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MRCP Physiology: Acid–Base Balance & Buffers (MRCP Part 1)

TL;DR

Acid–base balance is a core physiology topic in MRCP Part 1, repeatedly tested through ABG interpretation, buffer systems, and compensation logic. The exam rewards conceptual clarity over memorised formulas. Master the bicarbonate buffer system, recognise primary disorders, and avoid common traps to secure reliable marks.


Why acid–base balance matters in MRCP Part 1

Acid–base physiology is not confined to a single paper or system. It underpins respiratory medicine, nephrology, endocrinology, and acute care scenarios tested in MRCP Part 1. Questions often appear simple but are designed to expose weak conceptual understanding—particularly confusion between acidaemia and acidosis, or between compensation and mixed disorders.

The MRCP examiners expect candidates to interpret data logically rather than apply rote calculations. If you can consistently identify the primary disturbance and judge whether compensation is appropriate, you will score well across multiple question stems.

For a structured overview of how this topic fits into the exam, see the official MRCP(UK) syllabus:https://www.mrcpuk.org/mrcpuk-examinations/part-1/syllabus


Scope of acid–base physiology tested

At MRCP Part 1 level, acid–base questions focus on mechanisms and interpretation, not ICU-level management. The examinable scope includes:

  • Definitions: acidaemia vs acidosis, alkalaemia vs alkalosis

  • Buffer systems in blood

  • Respiratory control of pH via CO₂

  • Renal handling of hydrogen ions and bicarbonate

  • Recognition of compensated vs mixed acid–base disorders

You are not expected to memorise detailed correction equations or ventilator physiology.


High-yield concepts you must know (numbered list)

  1. pH depends on the ratio of bicarbonate to PaCO₂, not their absolute values.

  2. The bicarbonate buffer system is the most important extracellular buffer because CO₂ can be excreted by the lungs.

  3. Haemoglobin is a major buffer, particularly for hydrogen ions generated from CO₂ in tissues.

  4. Respiratory disturbances act fast (minutes), while renal compensation is slow (hours to days).

  5. Acidaemia describes blood pH, whereas acidosis describes the underlying process.

  6. Compensation moves pH toward normal but never overshoots.

  7. Normal pH does not exclude pathology—it may indicate a mixed disorder.

  8. Metabolic acidosis stimulates ventilation via peripheral chemoreceptors.

  9. Chloride balance influences bicarbonate handling, especially in vomiting and diarrhoea.

These principles are consistently tested and recur across multiple systems.


Postgraduate medical exam preparation for MRCP Part 1 physiology

The 5 most tested subtopics

1. Henderson–Hasselbalch relationship

You are not expected to calculate logarithms. Instead, understand directionality:

  • ↑ CO₂ → ↓ pH

  • ↓ HCO₃⁻ → ↓ pH

This conceptual framework is sufficient for MRCP Part 1.

2. Bicarbonate buffer system

Carbonic anhydrase allows rapid interconversion of CO₂ and carbonic acid. This is central to gas exchange in red blood cells and hydrogen ion secretion in renal tubules.

3. Respiratory acid–base disorders

  • Respiratory acidosis: hypoventilation → ↑ PaCO₂

  • Respiratory alkalosis: hyperventilation → ↓ PaCO₂

Common exam contexts include COPD, anxiety, and salicylate toxicity.

4. Metabolic acid–base disorders

  • Metabolic acidosis: loss of bicarbonate or gain of acid

  • Metabolic alkalosis: loss of hydrogen ions or gain of bicarbonate

The key is identifying the cause rather than listing differentials.

5. Compensation vs mixed disorders

If compensation is absent or excessive, suspect a mixed acid–base disorder—a frequent source of exam traps.


Summary table: acid–base disorders

Primary disorder

Primary change

Expected compensation

Respiratory acidosis

↑ PaCO₂

↑ HCO₃⁻ (renal)

Respiratory alkalosis

↓ PaCO₂

↓ HCO₃⁻ (renal)

Metabolic acidosis

↓ HCO₃⁻

↓ PaCO₂ (respiratory)

Metabolic alkalosis

↑ HCO₃⁻

↑ PaCO₂ (respiratory)

This table alone can answer a large proportion of MRCP physiology questions.

Short exam-style case (with explanation)

Question A 68-year-old man with severe COPD presents with increasing drowsiness. Arterial blood gas shows:pH 7.33, PaCO₂ 8.2 kPa, HCO₃⁻ 34 mmol/L.

What is the most likely explanation?

Answer Chronic respiratory acidosis with renal compensation.

Explanation The raised PaCO₂ explains the low pH. The elevated bicarbonate reflects renal retention over time, indicating a chronic process. This pattern is classically tested in MRCP Part 1 and should be recognised instantly.


Common pitfalls (and how to avoid them)

  • Confusing compensation with the primary disorder → Always identify the primary change first.

  • Assuming normal pH means normal physiology → Mixed disorders may normalise pH.

  • Ignoring time course → Acute and chronic respiratory disorders differ.

  • Over-interpreting numbers → Direction of change matters more than exact values.

  • Forgetting chloride loss → Vomiting causes metabolic alkalosis via chloride depletion.


Practical study-tip checklist

Use this checklist when revising acid–base balance:

  • Can I clearly identify the primary disturbance?

  • Do I know which organ compensates and how fast?

  • Can I recognise mixed disorders confidently?

  • Have I practised ABG interpretation under time pressure?

Targeted question practice is essential. Use a high-quality MRCP QBank such as the Crack Medicine platform:https://crackmedicine.com/qbank/

To simulate exam conditions, attempt full-length mocks here:https://crackmedicine.com/mock-tests/


FAQs

How important is acid–base balance in MRCP Part 1?

It is a high-yield physiology topic that appears repeatedly, often embedded within respiratory or renal questions.

Do I need to memorise formulas for MRCP Part 1?No. Conceptual understanding is prioritised over calculations.

What is the most common mistake candidates make?

Failing to distinguish compensation from mixed acid–base disorders.

Is ABG interpretation tested directly?

Yes, frequently, especially in COPD and metabolic scenarios.


Ready to start?

If you want consistent marks in physiology, master acid–base balance early. Revise it alongside respiratory and renal physiology, practise with high-quality questions, and test yourself under exam conditions using Crack Medicine’s MRCP resources:


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