2021A Question 14

Discuss how the body handles a metabolic acidosis.

Examiner Report

43.4% of candidates achieved a pass in this question.

In order to pass this question candidates needed to demonstrate an understanding of:

• The intracellular and extracellular buffering systems
• The role, mechanism, and limitations of respiratory compensation in handling a fixed acid.
• The role of the kidney in handling daily bicarbonate recycling, in removing fixed acid (by the formation of titratable acidity), and the production of new bicarbonate in the acidotic state by the ammonia/ammonium system.

The importance of the equation: CO2 + H2O ⇄ H2CO3 ⇄ H+ + HCO3 - to understanding the interlinking of these mechanisms cannot be overstated but was missed by many candidates.

Common errors in the answers included:

• No discussion of intracellular and extracellular buffering systems.
• Descriptions of renal bicarbonate recycling and renal acid handling were varied and often contained errors. Descriptions scored marks when they contained some detail, as found in the recommended texts. Exhaustive detail was not required, as this is a complex area
• Candidates wrote about the physiological effects of acidosis on multiple body systems, rather than body’s handling of a metabolic acidosis. These answers often prevented candidates from presenting enough specific information to pass.
• Errors in equations due to incorrect attribution of valence and unbalanced equations
• Confusion between glutamine, glutamate and glutathione
• It was frequently stated that movement of the equation CO2 + H2O ⇄ H2CO3 ⇄ H+ + HCO3- to the left causes an increase in bicarbonate when in fact the opposite is true

Model Answer

Structure:

• Introduction
• Dilution
• Buffering: ICF and ECF
• Compensation: Respiratory, renal, ion exchange

Introduction

Dilution

Buffering

Intracellular Buffering

Extracellular Buffering

Respiratory Compensation

Renal Compensation

Ion Exchange