# A student reads that the kidneys filter the blood and then selectively reabsorb useful substances back into the blood, while excreting waste products in urine. Explain how the kidneys carry out excretion, including what is filtered, what is reabsorbed, and what remains in the urine.

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

> The kidneys are organs of excretion. They remove metabolic waste products from the blood and regulate the water content of the body. Each kidney contains thousands of tiny filtering units.

## Mark scheme (5 marks)

1. The kidneys filter the blood (under high pressure) to produce a filtrate
2. Useful substances such as glucose and water are selectively reabsorbed back into the blood
3. Urea is not reabsorbed and remains in the filtrate / is excreted in urine
4. Urea is a waste product formed from the breakdown of excess amino acids (in the liver)
5. The amount of water reabsorbed is adjusted / regulated to control the water content of the blood

## Key terms

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

## 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-student-reads-that-the-kidneys-b895e375) · Published by Druglandscape Ltd.