The appropriate balance of fluid intake and output (along with dissolved salts essential for life). During a 24 hour period a 70-kg man normally takes in about 2,500 millilitres (ml) of fluid and excretes the same amount. Of this intake, 1,500 ml will be drunk, 800 ml will be in the food eaten, and 200 ml produced by food being metabolised during digestion. Excreted water is made up of 1,500 ml of urine, 800 ml insensible loss (sweating and in the breath) and 200 ml in the faeces. Cells contain about 28 litres (intracellular fluid) and are bathed in a further 14 litres of extracellular fluid. Blood constitutes 5 litres. Water content is controlled mainly by the sodium concentration in the body fluids via the release of ANTIDIURETIC HORMONE from the posterior part of the PITUITARY GLAND. In seriously ill people, close monitoring of fluid intake and output, along with measurements of PLASMA sodium and calcium concentrations, is an essential factor in treatment.