Osmoregulation in Fish.

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 …

…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.