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[Biochemistry](https://durgical.com/en/biochemistry/) · Molecule

# Urea

Urea is how the body gets rid of spare nitrogen: the liver makes it and the kidney excretes it.

Updated 2026-11-01

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## Key facts

- It carries two nitrogens and one carbon; it is neutral, very soluble and of low toxicity.
- It is the main form of nitrogen in urine.
- It is made only in the liver (arginase 1) and reported in blood as urea or blood urea nitrogen (BUN).
- It helps the kidney concentrate urine by recycling in the renal medulla.

**Formula**
CH₄N₂O

**PubChem**
[CID 1176](https://pubchem.ncbi.nlm.nih.gov/compound/1176)

## Structure

- A carbonyl (C=O) bonded to two amino groups; it is the diamide of carbonic acid.

## Clinical notes

- High BUN: kidney failure, dehydration, upper GI bleeding or a very high-protein diet.
- Low BUN: severe liver failure, malnutrition or urea cycle disorders.
- A BUN/creatinine ratio above 20 points to prerenal causes.
- Helicobacter pylori urease splits urea; the labelled-urea breath test is based on this.

## Pathways

- [Urea cycle](https://durgical.com/en/biochemistry/pathways/urea-cycle/)

## Related conditions

- [Chronic kidney disease](https://durgical.com/en/conditions/chronic-kidney-disease/)
- [Acute kidney injury](https://durgical.com/en/conditions/acute-kidney-injury/)
- [Helicobacter pylori gastritis](https://durgical.com/en/conditions/helicobacter-pylori-gastritis/)
- [Hyperammonaemia](https://durgical.com/en/conditions/hyperammonaemia/)

## Related

- [Ammonia (ammonium)](https://durgical.com/en/biochemistry/molecules/ammonia/)
- [Arginase 1](https://durgical.com/en/biochemistry/enzymes/arginase-1/)

## Test yourself

Think of the answer, then tap the question to see it.

## Which enzyme releases urea in the cycle?

Arginase 1, which hydrolyses arginine to urea and ornithine in the hepatocyte cytosol.

## Why does an upper GI bleed raise BUN?

Digested blood is a large protein load; the liver turns its nitrogen into urea.

## What is the breath test for Helicobacter pylori based on?

The bacterium's urease splits labelled urea and releases labelled CO₂, which is measured in exhaled air.

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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: [Urea (CID 1176)](https://pubchem.ncbi.nlm.nih.gov/compound/1176) (PubChem, NCBI, 2026) · Public domain; [BUN - blood test: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/003474.htm) (MedlinePlus, US National Library of Medicine, 2026); [Urea cycle](https://reactome.org/PathwayBrowser/#/R-HSA-70635) (Reactome, 2026).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/biochemistry/molecules/urea/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/biochemistry/molecules/urea/