Which of the following values corresponds most closely to the normal PO2 and PCO2 in the mixed venous blood returning to the lungs from the right side of the heart?
a. PO2 = 40 mm Hg; PCO2 = 46 mm Hg
b. PO2 = 40 mm Hg; PCO2 = 100 mm Hg
c. PO2 = 100 mm Hg; PCO2 = 40 mm Hg
d. PO2 = 100 mm Hg; PCO2 = 46 mm Hg
ANS: A
Venous blood returning to the lungs has a lower PO2 (40 mm Hg) than alveolar gas. Thus the pressure gradient for O2 diffusion into the blood is approximately 60 mm Hg (100 mm Hg – 40 mm Hg). Therefore, as blood flows past the alveolus, it takes up O2, leaving the capillary with a PO2 close to 100 mm Hg. Because venous blood has a higher PCO2 than alveolar gas (46 mm Hg vs. 40 mm Hg), the pressure gradient for carbon dioxide causes diffusion of carbon dioxide in the opposite direction, from the blood into the alveolus.
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