Disorders of water balance and sodium balance are common, but the pathophysiology is frequently misunderstood. As an example, the plasma sodium concentration is regulated by changes in water intake and excretion, not by changes in sodium balance. As will be described in the following sections, hyponatremia is primarily due to the intake of water that cannot be excreted, hypernatremia is primarily due to the loss of water that has not been replaced, hypovolemia represents the loss of sodium and water, and edema is primarily due to sodium and water retention. Understanding these basic principles is essential for appropriate diagnosis and treatment.
The general principles and disorders of water balance and sodium balance will be reviewed here. The causes and evaluation of hyponatremia, hypernatremia, hypovolemia, and edema are presented separately. (See "Causes of hyponatremia in adults" and "Evaluation of adults with hyponatremia" and "Etiology and evaluation of hypernatremia" and "Etiology, clinical manifestations, and diagnosis of volume depletion in adults" and "Clinical manifestations and diagnosis of edema in adults".)
The following terms are commonly used when discussing disorders of water and sodium balance. Understanding what these terms represent is essential for appropriate diagnosis and treatment.
Total body water — The total body water (TBW) as a percentage of lean body weight varies with age. Approximate normal values are 80 percent in premature infants, 70 to 75 percent in term infants, 65 to 70 percent in toddlers, and 60 percent after puberty (figure 1) . These values vary with the amount of fat since fat has a much lower water content than muscle. Thus, the TBW as a percentage of total body weight is lower in individuals with more fat. As examples, the TBW as a percentage of total body weight is lower in young adult females than in young adult males (50 versus 60 percent) and becomes progressively lower with increasing obesity or with loss of muscle mass.
The TBW has two main compartments: the extracellular fluid and the intracellular fluid, which are separated by the cell membrane. The relative size of the two main compartments varies with age. The extracellular fluid component is increased in infants and young children compared with older patients, which also contributes to the younger patients' greater TBW percentage of lean body weight (figure 1). The cell membranes are freely permeable to water but not electrolytes and therefore help to maintain the different solute composition of the two compartments: sodium salts in the extracellular fluid, with chloride and bicarbonate being the major anions; and potassium salts in the intracellular fluid, with large macromolecular organic phosphates being the main anions.