From Greek osmos push, pressure and Latin regulo directing.

Osmoregulation - physiological processes in the body of fish that regulate the osmotic pressure of the internal environment. In freshwater fish and lampreys, the osmotic pressure in tissues is much higher than the osmotic pressure of fresh water, so water constantly enters the body through the skin, gills and with food, and removed by the kidneys. In bony marine fish pressure of the internal environment is less than the pressure of the external, the water balance is regulated by the fact that fish dust sea water, and excess salts entering the body, excrete through the gills, skin and with feces. Osmoregulation is achieved by intake of salts and water into the body.

A set of processes occurring in a living organism (unicellular or multicellular) aimed at maintaining constant osmotic pressure. Osmoregulation is inherent in all freshwater organisms; among marine organisms, all vertebrates (except hagfish), some crustaceans, polychaetes, and mollusks possess osmoregulation.

In higher organisms, when the osmotic pressure of blood or intercellular fluid changes, osmoreceptors generate impulses that are transmitted to nerve centers. This controls the consumption of water and salts (and their excretion) by osmoregulating organs - nephridia, kidneys, salt glands, etc.

The mechanism of osmoregulation is called hyperosmotic, when the osmotic pressure in the fluid of the internal environment is greater than in the external environment, and hypoosmotic, if the internal pressure is less than the external pressure. In the hyperosmotic mechanism, excess water is excreted in fish mainly by the kidneys. In the hypoosmotic mechanism, the lack of water (with passive loss through the body coverings) is compensated for by the consumption of salt-rich water, and excess salts are excreted mainly through salt glands. All freshwater animals and marine cartilaginous fish have hyperosmotic regulation. The hypoosmotic mechanism is utilized by marine bony fishes, marine reptiles, and several other species.