Fish get rid of nitrogenous wastes in the form of ammonia. Some wastes diffuse through the gills. Others leave through the kidneys. The kidneys are composed of many tubules that filter nitrogenous wastes from the blood and concentrate them. Kidneys also help fish control the amount of water in their bodies. Fish who live in the salty environment of the oceans tend to loose water through osmosis. The salt pulls the liquid from the body. Kidneys in ocean dwelling fish work to concentrate the wastes so as to return as much water as possible to the body. The kidneys of fresh water fish dilute the urine and pumps out as much water as possible. In fresh water, a great deal of water enters the body through osmosis. Salmon, because they are able to move between fresh water and ocean environments, have the ability to control kidney function.