Disorders of Water Metabolism

Disorders of body fluids are among the most commonly encountered problems in the practice of clinical medicine. This is in large part because many different disease states can potentially disrupt the finely balanced mechanisms that control the intake and output of water and solute. Since body water is the primary determinant of the osmolality of the extracellular fluid (ECF), disorders of water metabolism can be broadly divided into hypoosmolar disorders, in which there is an excess of body water relative to body solute, and hyperosmolar disorders, in which there is a deficiency of body water relative to body solute. Because sodium is the main constituent of plasma osmolality (Posm), these disorders are typically characterized by hyponatremia and hypernatremia, respectively. Before discussing these disorders, this chapter will first review the regulatory mechanisms underlying water and sodium metabolism, the two major determinants of body fluid homeostasis.