Fish make use of gills in order to complete gas exchange. The gills are on the outside of their body. They lie free in the water, and are made up of many thin filaments. These filaments are further divided into thin foldings called lamellae, which greatly increases the surface area for gas exchange. This is because water contains much less oxygen than air. To further enhance gas exchange fish gulp in water, and then, by closing their mouth, force the water to exit over the gills.