# A patient's kidneys have stopped functioning. Doctors decide to treat the patient using a dialysis machine. Describe how a dialysis machine works to remove waste products and maintain the correct composition of the blood.

> WJEC A-Level Biology (Wales) — 3.7 Homeostasis and the kidney · Describe · 5 marks

## Mark scheme (5 marks)

1. Blood is passed from the patient through the dialysis machine / tubing
2. Blood is separated from the dialysis fluid by a partially permeable membrane
3. Urea / waste products diffuse from the blood into the dialysis fluid down a concentration gradient
4. The dialysis fluid contains the correct / normal concentration of useful substances (e.g. glucose, mineral ions) so these are not lost from the blood
5. Cleaned blood is returned to the patient's body

## Key terms

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

## Related

- [Revision notes for WJEC A-Level Biology (Wales)](https://www.gradenine.co.uk/learn)
- [How to answer "Describe" 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-s-kidneys-have-stopped-functioning-2287fa14) · Published by Druglandscape Ltd.