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the set of physicochemical and physiological processes that maintain the osmotic pressure of the intercellular fluids, lymph, and blood at a constant level in homoiosmotic animals.
Osmoregulation is found in organisms that inhabit environments with varying concentrations of osmotically active substances, chiefly salts, and in organisms whose level of water and salt utilization vary. Characteristic of all freshwater and terrestrial animals, it is also exhibited by some crustaceans and by all marine vertebrates, except members of the subclass Myxini. Its physiological mechanism is a reflex by which a change in osmotic pressure of the blood or intercellular fluid is perceived by osmoreceptors, which transmit impulses to the nerve centers that regulate the consumption and excretion of water and salts by the osmoregulatory organs, for example, the nephridia, kidneys, and salt glands.
Osmoregulation is hyperosmotic when the osmotic pressure of the internal medium is greater than that of the fluid of the environment and hypoosmotic when the internal osmotic pressure is less. In hyperosmotic regulation, the excess water is excreted by animals mainly through the kidneys and by plants through the stomata; in hypoosmotic regulation, animals replenish the water that is lost through the skin by drinking water that is rich in salts and by excreting the excess salt chiefly through the salt glands.
Osmoregulation in all freshwater animals and marine chondrichthians is hyperosmotic. In sharks and members of the suborder Batoidei, the need for hyperosmotic regulation is due to the high concentration of urea in the blood, and water enters the body across the osmotic gradient of the water-permeable portions of the teguments. In all animals, excess water is excreted by the kidneys or their analogs—the contractile vacuoles of protozoans and the nephridia; salts are absorbed from freshwater by the gills or—in amphibians—by the skin.
Organisms that lose water in the urine and through the integuments exhibit hypoosmotic regulation: these include marine teleosts and marine reptiles. To compensate for the loss, they drink seawater, which is freshened by their salt glands and other organs that excrete concentrated salt solutions. The main organ of osmoregulation in mammals in the kidney, which can excrete hypotonic urine when water is in excess and hypertonic urine when water is scarce. Migratory fish, for example, salmon, and some crustaceans exhibit both hyperosmotic and hypoosmotic osmoregulation and consequently can live in both freshwater and seawater.
In poikilosmotic animals—marine mollusks and echinoderms—the osmotic pressure of the blood varies with the osmotic pressure of the environment. Osmoregulation in these animals is cellular: when the osmotic pressure of the blood increases, the concentration of organic substances in the cells, mainly amino acids, to which the cell membrane is slightly permeable increases by the same amount. As a result, the salt concentration and water content of the cell do not change, and the osmotic pressure is equalized by the accumulation of osmotically active substances. A decrease in the osmotic pressures of the blood and environment decreases the concentration of organic substances in the cells. Thus, cellular osmoregulation provides for the limited adaptation of poikilosmotic animals to fluctuations of osmotic pressure in the environment.
REFERENCESFor references see ACTIVE ION TRANSPORT, WATER-SALT EXCHANGE, EXCRETION, EXCRETORY SYSTEM, THIRST.
IU. V. NATOCHIN