A critical role of the kidneys is to maintain the effective circulating volume and plasma osmolality within relatively narrow limits, as well as to maintain electrolyte homeostasis. The normal homeostatic mechanisms that maintain the effective circulating volume and plasma osmolality are discussed elsewhere. (See "General principles of disorders of water balance (hyponatremia and hypernatremia) and sodium balance (hypovolemia and edema)".)
Under normal circumstances, the kidneys can adjust to wide variations in dietary intake by appropriate variations in water and electrolyte excretion , which is particularly important when discussing maintenance fluid requirements.
Water balance — Water losses lead to an increase in serum sodium and osmolality, resulting in stimulation of thirst and increased release of antidiuretic hormone (ADH). In normal individuals, these changes will lead to increased water intake and reduced water excretion, which will restore normal water balance. Thus, patients who are alert, have an intact thirst mechanism, and access to water will not become hypernatremic. (See "Etiology and evaluation of hypernatremia".)
On a normal diet, the minimum water intake is estimated at 500 mL/day (assuming there are no increased losses). This value is based upon the balance of total water intake and production and the minimum rate of urinary loss. Individuals who can concentrate their urine to 1200 mosmol/kg who excrete 600 mosmol of solute (sodium and potassium salts and urea) per day will have a minimum urine output of 500 mL (600 mosmol ÷ 1200 mosmol/kg).
There are two other sources of water in addition to fluid ingestion: the water content of food (fruits and vegetables are almost 100 percent water by weight) and the water generated by oxidation of carbohydrates. There are also other sources of water loss in addition to the urine output: insensible losses and sweat.