Osmosis in the movement of water molecules through a selectively permeable membrane. It occurs from a region of high water concentration to a region of low water concentration. The control of water balance in animals is known as osmoregulation. Osmoregulation is a homeostatic mechanism. The body fluids of a salt water fish are hypotonic (higher in water concentration) compared with the surrounding sea water – the sea water is therefore hypertonic. Because of this they constantly lose water by osmosis through its selectively permeable gill and gut membranes. To replace its loses and maintain its water balance, the fish drinks the sea water.
The chloride secretory cells in the gills of the fish actively secrete the excess salt – gained from drinking the sea water – back out into the sea by an energy requiring process known as active transport, against a concentration gradient. The kidneys of salt water fish only contain a few small glomeruli, which results in a low filtration rate of blood and only a small volume of urine being formed. In addition, salt water fish convert their nitrogenous waste to a non-toxic form, trimethylamine oxide, which requires minimum amount of water for its removal.
Over millions of years of evolution, fresh water fish have adapted to carry out osmoregulation. The body fluids of a fresh water fish are hypertonic compared with the surrounding water and therefore they constantly gain water by osmosis. Fresh water fish gain water through the lining of their stomach and their gut – the same as salt water fish. In order to maintain a water balance, fresh water fish have to constantly remove excess water. Their kidneys possess a number of adaptations which make the constant intake of water easy to cope with; their kidneys possess many large glomeruli allowing rapid filtration of the blood and the tubules reabsorb mineral salts from the glomerular filtrate back into the bloodstream, resulting in a large volume of very dilute urine. Using chloride secretory cells in their gills, fresh water fish area able to actively uptake mineral salts from the water which are lost in the urine.