Osmosis in the motion of water particles through a selectively permeable membrane. It happens from a region of high water concentration to a region of low water concentration. The control of water balance in animals is called osmoregulation. Osmoregulation is a homeostatic mechanism. The body fluids of a seawater fish are hypotonic (higher in water concentration) compared to the surrounding sea water – the sea water is therefore hypertonic. Due to the fact that of this they constantly lose water by osmosis through its selectively permeable gill and gut membranes.
To change its loses and preserve its water balance, the fish drinks the sea water.
The chloride secretory cells in the gills of the fish actively produce the excess salt – gotten from consuming the sea water – back out into the sea by an energy requiring process referred to as active transportation, versus a concentration gradient. The kidneys of seawater fish just consist of a couple of little glomeruli, which leads to a low filtering rate of blood and just a little volume of urine being formed.
In addition, seawater fish transform their nitrogenous waste to a non-toxic kind, trimethylamine oxide, which needs minimum quantity 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.