# 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

> 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.

## 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

- [urea](https://www.gradenine.co.uk/glossary/urea)

## Related

- [Revision notes for OCR A-Level Biology A (H420)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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