Fishes exchange gases by using gills. Here is a step by step procedure:
- The alternate opening of mouth and operculum (thin layer covering the gills) forces the water (saturated with oxygen) over the gills.
- Gills contain gill filaments, which provide them with very large surface area. Filaments contain a large number of protrusions called lamella.
- Counter-current flow of blood and water ensures gas exchange between the two fluids. As the oxygen-deficient blood flows, it comes in contact with oxygen-saturated water and this dissolved oxygen diffuses into the blood by diffusion.
- Similar to oxygen diffusion into blood, carbon dioxide diffusion into water takes place. The blood is rich in carbon dioxide, whereas the water is deficient in it, this sets up a concentration gradient that causes diffusion of carbon dioxide into the water.
- The carbon dioxide containing water exits through the lungs.
Hope this helps.