# Explain how the structure of the gills of a bony fish allows efficient ventilation with water, ensuring a continuous flow of water over the gill surfaces.

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

> Bony fish (Osteichthyes) use a paired opercular pumping mechanism to drive water continuously and unidirectionally over their gills, even when the fish is stationary.

## Mark scheme (4 marks)

1. The buccal cavity and opercular cavity act as pressure pumps/suction pumps working in alternating phases to maintain near-continuous water flow.
2. The operculum (gill cover) opens and closes in a coordinated sequence with the mouth, maintaining a positive pressure differential that drives water across the gills.
3. Water flows unidirectionally over the gill lamellae, which maximises the time available for diffusion and maintains the concentration gradient for oxygen uptake / carbon dioxide removal.
4. The gill arch supports multiple gill filaments, which are arranged to offer a large surface area through which water must pass, maximising contact between water and the respiratory surface.

## Key terms

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

## 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-structure-of-the-70bc421e) · Published by Druglandscape Ltd.