Fish gills kind of work like human lungs. The purpose of the gills is to gather oxygen. Instead of getting oxygen from the air, fish get oxygen from the water. The fish will swallow water and force it through the gills. There are blood vessels that are close to the surface of the gills. Oxygen passes from the water and into the blood vessels in the gills. The blood cells carry this oxygen to the rest of the cells in the fish body.