Osmoregulation - a set of processes that occur in a living organism ( unicellular or multicellular ), aimed at maintaining constant osmotic pressure . Osmoregulation is inherent in all freshwater and terrestrial animals; among marine organisms, all vertebrates (except myxiniformes ), some crustaceans , polychaetes , and mollusks have osmoregulation.
In higher organisms, when the osmotic pressure of the blood or intercellular fluid changes, the osmoreceptors generate impulses that are transmitted to the nerve centers. At the same time, the consumption of water and salts (and their release) is controlled by osmoregulatory organs - nephridia, kidneys, salt glands, etc.
The osmoregulation mechanism is called hyperosmotic, when the osmotic pressure in the fluid of the internal medium is greater than in the external environment, and hypoosmotic if the internal pressure is less than the external. With the hyperosmotic mechanism, excess water is excreted in animals mainly by the kidneys, and in plants - by means of stomata. With the hypoosmotic mechanism, the lack of water (with passive loss through the integument of the body) is compensated by the consumption of salt-rich water, and the excess of salts is excreted mainly through the salt glands. All freshwater animals and marine cartilaginous fish have hyperosmotic regulation. The hypoosmotic mechanism is used by marine bony fishes, marine reptiles and some other species.