# Explain how countercurrent flow in the gills of bony fish increases the efficiency of oxygen uptake compared with concurrent flow.

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

## Mark scheme (4 marks)

1. In countercurrent flow, water and blood flow in opposite directions across the gill lamellae.
2. A concentration gradient favouring diffusion of oxygen into the blood is maintained along the entire length of the lamella.
3. In concurrent flow, blood and water equilibrate (reach the same oxygen concentration) partway along the lamella, so no further diffusion occurs.
4. Countercurrent flow therefore allows a greater proportion of the oxygen in the water to be absorbed (up to ~80%), making gas exchange more efficient than concurrent flow.

## Key terms

- [countercurrent flow](https://www.gradenine.co.uk/glossary/countercurrent-flow)
- [concurrent flow](https://www.gradenine.co.uk/glossary/concurrent-flow)

## Related

- [Revision notes for IB DP Biology Standard 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-countercurrent-flow-in-the-aaf0d29c) · Published by Druglandscape Ltd.