# Fish gills are adapted for efficient gas exchange. Describe the adaptations of fish gills that allow efficient gas exchange between the water and the blood.

> WJEC A-Level Biology (Wales) — 2.2 Adaptations for gas exchange · Describe · 5 marks

> Fish extract dissolved oxygen from water as it passes over their gills. The gills contain many gill filaments, each covered in smaller projections called lamellae. A dense network of capillaries runs through each lamella.

## Mark scheme (5 marks)

1. Large surface area (provided by the many gill filaments and lamellae) allows more molecules to diffuse across at once
2. Short diffusion distance — the walls of the lamellae / capillaries are very thin so oxygen and carbon dioxide do not have to travel far
3. Moist surfaces allow gases to dissolve before diffusing across the membrane
4. Large concentration gradient maintained — capillaries continuously carry oxygen away from the gill surface, keeping the concentration of oxygen in the blood low
5. Dense / extensive capillary network means blood flows close to the exchange surface, supplying blood to all parts of the gill

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