Circle System CO2 Absorption — FRCA Primary MCQ

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EasyPhysics & Clinical MeasurementCircle System CO2 AbsorptionFRCA Primary

In a circle breathing system, which component chemically removes carbon dioxide from exhaled gas before the remaining gas is rebreathed?

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Correct answer: E — Soda lime canister

The correct answer is E. The soda lime canister contains a carbon dioxide absorbent that chemically removes CO2 from exhaled gas, allowing the remaining oxygen and anaesthetic gases to be rebreathed. The net reaction converts carbon dioxide and calcium hydroxide to calcium carbonate and water and is exothermic; modern absorbent formulations vary in their additional alkali content. Unidirectional valves ensure one-way flow around the circle but do not remove CO2, although valve failure can cause abnormal rebreathing. The fresh gas inlet supplies new gas, the adjustable pressure-limiting valve controls circuit pressure and vents excess gas during manual or spontaneous ventilation, and the reservoir bag stores gas and permits manual ventilation. None of these substitutes for the carbon dioxide absorber.

Reference: Shelton CL, McBain SC, Mortimer F, White SM. Towards zero carbon healthcare: anaesthesia. BMJ. 2022;379:e069030. https://www.bmj.com/content/379/bmj-2021-069030