Basic Concepts of Fluid and Electrolyte Therapy

negative through insufficient intake or increased loss. It may also be
triggered by a high salt intake, which necessitates the intake and
retention of extra water in order to maintain the ECF sodium concentration
and osmolality in the normal range.
Although, in the elderly, the thirst mechanism becomes blunted, it
ensures, on the whole, that our intake matches the needs of bodily
functions, maintaining a zero balance in which intake and output are
equal and physiological osmolality (280-290 mOsm/kg) is maintained.
More than a century ago Claude Bernard coined the term ‘volume
obligatoire’ to describe the minimum volume of urine needed to
excrete waste products, e.g. urea, in order to prevent them accumulating
in the blood. This concept implies that, if sufficient fluid has
been drunk or administered to balance insensible or other losses and
to meet the kidney’s needs, there is no advantage in giving additional
or excessive volumes. Indeed, excessive intakes of fluid and electrolytes
may be hazardous under certain circumstances (see below)
and overwhelm the kidney’s capacity to excrete the excess and maintain
normal balance. Salt and water retention becomes clinically
apparent in the form of oedema when the ECF has been expanded by
at least 2-3 litres.
Output
Insensible loss: evaporation of water from the lungs and skin
occurs all the time without us being aware of it. In our temperate
climate the amount so lost is 0.5-1 litre/day. In a warm environment,
during fever, or with exertion, we produce additional sweat
containing up to 50 mmol/l of salt.
GI losses: normally, the intestine absorbs water and electrolytes
very efficiently so that fluid loss in the stool is as little as 100-
150 ml/day, although, in the presence of disease this may be greatly
increased (Table 3 and Fig. 3).
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