# Explain why the gills of a fish are an effective exchange surface for gas exchange between the water and the blood.

> OCR A-Level Biology A (H420) — 3.1 Exchange surfaces · Explain · 4 marks

> Fish obtain oxygen from water passing over their gills. The gill filaments are lined with thin, flattened epithelial cells and are supplied with an extensive network of blood capillaries. Water flows over the gills in a direction opposite to the flow of blood within the capillaries.

## Mark scheme (4 marks)

1. Large surface area increases the rate of diffusion
2. Short diffusion pathway / thin exchange surface
3. Countercurrent flow maintains a concentration gradient along the entire length of the gill
4. Good blood supply / ventilation maintains steep concentration gradient

## Related

- [Revision notes for OCR A-Level Biology A (H420)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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