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

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 excess amino acids and is subsequently removed from the body by the kidneys. The liver receives blood from two sources: the hepatic artery and the hepatic portal vein.

Model answer (4 marks)

1. Excess amino acids are deaminated in the liver, producing urea.
2. The urea formed is released directly into the hepatic blood, raising its concentration.
3. The blood then leaves the liver via the hepatic vein and is carried to the kidneys.
4. In the kidneys urea is filtered and excreted, so the blood leaving the liver has a lower urea concentration than the blood entering it.

Examiner tips

  • Use the command word ‘explain’ – give a clear cause–effect sequence.
  • Mention both the hepatic artery/portal vein and the hepatic vein to show the flow of blood.
  • Show that urea is added in the liver and removed in the kidneys.

Mark scheme (4 marks)

  1. Excess amino acids are deaminated in the liver, producing urea
  2. Urea produced by deamination is released directly into the hepatic blood, raising its concentration
  3. Urea passes out of the liver via the hepatic vein to the kidneys, where it is filtered / excreted
  4. Therefore, because urea is added in the liver and removed by the kidneys, blood entering the liver has a higher urea concentration than blood leaving it

Key terms in this question

urea

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