# 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

> 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.

## 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

- [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)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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