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sodium(Na)
(soŽde-әm)
a chemical element, atomic number 11, atomic weight 22.990. Sodium is the major cation of the extracellular fluid, constituting 90 to 95 per cent of all cations in the blood plasma and interstitial fluid; it thus determines the osmolality of the extracellular fluid. The serum sodium concentration is normally about 140 mEq/l, with hormones regulating its concentration to prevent hypernatremia (too high a concentration) and hyponatremia (too low a concentration).
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