High-yield map
Key points before the detail
- 01
Think of pH as a bicarbonate-to-CO2 relationship rather than isolated values.
- 02
Lungs control volatile acid through CO2 excretion; kidneys regenerate bicarbonate and excrete non-volatile acid.
- 03
Compensation is predictable but incomplete; values that move far beyond expectation suggest a mixed disorder.
- 04
The anion gap is a bookkeeping tool for unmeasured ions, not a diagnosis by itself.
The bicarbonate buffer system
Carbon dioxide combines with water to form carbonic acid, which dissociates into hydrogen ions and bicarbonate. Carbonic anhydrase accelerates this reaction in many tissues. Because the lungs can excrete CO2 and the kidneys can control bicarbonate and acid excretion, this buffer links respiratory and metabolic physiology.
Respiratory control
Alveolar ventilation is inversely related to PaCO2 when carbon-dioxide production is stable. Hyperventilation lowers PaCO2 and tends to raise pH; hypoventilation raises PaCO2 and tends to lower pH. Respiratory compensation for a metabolic disturbance begins quickly because ventilation can change within minutes.
Renal control
- Filtered bicarbonate is normally reclaimed rather than lost in large quantities.
- Hydrogen ions are secreted into tubular fluid and buffered by phosphate and ammonia systems.
- Generation and excretion of ammonium becomes increasingly important during sustained acid loads.
- Renal compensation is slower than respiratory compensation and depends on functioning kidneys.
Primary disorders and compensation
| Primary change | Expected compensatory direction |
|---|---|
| Metabolic acidosis: HCO3− falls | Ventilation rises and PaCO2 falls. |
| Metabolic alkalosis: HCO3− rises | Ventilation falls and PaCO2 rises, limited by the need to maintain oxygenation. |
| Respiratory acidosis: PaCO2 rises | Renal bicarbonate retention/regeneration increases, especially if chronic. |
| Respiratory alkalosis: PaCO2 falls | Renal bicarbonate falls, more noticeably if chronic. |
Why the anion gap helps
Plasma must remain electrically neutral, but routine chemistry does not measure every cation and anion. The anion gap estimates the difference between commonly measured cations and anions. A rise suggests accumulation of unmeasured anions such as lactate or ketoacids; a normal-gap acidosis often reflects bicarbonate loss with a compensatory rise in chloride. Albumin is an important unmeasured anion, so hypoalbuminaemia can conceal a raised gap.
Active recall
Close the notes and answer these
Try each question from memory before revealing the answer. These public prompts are a small preview of the integrated retrieval system inside SurgAtlas.
01Which system compensates more rapidly for a primary metabolic acid–base disturbance?
The respiratory system, by changing alveolar ventilation and PaCO2 within minutes.
02Can normal physiological compensation usually push pH past the normal range?
No. Apparent over-compensation should make you consider a second primary acid–base disorder.
03Why can low albumin hide a raised anion gap?
Albumin is a major unmeasured anion; reducing it lowers the baseline gap, so added pathological anions may be less obvious.
Sources & editorial basis
References & editorial basis
- SurgAtlas production chapter — Applied Physiology — Acid–Base & Renal Physiology. Primary source for this public lesson. Stable physiological principles have been condensed from the corresponding SurgAtlas production teaching.
- Guyton and Hall Textbook of Medical Physiology. Reference for stable acid–base regulation.
This lesson is derived from the corresponding SurgAtlas production teaching material. Where the source makes current management or guideline claims, the public lesson uses the cited contemporary guidance. It is written for education and examination preparation, not as patient-specific clinical advice.
Editorial details
Medical Doctor (MD) · MRCS Part A · Physician · Surgical Educator
SurgAtlas is an educational resource. For patient care, verify current national guidance, local antimicrobial and transfusion policies, specialty pathways and individual patient factors.
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