2 of about 40 to a P02 of about 138, as opposed to zone 3, where the PO2 goes from 40 to 95.