Biochemistry · Molecule
Ammonia (ammonium) !
Ammonia comes from breaking down amino acids. It is toxic to the brain, so the liver turns it into urea.
Updated 2026-11-01
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Key facts
- At body pH almost all of it is ammonium ion (NH₄⁺); the small NH₃ fraction crosses membranes, including the blood–brain barrier.
- Sources: deamination of glutamate (glutamate dehydrogenase), glutaminase in kidney and gut, and colonic bacteria.
- It travels in blood as glutamine (from almost every tissue) and as alanine (from muscle).
- The liver removes it as urea; the kidney excretes NH₄⁺ to get rid of acid.
- Also called
- NH₃ / NH₄⁺
- Formula
- NH₃
- PubChem
- CID 222
Structure
- A small pyramidal molecule: one nitrogen with three hydrogens and a lone pair that accepts a proton (pKa near 9.2).
Clinical notes
- In hyperammonaemia, astrocytes turn ammonia into glutamine, swell, and cerebral oedema follows.
- Causes: urea cycle disorders and organic acidaemias in children; cirrhosis and GI bleeding in adults.
- Lactulose acidifies the colon and traps NH₄⁺; rifaximin reduces the bacteria that make it.
- An ammonia sample is processed fast and on ice, because the level rises in the tube.
Drugs that act here
Pathways
Related conditions
Related
- Urea
- Glutamine
- Glutamate (glutamic acid)
- Glutamate dehydrogenase (GDH)
- Glutamine synthetase
- Carbamoyl phosphate synthetase I (CPS-I)
Test yourself
Think of the answer, then tap the question to see it.
Which form of ammonia crosses the blood–brain barrier?
Uncharged NH₃; although it is a small fraction at body pH, it passes membranes easily.
How does ammonia travel from tissues to the liver without poisoning them?
As glutamine (glutamine synthetase) and, from muscle, as alanine (glucose–alanine cycle).
Why is an ammonia sample carried on ice?
Because cells and time in the tube release ammonia and give falsely high values.
Molecules are shown in their neutral form, as the databases deposit them; in the cell the acids and phosphates are ionised.
Educational information, not medical advice. For any symptom, see a health professional.
Sources: Ammonia (CID 222) (PubChem, NCBI, 2026) · Public domain; Ammonia Levels: MedlinePlus Medical Test (MedlinePlus, US National Library of Medicine, 2026); Urea cycle (Reactome, 2026).
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