Explain why the concentration of urea in the blood supplying the kidneys is higher than the concentration of urea in the blood leaving the kidneys.

OCR A-Level Biology A (H420) — 5.2 Excretion · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Urea is produced in the liver from the breakdown of excess amino acids and is transported in the blood plasma to the kidneys, where it is filtered and excreted in urine. The concentration of urea differs between the renal artery and the renal vein.

Model answer (4 marks)

Urea is produced in the liver by deamination of excess amino acids, so it accumulates in the blood as it travels to the kidneys. The renal artery therefore carries a higher concentration of urea than the renal vein.

In the kidneys, urea is filtered out of the blood at the glomerulus/Bowman’s capsule by ultrafiltration. It is then excreted in the urine, so the blood leaving the kidney (renal vein) contains a lower concentration of urea.

Examiner tips

  • Use the word ‘deamination’ to show knowledge of urea synthesis.
  • Explain that ultrafiltration removes urea from the blood.
  • Mention that excretion into urine reduces the renal vein concentration.

Common mistakes

  • Confusing the direction of blood flow (artery vs vein).
  • Failing to mention ultrafiltration as the removal mechanism.
  • Using vague terms like ‘filtered’ without specifying the glomerulus.

Mark scheme (4 marks)

  1. Urea is produced in the liver by deamination of excess amino acids
  2. Urea accumulates in the blood as it travels to the kidneys, so the renal artery carries a higher concentration
  3. Urea is filtered out of the blood at the glomerulus / Bowman's capsule by ultrafiltration
  4. Urea is excreted in urine, so blood leaving the kidney (renal vein) contains a lower concentration of urea

Key terms in this question

urea

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