# A patient is diagnosed with kidney failure. Their kidneys can no longer carry out excretion properly. Explain why the build-up of urea in the blood is harmful, and describe how healthy kidneys normally remove urea from the body to maintain a safe concentration in the blood.

> OCR A-Level Biology B — Advancing Biology (H422) — 5.6 Excretion · Explain · 5 marks

> Urea is a toxic waste product produced in the liver from the breakdown of excess amino acids. It is transported in the blood to the kidneys, where it is removed and excreted in urine. If urea is not removed, it accumulates in the blood to dangerous levels.

## Mark scheme (5 marks)

1. Urea is a toxic / poisonous waste product that must be removed from the body
2. A build-up of urea would disrupt the body's normal functioning / damage cells / organs
3. The kidneys filter the blood to remove urea (filtration)
4. Useful substances (such as glucose and water) are reabsorbed back into the blood
5. The remaining urea (and excess water / ions) is excreted as urine

## Key terms

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

## Related

- [Revision notes for OCR A-Level Biology B — Advancing Biology (H422)](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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Source: [GradeNine](https://www.gradenine.co.uk/q/a-patient-is-diagnosed-with-kidney-b6c22b5f) · Published by Druglandscape Ltd.