Body Water Balance

As discussed in Chapter 25, the homeostatic regulation of any substance in the body requires a careful balance of input to the body, production by the body, and output from the body. The same is true with water balance. To maintain a constant amount of water, the body must eliminate as much water as is taken in and produced by metabolism. The intake of water can vary markedly from day to day, depending on our dietary habits and whether we are relatively sedentary or exercising.

The commonly accepted "classical" values for the daily intake and output of water by healthy adults under sedentary conditions in a temperate climate are given in Table 1. Data from studies before 1980 indicate that the average intake of water in this setting (as fluids that are drunk as well as the water in solid foods) ranges from 2200-2700 mL of water per day; however, as discussed in the following Clinical Note, the quantity of water drunk has increased in the last two decades. Metabolism of foodstuffs ultimately leads to the production of another ~300 mL, as well

TABLE 1 Daily Balance of Water Intake and Production in Sedentary Healthy Adults in a Temperate Climate

Intake and production (mL/day) Output

Urine

Water intake:

in fluids 1200-1700

drunk in solid foods 1000 Metabolic production 300 Feces 100

Insensible losses from 900 the skin and lungs

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