Sodium concentration is an expression of the relative numbers of
sodium molecules to water molecules, irrespective of the total
numbers. Sodium content represents the total number of sodium
molecules present in the extracellular fluid compartment Sodium
concentration is a transcellular fluid flux issue and is dealt
with in this chapter. Sodium content is a volume issue heralded
by physical indications of hypovolemia or dehydration/edema, and
is dealt with in chapters dealing with hypovolemia and
dehydration. Abnormalities in sodium concentration
(hyponatremia; hypernatremia) may occur in any combination with
abnormalities in sodium content (dehydration; edema). Laboratory
measurements (packed cell volume, total protein concentration,
and electrolyte concentrations) determine the nature of the
fluid loss and the nature of the fluid replenishment.
Polycythemia or hypernatremia do not define dehydration. Free
water is qualitatively expressed in terms of the sodium
concentration, compared to normal for the fluid