Sodium and water homeostasis. 
Disorders of sodium and water homeostasis are common occurrences in pediatric practice.  They reflect distinct problems in the regulation of total body sodium balance and water distribution, respectively.  Each of these groups of disorders has separate afferent and efferent mechanisms that are activated during disease states.  Optimal therapy of children with fluid and electrolyte problems requires accurate delineation of the ECF volume and water distribution disturbance and the design of therapeutic regimens that account for each component of the clinical condition.