Even in healthy young subjects, regional differences in pulmonary ventilation and blood flow result in the PaO2 being lower than the PAO2. Why is this so?
a. Most blood flows through the apexes of the lung.
b. Most blood flows through areas with high.
c. Most blood flows through the bases of the lung.
d. Most ventilation goes to the apexes of the lung.
ANS: C
As shown in Table 12-1, because of gravity most blood flows to the lung bases, where the PO2 is lower and the PCO2 is higher than normal. After leaving the lung, this relatively large volume of blood combines with the smaller volume coming from the middle and apical regions. The result is a mixture of blood with less O2 and more carbon dioxide than would come from an ideal gas ex-change unit.
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