# A fish obtains oxygen from water by passing water over its gills. Explain how the gills of a fish are adapted to allow efficient gas exchange.

> OCR A-Level Biology B — Advancing Biology (H422) — 2.4 Gas exchange surfaces · Explain · 5 marks

> Fish live in an aquatic environment where oxygen is dissolved in water. Water is taken in through the mouth and passed over the gill surfaces before leaving through the gill covers. Oxygen must move from the water into the blood inside the gills, whilst carbon dioxide moves in the opposite direction.

## Mark scheme (5 marks)

1. Gills have a large surface area
2. Large surface area increases the rate of diffusion of oxygen into the blood (and carbon dioxide out)
3. Gill surfaces are thin / one cell thick
4. A concentration gradient is maintained between the water and the blood
5. Gills have a rich blood supply / are well supplied with capillaries to remove oxygen quickly

## Key terms

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

## 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-fish-obtains-oxygen-from-water-01bac439) · Published by Druglandscape Ltd.