[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Physiology](https://durgical.com/en/physiology/) › [Renal, fluids and acid–base](https://durgical.com/en/physiology/#s-renal) › Metabolic acidosis, the anion gap and compensation [Physiology](https://durgical.com/en/physiology/) · [Renal, fluids and acid–base](https://durgical.com/en/physiology/#s-renal) # Metabolic acidosis, the anion gap and compensation - Mechanism - Exam priority: high In metabolic acidosis the blood loses bicarbonate or gains acid. The lungs breathe more to blow off CO₂ and the kidneys make new bicarbonate. Updated 2026-11-01 Also called: Kussmaul breathing, Winter's formula **See it in 3D:** [Kidneys](https://durgical.com/en/anatomy/kidney/) · [Medulla oblongata](https://durgical.com/en/anatomy/medulla-oblongata/) ## Key points - Expected compensation (Winter's): PaCO₂ = 1.5 × HCO₃⁻ + 8 ± 2. If PaCO₂ is higher, there is also respiratory acidosis; if lower, respiratory alkalosis. - Peripheral chemoreceptors (carotid body) sense H⁺ and raise ventilation: Kussmaul breathing, deep and fast. - High anion gap: an acid has been added with its anion (ketoacids, lactate, toxins, uremia). - Normal gap (hyperchloremic): HCO₃⁻ has been lost and Cl⁻ replaces it (diarrhea, renal tubular acidosis, large volumes of saline). - The kidney responds by excreting more NH₄⁺ (from glutamine) and regenerating HCO₃⁻; in chronic kidney disease that response fails. - Acidosis moves K⁺ out of cells and raises ionized calcium. ## What it is Metabolic acidosis is a primary fall in plasma HCO₃⁻, either because an acid is added (which consumes HCO₃⁻) or because HCO₃⁻ is lost through the gut or kidney. The body defends itself in three stages: buffers within seconds, hyperventilation within minutes to hours and increased renal acid excretion over days. The anion gap calculation (Biochemistry page) separates the two big groups of causes. ## How it works, step by step - A strong acid enters (e.g. ketoacids): its H⁺ reacts with HCO₃⁻ to form CO₂ and water, and HCO₃⁻ falls. - The acid's anion (β-hydroxybutyrate, lactate) stays in plasma unmeasured, raising the anion gap. - Proteins, phosphate and bone buffer part of the H⁺ over seconds to hours. - H⁺ stimulates peripheral chemoreceptors; ventilation rises, PaCO₂ falls and pH moves toward normal without reaching it. - In the proximal tubule, glutamine is converted to NH₄⁺ and new HCO₃⁻; NH₄⁺ is excreted and HCO₃⁻ returns to blood. - In the collecting duct, α-intercalated cells secrete more H⁺, excreted bound to NH₃ and phosphate. - If the cause is HCO₃⁻ loss (diarrhea), the kidney retains Cl⁻ and the gap stays normal. ## Equations **Expected respiratory compensation (Winter's)**Expected PaCO₂ = 1.5 × HCO₃⁻ + 8 ± 2 **PaCO₂** mmHg **HCO₃⁻** mEq/L Shortcut: PaCO₂ usually resembles the last two digits of the pH (pH 7.25 → PaCO₂ ≈ 25). **Anion gap**AG = Na⁺ − (Cl⁻ + HCO₃⁻) **AG** ≈ 8–12 mEq/L (laboratory dependent) Add ≈ 2.5 for every 1 g/dL that albumin is below 4. Details and the delta ratio are on the Biochemistry page. ## Two families of metabolic acidosis | Type | What happens | Causes | Clue | | --- | --- | --- | --- | | [High anion gap](https://durgical.com/en/conditions/high-anion-gap-metabolic-acidosis/) | An acid with an unmeasured anion is added | Ketoacidosis, lactic acidosis, uremia, methanol, ethylene glycol, salicylates | High unmeasured anion; high osmolar gap with methanol or ethylene glycol | | [Normal gap: gut loss](https://durgical.com/en/conditions/acute-gastroenteritis/) | HCO₃⁻ is lost through the bowel | Diarrhea, fistulas | Negative urine anion gap (high NH₄⁺) | | [Normal gap: renal](https://durgical.com/en/conditions/normal-anion-gap-metabolic-acidosis/) | The kidney loses HCO₃⁻ or fails to excrete H⁺ | Renal tubular acidosis, carbonic anhydrase inhibitors | Positive urine anion gap | | [Normal gap: dilutional](https://durgical.com/en/conditions/normal-anion-gap-metabolic-acidosis/) | Lots of Cl⁻ without HCO₃⁻ | Large volumes of 0.9% saline | High Cl⁻ after resuscitation | ## Clinical notes - Diabetic ketoacidosis presents with Kussmaul breathing, fruity breath, dehydration and a high anion gap. - In shock, hypoperfusion produces lactic acidosis; a high lactate marks severity. - Acute diarrhea in children can cause hyperchloremic acidosis with hypokalemia. - In chronic kidney disease, lower NH₄⁺ production leaves an acidosis that harms bone and muscle. ## Memory aid 'Acid arriving brings its anion; bicarbonate leaving leaves its chloride': high gap if an acid arrived, normal gap if bicarbonate left. ## Test yourself Think of the answer, then tap the question to see it. ## A patient has HCO₃⁻ 10 mEq/L and PaCO₂ 32 mmHg: is compensation appropriate? No. Winter's predicts 1.5 × 10 + 8 = 23 ± 2 mmHg. A PaCO₂ of 32 is higher than expected, so there is also respiratory acidosis (ventilation is not keeping up). ## Why does diarrhea cause a normal anion gap acidosis and ketoacidosis a high gap? In diarrhea HCO₃⁻ is lost and the kidney retains Cl⁻, which takes its place. In ketoacidosis an acid is added whose anion (β-hydroxybutyrate) is not measured and opens the gap. ## How does the kidney generate new bicarbonate in metabolic acidosis? The proximal tubule metabolises glutamine to NH₄⁺ and HCO₃⁻; it excretes NH₄⁺ and returns HCO₃⁻ to blood. The collecting duct secretes H⁺, excreted as NH₄⁺ and titratable acid. ## Why can metabolic acidosis raise serum potassium? Excess H⁺ enters cells and K⁺ leaves to keep electrical neutrality; its secretion in the collecting duct also falls. [Add to my cards (4)](https://durgical.app/estudiar/?add=/en/physiology/metabolic-acidosis-the-anion-gap-and-compensation/) ## See it in 3D The structures of Durgical's 3D anatomy where it happens. - [Kidneys](https://durgical.com/en/anatomy/kidney/) - [Medulla oblongata](https://durgical.com/en/anatomy/medulla-oblongata/) ## Related topics - [Reading an arterial blood gas step by step](https://durgical.com/en/physiology/reading-an-arterial-blood-gas-step-by-step/) - [Renal handling of H⁺ and bicarbonate](https://durgical.com/en/physiology/renal-handling-of-h-and-bicarbonate/) - [Buffers and the Henderson–Hasselbalch equation](https://durgical.com/en/physiology/buffers-and-the-henderson-hasselbalch-equation/) - [Mixed acid–base disorders](https://durgical.com/en/physiology/mixed-acid-base-disorders/) - [Control of breathing, chemoreceptors and lung reflexes](https://durgical.com/en/physiology/control-of-breathing-chemoreceptors-and-lung-reflexes/) - [Potassium balance](https://durgical.com/en/physiology/potassium-balance/) ## Related conditions - [High anion gap metabolic acidosis](https://durgical.com/en/conditions/high-anion-gap-metabolic-acidosis/) - [Normal anion gap (hyperchloraemic) metabolic acidosis](https://durgical.com/en/conditions/normal-anion-gap-metabolic-acidosis/) - [Diabetic ketoacidosis](https://durgical.com/en/conditions/diabetic-ketoacidosis/) - [Lactic acidosis](https://durgical.com/en/conditions/lactic-acidosis/) - [Methanol poisoning](https://durgical.com/en/conditions/methanol-poisoning/) - [Ethylene glycol poisoning](https://durgical.com/en/conditions/ethylene-glycol-poisoning/) - [Chronic kidney disease](https://durgical.com/en/conditions/chronic-kidney-disease/) - [Acute (viral) gastroenteritis](https://durgical.com/en/conditions/acute-gastroenteritis/) ## In Biochemistry - [Anion gap and delta ratio](https://durgical.com/en/biochemistry/topics/anion-gap-and-delta-ratio/) - [Albumin](https://durgical.com/en/biochemistry/enzymes/albumin/) - [Bicarbonate–CO₂ buffer system](https://durgical.com/en/biochemistry/topics/bicarbonate-co-buffer-system/) - [Ammonia (ammonium)](https://durgical.com/en/biochemistry/molecules/ammonia/) - [Glutamine](https://durgical.com/en/biochemistry/molecules/glutamine/) - [Acid–base compensation rules](https://durgical.com/en/biochemistry/topics/acid-base-compensation-rules/) [← All of physiology](https://durgical.com/en/physiology/) Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given. Sources: [26.5 Disorders of Acid-Base Balance](https://openstax.org/books/anatomy-and-physiology-2e/pages/26-5-disorders-of-acid-base-balance) (OpenStax (Rice University), 2022); [Metabolic acidosis: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/000335.htm) (MedlinePlus, US National Library of Medicine, 2026); [Diabetic ketoacidosis: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/000320.htm) (MedlinePlus, US National Library of Medicine, 2026); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 31). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016). ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/physiology/metabolic-acidosis-the-anion-gap-and-compensation/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/physiology/metabolic-acidosis-the-anion-gap-and-compensation/