Disorders of Sodium and Water Homeostasis

This chapter reviews the pathophysiology of changes in volume and composition of the body fluid compartments associated with hyponatremia and hypernatremia as well as a clinical approach to fluid therapy that takes into account these changes in sodium concentration. Hyponatremia is the most common sodium disturbance in small animals. It can result from an excess of free water or from loss of sodium, which is rare because sodium usually is followed by water. Serum sodium concentration is regulated by changes in water balance, and not by changes in sodium or volume. Hypernatremia reflects a relative excess of sodium compared to water, arising either by loss of water or from addition of salt, the latter being less common. Loss of water can be a consequence of pure water loss or hypotonic fluid loss. The chapter also discusses causes, clinical manifestations, consequences, and management principles of hyponatremia and hypernatremia.