Basic Concepts of Fluid and Electrolyte Therapy

The external fluid and electrolyte balance between the body and its
environment is defined by the intake of fluid and electrolytes versus
the output from the kidneys, the gastrointestinal tract, and the skin
and lungs (insensible loss). Since the external and internal balances
may be disturbed by disease, it is important to understand normal
physiology in order to appreciate the disorders, which may occur in
patients.
External balance
Values for the normal daily intake and output of fluid and electrolytes
are shown in Tables 1 and 2. These are only an approximate guide and
may have to be modified in the presence of excessive losses, e.g. of
water and salt through increased sweating and insensible loss in hot
climates. They may also need to be modified in the presence of disease,
e.g. gastroenteritis, which causes abnormal losses of fluid and
electrolyte from the GI tract (Fig. 3 and Table 3).
Table 1: Approximate daily water balance in health
Intake (ml)
Output (ml)
Water from beverages 1200 Urine 1500
Water from solid food 1000
Insensible losses from skin
and lungs
900
Metabolic water from oxidation 300 Faeces 100
13