# Describe the process of ultrafiltration in the kidney and explain why not all substances filtered from the blood are excreted in urine.

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

## Mark scheme (5 marks)

1. High pressure in the glomerulus forces small molecules out of the blood / into the Bowman's capsule
2. Small molecules such as water, glucose, urea and ions pass into the filtrate / large molecules such as proteins and blood cells are too large to pass through
3. Glucose is selectively reabsorbed back into the blood from the tubules
4. Water is reabsorbed by osmosis from the tubules back into the blood / amount reabsorbed varies according to body's needs
5. Urea is not reabsorbed and remains in the filtrate to be excreted in urine as a waste product

## Key terms

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

## Related

- [Revision notes for WJEC A-Level Biology (Wales)](https://www.gradenine.co.uk/learn)
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Source: [GradeNine](https://www.gradenine.co.uk/q/describe-the-process-of-ultrafiltration-in-ab4be267) · Published by Druglandscape Ltd.