In most steady state situations, water intake matches water losses through all sources. Water balance involves adjusting the effects of intake, which is determined by thirst; renal dilution or concentration of urine; and losses, via the skin, kidney, gastrointestinal tract, respiratory tract.
The usual sources of water intake are ingested liquids, foods (fruits and vegetables) and endogenous metabolic water production. The sources of water output comprise:
Sensible losses: urine, stools and sweat. The ability to dilute and to concentrate urine allows for a wide flexibility in urine flow. Insensible losses: skin loss (insensible perspiration); exhaled air from the respiratory tract.
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