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Alveolar Gas Equation — FRCA Primary MCQ

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HardPhysiology - RespiratoryAlveolar Gas EquationFRCA Primary

On room air at sea level, PaCO2 is 5.3 kPa and respiratory quotient is 0.8. Using PAO2 = FiO2(PB − PH2O) − PaCO2/R with PB 101.3 kPa and PH2O 6.3 kPa, what PAO2 is closest?

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Correct answer: B — 13.3 kPa from 19.95 minus 6.625

The best answer is “13.3 kPa from 19.95 minus 6.625”. Inspired oxygen pressure is 0.21 × (101.3 − 6.3) = 19.95 kPa. The carbon-dioxide term is 5.3/0.8 = 6.625 kPa, so PAO2 is 13.33 kPa. The calculation shows why normal alveolar oxygen tension exceeds arterial oxygen tension despite the physiological alveolar–arterial gradient.

Reference: Resuscitation Council UK, Adult Advanced Life Support Guidelines 2025: https://www.resus.org.uk/cy/node/36438