Healthy adults take in and lose roughly 2.5 liters of water a day. The brain keeps the blood's concentration within a narrow range by adjusting thirst and urine output.
Water in
Most water comes from drinks and food, with a small amount made by metabolism. Thirst is the main signal to drink.
Water out
Urine is the largest and only controlled loss. Water is also lost through the skin, breath and stool; these insensible losses rise with fever, fast breathing and heat.
The sensors
Osmoreceptors in the hypothalamus detect when the blood becomes too concentrated. They trigger thirst and the release of ADH, so the person drinks and the kidney keeps water.
When it fails
Older adults feel less thirst and become dehydrated easily. Too much ADH causes water retention and low sodium, and too little ADH causes large volumes of dilute urine.
| Route | Water in (approx.) | Water out (approx.) |
|---|---|---|
| Drinks and urine | 1.5 L from drinks | 1.5 L as urine |
| Food and skin or breath | 0.7 L from food | 0.8 L through skin and breath |
| Metabolism and stool | 0.3 L from metabolism | 0.2 L in stool |
At the bedside
Intake and output charts and daily weights track water balance; 1 kg of weight change in a day is about 1 L of fluid. Assess hydration with mucous membranes, skin turgor, pulse, blood pressure and urine output.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).