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 in question.
Free water must be added to reduce a sodium concentration from 145 mEq/L to 120 mEq/L, for instance. Free water excess is
synonymous with hyponatremia and is caused by a free water gain. Hypernatremia is a free water deficit and is caused by free
water loss. Free water may be lost via evaporation (lungs and skin) or by losing fluids which are low in sodium compared
to normal ECF (diarrhea, vomitus, urine). Free water may be gained by drinking water or may be administered in the form of
5% dextrose in water.
Urine sodium concentration normally varies between 60 and 100 mEq/L. Lower urine sodium values represent free water loss
(which may result in a free water deficit in the ECF - hypernatremia), while higher values represent a lack of free water
loss (which may result in a free water excess in the ECF - hyponatremia).
Free water losses and gains change sodium concentration with minimal changes in blood volume (e.g. free water loss will cause
hypernatremia but not much hypovolemia). Sodium concentration and sodium content are different in concept and they do not
overlap; concentration and content abnormalities commonly occur together. Sodium concentration abnormalities are water abnormalities.
Edema/dehydration are sodium content or "saline" or "crystalloid" abnormalities.
ECF sodium content (aka ECF volume)
In contrast to sodium concentration which refers to the number of sodium molecules compared to the number of water molecules,
sodium content refers to the total number of sodium molecules (and the associated anion molecules) and the water molecules
in the extracellular fluid compartment. Clinical assessment of ECF sodium content is the evaluation of hydration (dehydrated,
normally hydrated, edematous). Variations in sodium concentration and sodium content (ECF volume) can occur together in any
combination (i.e. dehydrated patients may be hypernatremia, normonatremic, or hyponatremic). "Dehydration" is defined as
the loss of a crystalloid (a fluid which contains sodium and an associated anion such as chloride or bicarbonate) from the
ECF; "Edema" is defined as the gain of a crystalloid (a fluid which contains sodium and an associated anion such as chloride
or bicarbonate) from the ECF. These fluids do not have to contain the same concentration of sodium or any other electrolyte
as is normally present in the plasma or ECF. When an animal loses, for instance, a fluid with a relatively low sodium concentration,
the fluid left behind is high in sodium (the reason why most dehydrated patients are hypernatremic). If that same animal
drinks a lot of water, it may dilute the sodium concentration and may induce hyponatremia, even though they are still dehydrated.
A dehydrated patient is, by definition, deficient in ECF crystalloid; an edematous patient has an excess of ECF crystalloid.
Whether or not this patient is hypo/normo/hypernatremic depends upon the nature of the crystalloid loss or gain that caused
the dehydration or edema.