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.

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

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.

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.

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) (PubChem, NCBI, 2026) · Public domain; BUN - blood test: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Urea cycle (Reactome, 2026).

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