17 5A.4 Water balance and dehydration
11–2 %
Effect
- thirst, reduced concentration
25 %
Effect
- tiredness, headache, fast pulse
310 %
Effect
- dizziness, kidney strain, danger
415–20 %
Effect
- usually fatal
- An adult loses 2–3 litres per day (urine about 1.5 L, sweat 0.5 L or much more in heat, breath 0.4 L, faeces 0.1 L).
- The loss must be replaced by drinking, food and metabolic water from respiration.
✍️ Worked examples
Worked example 8.
- Problem. A 70 kg athlete loses 2.8 kg in sweat during a match.
- What percentage of body mass is that, and what could be the symptoms?
- How much should he drink?
Solution. 2.8/70 × 100 = 4 %: near the level of tiredness and headache. He should drink about 1.5 times the lost mass (4.2 L) in the following hours, with some salts, because water is lost together with ions.