Definition
- ECF volume and osmolality are held constant by matching water gain to water loss, sensed via plasma osmolality and blood volume.
- Effective circulating volume (ECV), about 20% of blood volume, is the arterial volume producing effective tissue perfusion.
Water Balance
- Water gain is exogenous (drinking and food) and endogenous (metabolic water from tissue oxidation); water loss occurs via urine, feces, sweat and insensible loss.
Normal daily water balance
Route
Exogenous gain: drinking
- Volume/day
- 0.5 to 8 L
Exogenous gain: food
- Volume/day
- 1 to 5 L
Endogenous (metabolic) gain
- Volume/day
- 300 to 500 mL
Loss: urine
- Volume/day
- About 1 L (range 0.5 to 18 L, ADH-dependent)
Loss: feces
- Volume/day
- About 200 mL
Loss: sweat
- Volume/day
- 0.1 L at 23°C to 5 L at 45°C