Sodium

Sodium (Na⁺) plays a central role in water and osmotic regulation in the body. It is the most important osmotically effective solute in the ECF and is the chief determinant of TBW. A solute is considered osmotically effective if it is capable of causing a water shift because it does not freely pass from the ECF to the ICF; in contrast urea is considered an ineffective osmole because it does pass freely from the ECF to the ICF. The main source of Na⁺ is dietary, and regulation is by the kidney through the effect of aldosterone on the renal tubules. Diets of herbivores usually must be supplemented with Na⁺. Serum Na⁺ is a good indicator of total body Na⁺ since it is primarily an ECF ion (i.e. very little Na⁺ is present intracellularly; see Table 6.1).

Hypernatremia

Hypernatremia may be absolute due to Na+ gain or relative due to water loss. Na⁺ gain can occur iatrogenically, for example, through high Na⁺-containing intravenous (IV) fluids (e.g. hypertonic saline or Na⁺ bicarbonate) or Na⁺-rich enema preparations. Increased Na⁺ intake, without concurrent adequate water intake, occurs with salt water ingestion and ingestion of other high Na⁺ products, such as modelling clay (play dough). Hyperaldosteronism is rare but will result in hypernatremia. Hypernatremia is not expected with hyperadrenocorticism since hyperaldosteronism does not accompany this endocrinopathy. This is unlike the situation with hypoadrenocorticism, which is usually accompanied by hypoaldosteronism and hyponatremia.

Water loss in excess of solute loss, resulting in relative hypernatremia, occurs with central or renal diabetes insipidus (DI). There is decreased water reabsorption by the kidney due to a lack of ADH (central DI) or an inability of the kidney to respond to ADH (renal DI). Hyperventilation, extensive skin denudation, and decreased water intake as with water deprivation or central nervous system disease, can also result in relative hypernatremia.

Hyponatremia

Hyponatremia can occur with decreased dietary intake, particularly in large herbivores. High producing dairy cows can lose large amounts of Na⁺ through milk production, and working horses can lose large amounts through sweating. Excessive Na⁺ loss can occur with vomiting, diarrhea, hypoadrenocorticism, acute hemorrhage, or losses of Na⁺-rich fluid through drainage from a third space, for example, drainage of chylothorax. An animal that is vomiting electrolyte-rich fluid but maintaining water intake will develop hyponatremia as well as an inability to excrete free water due to a heightened ADH effect. Expansion of a third space due to ascites or edema will result in hyponatremia. With ascites there is loss of fluid from the vascular space into the abdominal cavity, and with edema the loss is into peripheral tissues, both causing hypovolemia which in turn stimulates increased water intake and increased ADH release, hence a dilutional hyponatremia.

With chronic hyperglycemia (i.e. due to diabetes mellitus), water enters the ECF because of the osmotic effect of the glucose. This shift in water from the intracellular to the ECF space results in a relative decrease in Na⁺; however, there will also be absolute renal losses of Na⁺ resulting from osmotic diuresis associated with glucosuria.