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Zoology the adjustment of the osmotic pressure of a cell or organism in relation to the surrounding fluid



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.




References in periodicals archive ?
Nevertheless, their low CI and dwarfed appearance clearly substantiate the theory that those mussels live under suboptimal conditions, with a probable continuous loss of energy as a result of permanent osmoregulation.
During the animal physiology section, muscle physiology and osmoregulation changes from saltwater to freshwater can be incorporated.
However, osmoregulation in this species has not been examined in great detail.
Many anurans are sensitive to low pH, which negatively impacts osmoregulation at the tadpole life-stage (Rowe et al.
Findings from this preliminary data suggest that further research is needed for the future, including population genetics, artificial spawning, domestication for future aquaculture activities, natural behaviour during migration and spawning, as well as ionic balance and osmoregulation.
Water and osmoregulation has been a challenge since the beginning of cellular life.
1985), detoxification (Huxtable, 1992), antioxidation (Gordon and Heller, 1992), osmoregulation (Thurston et al.
Commercial fluid solutions have been determined based on human osmolality and osmoregulation and, although appropriate in domestic mammals, may not be appropriate in birds.
The disorders of water metabolism and osmoregulation are often associated with surgery in the sellar region probably due to manipulation or vascular alterations of the neurohypophysis (1).
For example, urea synthesis enables rapid osmoregulation (control of minerals and salts in the blood) in animals such as sharks, skates, rays and bony fish; and ammonia detoxification associated with water retention in amphibians and mammals.
While thymol exposure causes cell death in giardia via plasma membrane alterations which impede osmoregulation, no cytotoxic effects have been observed in mammalian cells (Machado 2010).
A high content of dissolved solids elevates the density of water, influences osmoregulation of fresh water organism, reduces solubility of gases like oxygen and reduces utility of water for drinking, irrigation and industrial purposes (S.