# Explain how the countercurrent flow of water and blood in fish gills maintains a concentration gradient for oxygen along the entire length of a gill lamella.

> IB DP Biology Higher Level (2023 syllabus) — B3.1 Gas exchange · Explain · 4 marks

## Mark scheme (4 marks)

1. In countercurrent flow, water and blood move in opposite directions across the gill lamella.
2. Blood that is already partially oxygenated meets water that is even more oxygen-rich, so a diffusion gradient is maintained along the entire length of the lamella.
3. In a parallel/concurrent arrangement the two fluids would reach equilibrium part-way along, stopping net diffusion before the blood leaves the lamella.
4. As a result, up to ~80% of dissolved oxygen can be extracted from the water, making gas exchange highly efficient.

## Key terms

- [countercurrent flow](https://www.gradenine.co.uk/glossary/countercurrent-flow)
- [gill lamella](https://www.gradenine.co.uk/glossary/gill-lamella)
- [concentration gradient](https://www.gradenine.co.uk/glossary/concentration-gradient)

## Related

- [Revision notes for IB DP Biology Higher Level (2023 syllabus)](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-the-countercurrent-flow-of-5946441b) · Published by Druglandscape Ltd.