Over the course of 24 hours, the body loses fluids in more ways than one. Firstly, through evaporation, the body loses roughly 350ml fluid via evaporation. This is without additional exercise and illness etc.
The body loses an additional 350ml through respiratory water vapour and another 200ml through faecal losses.
Already this is a 900ml output on and does not take into consideration exercise, temperatures (pyrexia), altered skin integrity (eg burns) and diarrhoea or vomiting (higher output from a stoma bag).
The kidney produces approximately 1200ml-2000ml of urine per day meaning the average 70kg person needs a fluid intake of at leased 2000ml-2500ml per day. This is calculated as ‘replacement volume’ 40ml/kg/day.
However, the kidneys can still perform their metabolic functions whilst reducing urine output to as little as 500ml-700ml per day during a fluid depletion.