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Alveolar oxygen tension at altitude — FRCA Primary MCQ

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ModeratePhysiologyAlveolar oxygen tension at altitudeFRCA Primary

A healthy volunteer ascends rapidly to high altitude. Barometric pressure falls while inspired oxygen fraction remains 0.21. Which change most directly lowers alveolar oxygen tension?

Educational content. Not a substitute for clinical judgement or local policy.

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Correct answer: D — Reduced inspired oxygen partial pressure

At altitude, FiO2 remains about 0.21 but barometric pressure falls, lowering inspired oxygen partial pressure and therefore alveolar oxygen tension. Water vapour pressure at body temperature is constant and subtracts from inspired pressure. Hyperventilation lowers PaCO2 and partly offsets the fall in PAO2. The pearl is that oxygen fraction and oxygen partial pressure are not the same.

Reference: RCoA Primary FRCA Syllabus 2026; Ganong's Review of Medical Physiology; West's Respiratory Physiology