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In osmoregulation osmoregulators tightly regulate their body osmolarity, which always stays constant, and are more common than osmoconformers in the animal kingdom. Osmoregulators actively control salt concentrations despite the salt concentrations in the environment. An example is freshwater fish. The gills actively uptake salt from the environment by the use of mitochondria-rich cells. Water will diffuse into the fish, so it excretes a very hypotonic (dilute) urine to expel all the excess water. A marine fish has an internal osmotic concentration lower than that of the surrounding seawater, so it tends to lose water and gain salt. It actively excretes salt out from the gills. Most fish are stenohaline, which means they are restricted to either salt or fresh water and cannot survive in water with a different salt concentration than they are adapted to. However, some fish show a tremendous ability to effectively osmoregulate across a broad range of salinities; fish with this ability are known as euryhaline species, e.g., salmon. Salmon has been observed to inhabit two utterly disparate environments — marine and fresh water — and it is inherent to adapt to both by bringing in behavioral and physiological modifications.
Salt and water balance in animals
Animal water intake - Hypertonicity - Isotonicity - Osmoreceptors - Osmoregulation - Homeostasis - Halotolerance (Halophile) - Osmoconformer - Osmoregulation - Renal medulla - Renin-angiotensin system - Salt gland - Supraorbital gland - Thirst - Water intake -