Determinants of water loss
- Respiratory water loss is a function of the amount of air that is moved in and out of the lungs, or breathed, per unit time. Unless the ambient relative humidity is 100%, this air needs to be humidified, and the water used for humidification is breathed out.
- The amount of air breathed depends on oxygen need and carbon dioxide production, which in turn depend on the metabolic rate (MR). The MR does not rise linearly with increase in body weight. MR/kg is highest in newborns and lowest in adults. Respiratory water loss/kg follows the same pattern.
- Transcutaneous evaporative water loss depends on body surface area (BSA). The BSA/kg body weight is highest in newborns and lowest in adults. Transcutaneous evaporative water loss/kg follows the same pattern.
Of course, these statements assume normal body temperature and identical ambient temperature and humidity.
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- Obligatory urine volume depends on the amount of waste solute that needs to be excreted and the ability to concentrate the urine. Urinary solutes are derived largely from ingested protein and electrolytes. The intake of these substances is higher in younger children than in adults. In addition, young infants cannot concentrate urine to the same extent as older children. For these reasons, obligatory urine output is higher in infants and children as compared to adults.
Putting it all together, the rate of water loss from all three routes is highest in the smallest children and does not rise in direct proportion to increase in body weight.