Conditions · Metabolism (biochemistry)
Metabolic alkalosis !
Blood bicarbonate rises, usually from vomiting or diuretics; the kidney will not shed it while volume, chloride or potassium are short.
Updated 2026-11-01
Structures involved
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In biochemistry
In embryology
In physiology
- Gastric acid secretion
- Gastric motility and emptying
- Vomiting and its control
- Transport in the loop of Henle
- Distal tubule and collecting duct
- Reading an arterial blood gas step by step
- Where and how diuretics act
- Renal handling of H⁺ and bicarbonate
- Metabolic alkalosis and its compensation
- Mixed acid–base disorders
Why it happens
What it is
A primary rise in plasma bicarbonate with high pH, sustained because the kidney reabsorbs it when volume, chloride or potassium are depleted or mineralocorticoids are in excess.
Who gets it
It is the most common acid–base disorder in hospital patients, mainly from diuretics and gastric aspiration or vomiting. In infants, hypertrophic pyloric stenosis is the classic cause.
Causes
- Loss of gastric acid: vomiting, nasogastric drainage, pyloric stenosis
- Loop and thiazide diuretics
- Mineralocorticoid excess: primary hyperaldosteronism, Cushing syndrome, liquorice
- Bartter and Gitelman syndromes
- Alkali load (bicarbonate, citrate in massive transfusion, milk–alkali syndrome) and post-hypercapnic alkalosis
Risk factors
- Prolonged vomiting (gastroenteritis, hyperemesis, bulimia)
- Diuretic use
- Resistant hypertension with hypokalaemia (aldosteronism)
- Ventilation that rapidly corrects chronic hypercapnia
Pathophysiology
Generation: H⁺ is lost (stomach, kidney) or HCO₃⁻ is gained. Maintenance: with volume and chloride depletion, the kidney reabsorbs sodium with bicarbonate in the proximal tubule; aldosterone drives H⁺ and K⁺ secretion in the collecting duct, so paradoxical aciduria and hypokalaemia appear, and hypokalaemia in turn increases HCO₃⁻ reabsorption. Urine chloride separates the causes: low in saline-responsive ones (vomiting, diuretics already stopped) and high in unresponsive ones (mineralocorticoid excess, Bartter, Gitelman, current diuretic). Respiratory compensation (hypoventilation) is limited. Alkalaemia lowers ionised calcium and favours arrhythmias.
Classification
- Low urine chloride: chloride/volume-responsive (vomiting, gastric drainage, past diuretic, post-hypercapnic)
- High urine chloride: unresponsive (hyperaldosteronism, Cushing, Bartter, Gitelman, current diuretic, severe hypokalaemia)
Symptoms and signs
- Symptoms of the cause (vomiting, dehydration)
- Cramps, weakness and paraesthesia
- Tetany or carpopedal spasm from low ionised calcium
- Palpitations
- Confusion in severe cases
Diagnosis
- Clinical
- History of vomiting, tubes, diuretics or hypertension; assessment of volume status and blood pressure.
- Tests
- Gases with high HCO₃⁻ and pH, electrolytes (low chloride and potassium), urine chloride, renin and aldosterone if hypertensive, magnesium.
- Imaging
- Pyloric ultrasound in a vomiting infant; adrenal CT if primary aldosteronism is confirmed.
Treatment
- Medical
- In the chloride-responsive form: saline to restore volume and chloride, plus potassium
- Medical
- Correct hypokalaemia and hypomagnesaemia, which sustain the alkalosis
- Medical
- In the unresponsive form: treat mineralocorticoid excess (aldosterone antagonists, surgery for an adenoma) and adjust diuretics; acetazolamide in selected cases
- Follow-up
- Monitoring of gases, electrolytes and heart rhythm
Complications
- Arrhythmias
- Tetany and seizures
- Hypoventilation with hypoxaemia
- Severe hypokalaemia
Prognosis
Good once the cause is corrected; severe alkalaemia (very high pH) is linked to higher mortality in critically ill patients.
Test yourself
Think of the answer, then tap the question to see it.
Why does vomiting cause metabolic alkalosis?
Gastric HCl is lost, bicarbonate is left in the blood, and volume and chloride depletion make the kidney reabsorb it instead of excreting it.
Which test separates saline-responsive from unresponsive alkalosis?
Urine chloride: low in responsive cases (vomiting) and high in unresponsive ones (mineralocorticoid excess, Bartter, Gitelman).
What is paradoxical aciduria?
Acid urine despite alkalosis, because aldosterone and hypokalaemia force the kidney to secrete H⁺ to retain sodium.
Why can alkalosis cause tetany?
High pH increases calcium binding to albumin and lowers ionised calcium.
Related conditions
- Hypokalaemia
- Infantile hypertrophic pyloric stenosis
- Cushing syndrome
- Heart failure
- Chronic obstructive pulmonary disease (COPD)
Educational information, not medical advice. For any symptom, see a health professional.
Sources: Alkalosis: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Chloride - urine test: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Low blood potassium: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).
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