osmoregulation(redirected from Water-electrolyte balance)
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osmoregulationthe control of OSMOTIC POTENTIAL or WATER POTENTIAL in organisms. Water molecules tend to move from an area of high osmotic or high water potential (low osmotic pressure) to an area of low osmotic potential or low water potential (highosmotic pressure), when separated by a differentially permeable membrane. Where cells are bathed in a solution, water tends to cross the cell membrane in order to equalize the water potential on either side.
In a dry atmosphere or the marine environment, organisms tend to lose water to their surroundings. In a wet atmosphere or in freshwater, organisms may have difficulty in losing water. Various means are thus adopted to maintain a correct water balance. Xerophytic plants (see XEROPHYTE).may reverse the normal stomatal rhythm, develop a waxy cuticle, fold leaves, store water (succulent plants) or survive dry periods as spores or seeds. Excess water may be removed by GUTTATION in some plants, and by CONTRACTILE VACUOLES in some unicellular organisms. In mammals osmoregulation is carried out by the kidney, water passing out of the blood stream into the kidney tubule via the BOWMAN'S CAPSULE and reabsorbed, where necessary, in the tubule itself. Marine fish, reptiles and birds are able to eliminate salt through special excretory cells. Different groups of animals remove NITROGENOUS WASTE in different ways - all associated with osmoregulation. Many marine organisms have blood that is ISOTONIC with sea water, and so do not have an osmoregulatory problem.