Exchange substances in fish

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