Which of the following best represents the partial pressures of all gases in the normally ventilated and perfused alveolus when breathing room air at sea level?
a. PO2 = 40 mm Hg; PCO2 = 100 mm Hg; PN2 = 573 mm Hg; PH2O = 47 mm Hg
b. PO2 = 100 mm Hg; PCO2 = 40 mm Hg; PN2 = 573 mm Hg; PH2O = 47 mm Hg
c. PO2 = 100 mm Hg; PCO2 = 40 mm Hg; PN2 = 713 mm Hg; PH2O = 47 mm Hg
d. PO2 = 149 mm Hg; PCO2 = 40 mm Hg; PN2 = 573 mm Hg; PH2O = 47 mm Hg
ANS: B
Nitrogen is inert and plays no role in gas exchange. However, nitrogen does occupy space and exert pressure. According to Dalton's law, the partial pressure of alveolar nitrogen must equal the pressure it would exert if it alone were present. Thus, to compute the partial pressure of alveolar nitrogen, subtract the pressures exerted by all the other alveolar gases, as follows:
PAN2 = PB – (PAO2 + PACO2 + PH2O)
PAN2 = 760 mm Hg – (100 mm Hg + 40 mm Hg + 47 mm Hg)
PAN2 = 760 mm Hg – 187 mm Hg
PAN2 = 573 mm Hg
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