Basic Concepts of Fluid and Electrolyte Therapy

In the presence of water lack, the urine to plasma urea or osmolality
ratio is, therefore, a measure of the kidney’s concentrating
capacity. Age and disease can impair the renal concentrating
capacity so that a larger volume of urine is required in order to
excrete the same amount of waste products. Also if protein
catabolism increases due to a high protein intake or increased
catabolism, a larger volume of urine is needed to clear the
resulting increase in urea production.
To assess renal function, therefore, measurement of both urinary
volume and concentration (osmolality) are important, and
the underlying metabolic circumstances taken into account. If
serum urea and creatinine concentrations are unchanged and
normal, then, urinary output over the previous 24 hours has
been sufficient, fluid intake has been adequate, and the urinary
‘volume obligatoire’ has been achieved.
Sodium
Since the integrity of the ECF volume and its proportion of the
total body water are largely dependent on the osmotic effect of
Na + and its accompanying anions, it is important that the kidneys
maintain Na + balance within narrow limits. If salt depletion
occurs, then the ECF, and with it the plasma volume, falls.
Pressure sensors in the circulation are then stimulated and
these excite renin secretion by the kidney. This, in turn, stimulates
aldosterone secretion by the adrenal gland, which acts on
the renal tubules, causing them to reabsorb and conserve Na + .
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