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Osmoregulation
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Diagram comparing osmotic regulation in freshwater and marine teleost fish, showing water and ion movement through skin and gills, urine concentration, and active uptake or secretion processes.
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Osmoregulation

biology
Also known as: osmotic regulation
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Osmoregulation, in biology, maintenance by an organism of an internal balance between water and dissolved materials regardless of environmental conditions. In many marine organisms osmosis (the passage of solvent through a semipermeable membrane) occurs without any need for regulatory mechanisms because the cells have the same osmotic pressure as the sea. Other organisms, however, must actively take on, conserve, or excrete water or salts in order to maintain their internal water-mineral content.