Body Fluid Composition

Educational Gap

Body fluid composition is maintained in a normal physiologic range by regulatory mechanisms that control sodium and water metabolism. A detailed knowledge of the homeostatic mechanisms will help in understanding the pathogenesis and management of disorders of sodium and water balance.

Objectives

After completing this article, readers should be able to:

1. Understand the distribution of fluid and solute in different body compartments.

2. Demonstrate the homeostatic mechanisms involved in maintaining sodium and water metabolism.

3. Calculate osmolality and recognize the clinical importance of maintaining osmotic equilibrium.

4. Recognize common disorders of hypernatremia or hyperosmolality and evaluate and understand the role of calculating free water deficit in the treatment of these disorders.

5. Recognize common disorders of hyponatremia or hypo-osmolality, appreciate the role of urine sodium and urine osmolality in evaluation, and understand the importance of slow correction of these disorders.

Case Scenario

A 6-month-old infant presents to the emergency department with vomiting for 3 days. He is lethargic and has a weak cry. His vital signs reveal an elevated heart rate (140 beats per minute), and physical examination findings are remarkable for dry mucous membranes. His capillary refill is more than 2 seconds. His initial laboratory values are as follows: serum sodium, 122 mEq/L (122 mmol/L); blood urea nitrogen (BUN), 28 mg/dL (10 mmol/L); serum creatinine, 0.4 mg/dL (35 μmol/L); serum glucose, 90 mg/dL (5.0 mmol/L); and serum osmolality (Sosm), 260 mOsm/kg (260 mmol/kg). Urinalysis reveals a specific gravity of 1.030, pH 6.5, and negative results for blood, protein, leukocyte esterase, or nitrite. Additional urine studies reveal a urine osmolality of 900 mOsm/kg (900 mmol/kg) and a urine sodium level of 6 mEq/L (6 mmol/L). A detailed history also …