Water Balance: Hypo- and Hypernatremia

The adult human body is made up of 60% water and is divided into extracellular and intracellular fluid compartments. Extracellular fluid is present outside the cells and makes up â…“ of the total body water. Intracellular fluid is present inside the cells and makes up â…” of the total body water. Intracellular and extracellular fluids are separated into compartments by semipermeable membranes, and the transport of fluid and ions is maintained by channels in the cell membrane. Each compartment contains different concentrations of ions and osmolar molecules. The relative charge and osmolarity are maintained rigorously by the transport of water and substances between compartments. Hypernatremia, hyponatremia, and edema are the clinical conditions arising from disturbances in the maintenance of osmolarity of the body fluid compartments.

Hyponatremia is defined as a decreased serum sodium (sNa+) concentration less than 135 mmol/L. Serum sodium is the greatest contributor to plasma osmolality, which is very tightly controlled via antidiuretic hormone (ADH) release from the hypothalamus and by the thirst mechanism. The basic pathophysiology of all etiologies of hyponatremia is an abnormal increase in total body water (TBW), which dilutes the total body sodium (TBNa+) concentration. The clinical presentation varies greatly, from asymptomatic to subtle cognitive deficits, seizures, and death. Management is guided by etiology, acuity, and duration of symptoms, usually involving oral fluid restriction or administration of IV fluids that contain Na. Sodium must be replaced slowly, as overly rapid correction of hyponatremia can lead to irreversible neurologic complications and death, known as the osmotic demyelination syndrome (ODS).

Hypernatremia is an elevated serum sodium concentration > 145 mmol/L. Serum sodium is the greatest contributor to plasma osmolality, which is very tightly controlled by the hypothalamus via the thirst mechanism and antidiuretic hormone (ADH) release. Hypernatremia occurs either from a lack of access to water or an excessive intake of sodium. The total volume of water lost (usually via GI or renal routes) is regained through normal oral intake. Therefore, if a patient has access to water and an intact thirst mechanism, many etiologies of hypernatremia may remain hidden. The etiology of hypernatremia is often easily determined by clinical history. Treatment is primarily a replacement of the free water deficit by IV or oral routes.

Renal Na+ and water regulation work in tandem to control how fluid is distributed throughout the compartments of the body. Sodium is the body’s dominant extracellular solute, and is responsible for the osmotic force that keeps differing amounts of water in each compartment. Changes in Na+ balance are sensed by the body through changes in blood volume. Changes in water balance are sensed by the body through changes in plasma osmolality. Both ultimately send feedback signals to the kidneys to ensure that homeostasis is maintained. Abnormalities in these processes can result in problems in volume status (e.g., hypertension, pulmonary edema, pitting edema) and dysnatremias (hyponatremia and hypernatremia).