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The concept of dehydration did not enter clinical medicine until the 1830s and was not scientifically defined for some years after that. Although the word dehydration in general English usage means loss of water, in physiology and medicine, the unmodified word means a loss of water and salt or extracellular fluid (ECF), the most common of the clinically recognized types of dehydration. Depending on the type of pathophysiologic process, water and salts (primarily sodium chloride) may be lost in physiologic proportion or lost disparately, with each type producing a somewhat different clinical picture. We have found it useful to designate these types as isonatremic (classical), hypernatremic (hypertonic), or hyponatremic (hypotonic). The differential losses produce different clinical features because of the functional impermeability of the Na+ and Cl− ions of the ECF to the adjacent intracellular fluid (ICF). We use sodium rather...

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