RT Book, Section A1 Srivatsa, Abhinash A1 Majzoub, Joseph A. A1 Kappy, Michael S. A2 Kappy, Michael S. A2 Allen, David B. A2 Geffner, Mitchell E. SR Print(0) ID 1107892950 T1 Posterior Pituitary and Disorders of Water Metabolism T2 Pediatric Practice: Endocrinology, 2e YR 2013 FD 2013 PB McGraw-Hill Education PP New York, NY SN 9780071813174 LK accesspediatrics.mhmedical.com/content.aspx?aid=1107892950 RD 2024/10/08 AB Maintenance of the tonicity of extracellular fluids within a very narrow range is crucial for proper cell function because extracellular osmolality regulates cell shape, as well as intracellular concentrations of ions and other osmolytes.1,2 Furthermore, proper extracellular ionic concentrations are necessary for the correct function of ion channels, action potentials, and other modes of intercellular communication. Normal blood tonicity is maintained over a 10-fold variation in water intake by a coordinated interaction among the vasopressin, thirst, and renal systems. Dysfunction in any of these systems can result in abnormal regulation of blood osmolality, which, if not properly recognized and treated, may cause life-threatening hyperosmolality or hypo-osmolality.