# Explain how gas exchange occurs in fish gills and why the counter-current system makes this process more efficient than if water and blood flowed in the same direction.

> AQA A-Level Biology (7402) — 3.3.2 Gas Exchange · Explain · 5 marks

> Fish extract dissolved oxygen from water as it passes over their gills. Blood flowing through the gill filaments picks up oxygen and releases carbon dioxide.

## Mark scheme (5 marks)

1. Water flows over the gill filaments / lamellae in one direction
2. Blood flows in the opposite direction to water (counter-current)
3. A concentration gradient is maintained along the entire length of the lamella / oxygen always diffuses from water into blood
4. More oxygen is absorbed / diffuses into the blood compared with parallel flow
5. In parallel flow the concentrations would equalise / reach equilibrium so diffusion would stop (before all available oxygen is absorbed)

## Key terms

- [counter-current system](https://www.gradenine.co.uk/glossary/counter-current-system)

## Related

- [Revision notes for AQA A-Level Biology (7402)](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/explain-how-gas-exchange-occurs-in-4a294b3c) · Published by Druglandscape Ltd.