Chapter 74 - Physiologic Principles in the Clinical Evaluation of Electrolyte, Water, and Acid–Base Disorders

Abstract

The kidneys play a fundamental role in the regulation of water and electrolyte excretion as well as in the maintenance of acid base balance. Water and electrolyte homeostasis is accomplished almost exclusively through changes in the volume and composition of urine. Routes of external loss other than from the kidneys play a limited role in regulation, even though they may display some response to the needs of the organism. Intake of the various electrolytes and of water can vary extremely widely and almost independently of each other. Balance (used as a synonym for external balance) is the difference between intake and excretion by all routes. Internal balance, on the other hand, means the rate of transfer of a substance between the intra- and extracellular fluids, a concept that has proven especially useful with regard to potassium homeostasis. In this chapter, we will limit the discussion to external balance as we review the physiology underlying the clinical approach to the evaluation of water and electrolyte excretion by the kidneys. The objective of this chapter is to summarize the rationale and physiologic principles behind the use of urinary indices and clinical tools available in the evaluation of normal and pathophysiologic states related to sodium, water, potassium, and acid-base disturbances.