Basic Concepts of Fluid and Electrolyte Therapy

Kidney: this is the main organ for regulating fluid and electrolyte
balance as well as excreting the waste products of metabolism, e.g.
urea. In this function, its activity is controlled by pressure and
osmotic sensors and the resulting changes in the secretion of hormones.
The modest daily fluctuations in water and salt intake
cause small changes in plasma osmolality which trigger osmoreceptors.
This in turn causes changes in thirst and also in renal
excretion of water and salt. If blood or ECF volumes are threatened
by abnormal losses, volume receptors are triggered (see below) and
override the osmoreceptors. In the presence of large volume
changes, therefore, the kidney is less able to adjust osmolality,
which can be important in some clinical situations.
Water
Organs, which sense the changes in osmolality of plasma
(osmoreceptors), are located in the hypothalamus and signal
the posterior pituitary gland to increase or decrease its secretion
of vasopressin or antidiuretic hormone (ADH). Dilution of
the ECF, including plasma, by intake of water or fluid of osmolality
lower than plasma, causes ADH secretion to fall, so that
the distal tubules of the renal glomeruli excrete more water and
produce a dilute urine (this dilution requires the permissive
effect of glucocorticoid upon the distal tubules and is, therefore,
lost in adrenal insufficiency - one of the reasons for the
hyponatraemia of Addison’s Disease). Conversely, dehydration
causes the ECF to become more concentrated, ADH secretion
rises and the renal tubules reabsorb more water, producing a
concentrated urine. In response to dehydration, the normal
kidney can concentrate urea in the urine up to a hundred-fold,
so that the normal daily production of urea during protein
metabolism can be excreted in as little as 500 ml of urine.
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