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Bicarbonate Buffer System — FRCA Primary MCQ

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ModeratePhysiology - RespiratoryBicarbonate Buffer SystemFRCA Primary

Which of the following is the principal extracellular buffer system in plasma, with a volatile component regulated by ventilation and a base component regulated mainly by the kidneys?

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Correct answer: D — Bicarbonate buffer system

The correct answer is D, the bicarbonate buffer system: CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3−. It is the principal extracellular buffer because it operates as an open system. Ventilation rapidly alters the volatile component, PaCO2, while the kidneys reabsorb filtered bicarbonate, generate new bicarbonate and excrete non-volatile acid. This compensates for its pKa of approximately 6.1 being below physiological pH. Haemoglobin is an important non-bicarbonate buffer, particularly inside erythrocytes, so an unqualified reference to whole blood can be misleading. Plasma proteins also buffer hydrogen ions but are less important systemically. Phosphate has a greater role intracellularly and in renal tubular fluid, while ammonia primarily facilitates renal acid excretion.

Reference: Fraser A, Ong YE. Interpreting arterial blood gases: Acid-base disturbances. BMJ. 2012;344:e818. https://www.bmj.com/content/344/sbmj.e818.abstract