Fish - gills have a larger surface area for gas exchange

• 1. The gills are the gas exchange surface in fish
• 2. Water (containing oxygen) enters the fish through its mouth and passes out through the gills. As this happens, oxygen diffuses from the water into the blood in the gills and carbon dioxide diffuses from the blood into the water.
• 3. Each gill is made of lots of thin plates called gill filaments, which give a big surface area for exchange of gases
• 4. The gill filaments are covered in lots of tiny structures called lamellae, which increase the surface area even more
• 5. The lamellae have lots of blood capillaries to speed up diffusion
• 6. They also have a thin surface layer of cells to minimise the distance that the gases have to diffuse
• 7. Blood flows through the lamellae in one direction and water flows over in the opposite direction. This maintains a large concentration gradient between the water and the blood
• 8. The concentration of oxygen in the water is always higher than that in the blood, so as much oxygen as possible diffuses from the water into the blood