Disorders of Extracellular Fluid Volume: Basic Concepts

Abstract

In Chap. 3, we discussed how NaCl and water are handled by various segments of the nephron. Since Na⁺ is the major extracellular electrolyte, the total amount of this electrolyte and its accompanying anion (Cl⁻) determine the extracellular fluid (ECF) volume. Therefore, retention or excretion of Na⁺ by the kidneys is critical for the regulation of ECF volume. This regulation of NaCl is precise in normal individuals. In a steady state, urinary Na⁺ approximates dietary Na⁺, as the kidneys are major excretory organs of Na⁺ besides gastrointestinal tract and skin. Low salt intake results in low excretion of Na⁺. Conversely, high salt intake results in high excretion of Na⁺. Any disturbance in this regulation activates or inhibits neural and hormonal mechanisms, leading to appropriate change in Na⁺ excretion by the kidneys. This chapter discusses the basic concepts that characterize ECF volume depletion and conditions that are associated with the development of volume expansion and edema formation.