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Oxygen content in anaemia — FRCA Primary MCQ

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HardPhysiologyOxygen content in anaemiaFRCA Primary

A 58-year-old woman awaiting urgent laparotomy has Hb 72 g L-1, SaO2 98%, PaO2 12.8 kPa and cardiac output 5.0 L min-1. Despite a normal oxygen saturation, her lactate is rising. What is the most likely explanation?

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Correct answer: A — Reduced arterial oxygen content from low haemoglobin concentration

Arterial oxygen content depends mainly on haemoglobin-bound oxygen, so severe anaemia can markedly reduce oxygen delivery despite normal SaO2 and PaO2. PaO2 contributes little to total oxygen content because dissolved oxygen is a small fraction under normal pressure. A left shift can impair unloading, but the dominant abnormality here is low haemoglobin. The clinical pearl is that saturation is not a measure of oxygen content.

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