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Abstract

Changes in volume and composition of body fluids due to disorders of fluid and electrolyte balance cause various common clinical illnesses. The rationale for reviewing the diagnosis and management of fluid and electrolyte disorders was eloquently denoted by Dr Altemeier, when he suggested that this knowledge belongs among the core concepts needed by the "keepers of the gates," that is, primary care pediatricians.1 In the body, homeostasis is maintained by the coordinated action of behavioral, hormonal, renal, and vascular adaptations to volume and osmotic changes. These core issues have been outlined in a previous article in this journal by Dr Hellerstein, and the current article proceeds from that discussion.2 Following introductory comments about body fluid volume and composition, we provide an overview of some of the etiologies of the disorders of volume, tonicity, and composition of body fluids and of the therapy to correct these disorders. Sodium, Osmolality, and the Volume of Body Fluids Total body water, which is 55% to 72% of body mass, varies with sex, age, and fat content and is distributed between the intracellular and extracellular spaces. The extracellular fluid (ECF), which comprises about one third of total body water, includes the intravascular plasma fluid and the extravascular interstitial fluid.