Breathing room air, a normal PAO2 – PaO2 of 5 to 10 mm Hg exists due to which of the follow-ing?
1. Anatomical shunts in the pulmonary and cardiac circulations
2. Normal limitations to O2 diffusion in the lung
3. Regional differences in pulmonary ventilation and blood flow
a. 1 and 3 only
b. 2 and 4 only
c. 1, 2, and 3
d. 1 and 2 only
ANS: A
Two factors account for the normal PAO2 – PaO2: (1) right-to-left shunts in the pulmonary and cardiac circulation and (2) regional differences in pulmonary ventilation and blood flow.
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