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solutes must be excreted per day to maintain electrolyte balance; thus, if the urine is maximally concentrated (Uosm approximately 1,200 mOsmol/kg water), the minimum urine output is approximately 500 mL/day. For dehydrated subjects living in hot weather, minimum daily urine outputs can be less than 500 mL/day (Adolph, 1947b).

Urine output generally averages 1 to 2 L/day but can reach 20 L/day in those consuming large quantities of fluid (West, 1990). Healthy older individuals, however, cannot concentrate urine as well as young individuals and thus have a higher minimum urine output. For example, older men and women (mean age 79 years) had lower maximal urine osmolalities of 808 and 843 mOsm/kg, respectively, compared with 1,089 mOsm/kg for young men (mean age 24 years). This corresponds to higher minimum urine outputs of 700 and 1,086 mL/day for the older men and women compared with 392 mL/day for the young men (Dontas et al., 1972).

Urine output varies inversely with body hydration status. Figure 4-4 depicts the hyperbolic relationship between urine output and

FIGURE 4-4 Relation of urine output to body hydration status. Reprinted with permission, from Lee (1964). Copyright 1964 Handbook of Physiology, Section 4, American Physiological Society.

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