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Central Chemoreceptor Stimulus — FRCA Primary MCQ

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HardPhysiology - RespiratoryCentral Chemoreceptor StimulusFRCA Primary

A sudden rise in PaCO2 occurs in a patient with an intact blood–brain barrier. Which sequence best describes the dominant central-chemoreceptor stimulus?

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Correct answer: E — Carbon dioxide enters brain fluid and generates local hydrogen ions

The best answer is “Carbon dioxide enters brain fluid and generates local hydrogen ions”. CO2 diffuses readily across the blood–brain barrier and is hydrated in brain extracellular fluid, increasing H+ concentration and lowering pH; this stimulates central chemoreception. Arterial H+ crosses poorly, so it is not the principal direct signal. Hypoxaemia is sensed mainly by peripheral chemoreceptors, and pulmonary stretch receptors serve a separate mechanoreceptor function.

Reference: RCoA, Primary FRCA syllabus: https://www.rcoa.ac.uk/sites/default/files/documents/2026-01/Primary-FRCA-Syllabus.pdf; Central chemoreceptors: locations and functions: https://pmc.ncbi.nlm.nih.gov/articles/PMC4802370/; The respiratory system and acid–base disorders: https://pmc.ncbi.nlm.nih.gov/articles/PMC10201398/