Maintaining water and ionic balance is a fundamental physiological process for animals because animal cells can only function effectively over a specific, relatively constant range of body fluid composition. For unicellular animals, the intercellular environment is juxtaposed with the external environment. For multicellular animals, the extracellular space is a buffer between the intracellar and external environments. In all animals, the intracellu-lar environment has a different ionic composition from the external environment. For some animals there are osmotic differences, but their intracellular and extracellular environments must have the same osmotic concentration to maintain constant cellular volume (but invariably they have different ionic concentrations). The challenges associated with maintaining osmotic and ionic homeostatis differ with the external environment of the animal, and so there are various strategies for animals to maintain fluid and ion balance.
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