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The liver, deamination and urea

Excretion in humans · Excretion in humans · note 3 of 3

Spec 13.1.6, 13.1.7, 13.1.8, 13.1.9
Supplement (what this means)Supplement: only for the Extended papers (2 and 4). Core students can skip it. What the labels mean
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The liver, deamination and ureaSpec 13.1.6, 13.1.7, 13.1.8, 13.1.9

In short

Deamination is the removal of the nitrogen-containing part of amino acids to form urea. It happens in the liver, which breaks down amino acids in excess of requirements. The urea passes into the blood and is carried to the kidneys, which excrete it in urine. Urea must be excreted because it is toxic and would build up.

Assimilation of amino acids

Digestion breaks proteins down into amino acids, which are absorbed into the blood. The blood carries them to the liver. The liver takes up the amino acids it needs and converts them into proteins. This is the assimilation of amino acids.

Deamination and urea

The body does not need all of the amino acids it takes in. The excess amino acids are broken down in the liver. Deamination is the removal of the nitrogen-containing part of amino acids to form urea.

  1. Amino acids that are in excess of requirements reach the liver.
  2. The liver carries out deamination: the nitrogen-containing part of each amino acid is removed.
  3. The nitrogen-containing part forms urea.
  4. Urea passes into the blood and is carried to the kidneys.
  5. The kidneys filter urea from the blood and it leaves the body in urine.
Flow diagram: amino acids absorbed in the small intestine travel in the blood to the liver, where amino acids that are needed are converted into proteins (assimilation) and excess amino acids are deaminated to form urea; urea travels in the blood to the kidneys and leaves the body in urine. (opens full size in a new tab)
The liver converts the amino acids it needs into proteins and deaminates excess amino acids to form urea, which the kidneys excrete.

Why excretion of urea matters

Urea is toxic. If it were not removed, it would build up in the blood and body cells and cause harm. Excretion is important because it removes this toxic waste from the body.

Exam tip:

If asked why urea must be excreted, say it is toxic (poisonous) and would build up. Do not say urea is harmless or safe.

Common mistake:

Deamination happens in the liver, not the kidneys. The kidneys only remove the urea from the blood. Deamination is not digestion.

Quick check

  1. Which organ excretes carbon dioxide?

    Show answer

    The lungs.

  2. Name the tube that carries urine from the bladder out of the body.

    Show answer

    The urethra.

  3. Supplement Name the two regions seen in a section through a kidney.

    Show answer

    The cortex (outer) and the medulla (inner).

  4. Supplement Which substance is all reabsorbed into the blood from the nephron?

    Show answer

    Glucose.

  5. Supplement Where is urea formed, and from what?

    Show answer

    In the liver, from excess amino acids.

Written and checked against the Cambridge IGCSE Biology (0610) specification · Updated October 2026

Frequently asked questions

How does the kidney filter blood?

Supplement The kidney filters blood in tiny tubes called nephrons. Blood in the glomerulus is under high pressure, so water, glucose, urea and ions are forced out into the nephron, while proteins and blood cells stay in the blood. All the glucose, some ions and most of the water are reabsorbed, and what is left becomes urine.

What is the difference between the ureter and the urethra?

The ureter carries urine from the kidney to the bladder, while the urethra carries urine from the bladder out of the body. Each kidney has its own ureter, and the bladder stores the urine until it leaves. Students often mix the two up, so link the ureter with the kidney and the urethra with the way out.

Why is there no glucose in urine?

Supplement There is normally no glucose in urine because all of it is reabsorbed into the blood from the nephron. Glucose is small enough to be filtered out of the blood at the glomerulus, so it does enter the nephron. It is then all taken back into the blood as the filtered liquid passes along the nephron.

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