# Fish gills are adapted for efficient gas exchange between water and the blood. Describe the features of fish gills that make them well-adapted for gas exchange.

> Eduqas A-Level Biology — 2.2 Adaptations for gas exchange · Describe · 5 marks

> Fish extract dissolved oxygen from water as it flows over their gills. The gills contain thin, plate-like structures called lamellae, which are richly supplied with blood capillaries. Water flows over the lamellae in the opposite direction to the blood flowing through the capillaries.

## Mark scheme (5 marks)

1. Large surface area (provided by the lamellae) allows more molecules to diffuse across at once
2. Thin walls / short diffusion distance so oxygen and carbon dioxide can diffuse quickly across the membrane
3. Rich blood capillary supply carries oxygen away from the gill surface, maintaining a large concentration gradient
4. Water flows in the opposite direction to blood (countercurrent flow) so a concentration gradient is maintained along the entire length of the lamella
5. Moist surface allows gases to dissolve before diffusing across the membrane

## Related

- [Revision notes for Eduqas A-Level Biology](https://www.gradenine.co.uk/learn)
- [How to answer "Describe" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/fish-gills-are-adapted-for-efficient-4036837e) · Published by Druglandscape Ltd.