The appropriate balance of ﬂuid input and output (along with dissolved salts essential for life) over 24 hours. During this period, about 2,500 millilitres (ml) of ﬂuid should be taken in by a 70-kg man and the same amount excreted; of this, 1,500 ml will be drunk, 800 ml will be in the food eaten, and 200 ml produced by food metabolism. Excreted water is made up of 1,500 ml of urine, 800 ml insensible loss and 200 ml in the faeces. A 70-kg man’s total body ﬂuid is 42 litres – 60 per cent of body weight. Intracellular ﬂuid comprises 28 litres, extracellular, 14 litres and blood, 5 litres. Water is controlled mainly by the sodium concentration in the body ﬂuids via the release of antidiuretic hormone (ADH – see VASOPRESSIN) 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.
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