Fish have a large surface area for gas exchange called gills, water enters the fish through the mouth and goes out through the gills, the oxygen is transported from the water to the blood by diffusion, carbon dioxide diffuses from the blood to the water, each gill has gill filaments which give the gills a large surface area, lamellae cover each gill filament to further increase the surface area for more gas exchange, they have a thin surface layer and capillaries for good blood supply which helps with diffusion
