Oxygen Cascade Largest Drop — FRCA Primary MCQ

Instant feedback + full explanation. One question, done properly.

HardPhysiology - RespiratoryOxygen Cascade Largest DropFRCA Primary

In a healthy resting adult at sea level breathing room air, which pair of compartments encompasses the greatest overall decrease in PO2 along the oxygen cascade?

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

Reveal the answer and explanation

Correct answer: C — Systemic arterial blood and mitochondria

The correct answer is C. Representative normal values are approximately 13.3 kPa in systemic arterial blood and 1–5 kPa at the mitochondria, giving an overall decrease of about 8–12 kPa. This exceeds the approximately 7 kPa fall from dry atmospheric air (about 21 kPa) to alveolar gas (about 14 kPa). Humidification from atmosphere to trachea produces only a small fall, and the normal alveolar–arterial gradient is also small. The capillary-to-interstitial gradient is an important component of tissue oxygen diffusion but is less than the total arterial-to-mitochondrial decrease. The wording specifies an overall decrease because arterial blood to mitochondria encompasses circulatory unloading and tissue diffusion rather than one adjacent step.

Reference: O’Driscoll BR, Howard LS, Earis J, Mak V; British Thoracic Society Emergency Oxygen Guideline Group. BTS guideline for oxygen use in adults in healthcare and emergency settings, section 3.1, 2017. https://thorax.bmj.com/content/72/Suppl_1/ii1