section 39.4
Electrolyte Balance
933
of all the ions and molecules it contains. A solution that
has the same effective osmolality as plasma is said to be
isotonic,
e.g., 0.9% saline, 5% glucose, and Ringer’s and
Locke’s solutions. If a solute can permeate a membrane
freely, then a solution of that solute will behave like pure
water with respect to the membrane. Thus, a solution of
urea will cause red cells to swell and burst as does pure
water because urea moves freely across erythrocyte mem-
branes.
The osmolality of urine can differ markedly from that
of plasma because of active concentrative processes in the
renal tubules. The membranes of renal collecting ducts
show varying degrees of water permeability and permit
removal of certain solutes without simultaneous uptake of
water.
Plasma osmolality can be calculated from the con-
centrations of plasma Na+, glucose, and serum urea
nitrogen:
,
glucose (mg/dL)
Osmolality = 1.86(Na+ mEq/L) -|----------- —---------
serum urea nitrogen (mg/dL)
+
Z8
The numerical denominators for glucose and urea nitro-
gen convert the concentrations to moles per liter. Such
an estimated osmolality is usually 6-9 mosm less than
the value determined by freezing point or vapor pressure
measurements. If the latter value is much greater than the
estimated value, molecules other than Na+, glucose, and
urea must account for the difference. Such “osmolal gaps”
occur in individuals suffering from drug toxicity (alcohol,
barbiturates, salicylates), acute poisoning due to unknown
substances, and acidosis (keto-, lactic, or renal). Deter-
mination of osmolality is helpful in management of pa-
tients with fluid and electrolyte disorders, e.g., chronic
renal disease, nonketotic diabetic coma, hypo- and hy-
pernatremia, hyperglycemia, hyperlipemia, burns, seque-
lae to major surgery or severe trauma (particularly seri-
ous head injuries), hemodialysis, or diabetes insipidus.
Changes of about 2% or more are detected by the hypotha-
lamic osmoreceptors (Chapter 31) and elicit a sensation
of thirst and production of hypertonic urine. Under condi-
tions of fluid restriction, urine osmolality can reach 800-
1200 mosm/kg (normal is 390-1090 mosm/kg), or three
to four times the plasma levels. Decrease in plasma osmo-
larity (as in excessive water intake) produces urine with
decreased osmolality. Water losses from skin and lungs
are not subject to controls of this type.
ADH acts at the renal tubules and collecting ducts
to raise cAMP levels. Urinary levels of cAMP are in-
creased by ADH. Factors other than plasma hypertonicity
may stimulate ADH secretion. Thus, in acute hemorrhage
extracellular fluid volume drops abruptly, and ADH is
secreted to increase the volume at the expense of a drop
in osmolarity.
Inappropriate ADH secretion can occur in the presence
of water overload and a decline in plasma Na+ concen-
tration and osmolality. Fear, pain, and certain hormone-
secreting tumors can cause inappropriate ADH secretion.
It leads to hyponatremia and water retention. Morphine
and barbiturates increase, and ethanol decreases, secretion
of ADH.
In
diabetes insipidus
due to defective ADH receptors
or to diminished ADH secretion, renal tubules fail to re-
cover the water from the glomerular filtrate. In cases of
deficiency of ADH, hormone replacement by
8
-lysine va-
sopressin or l-deamino-
8
-D-arginine vasopressin (admin-
istered as a nasal spray or subcutaneously) is effective.
In osmotic diuresis, e.g., in
diabetes mellitus
with se-
vere glycosuria, the solute load increases the osmolal-
ity of the glomerular filtrate and impairs the ability of
the kidney to concentrate the urine. Extracellular fluid
volume in a normal adult is kept constant; body weight
does not vary by more than a pound per day despite
fluctuations in food and fluid intake. A decrease in
extracellular fluid volume lowers the effective blood
volume and compromises the circulatory system. An
increase may lead to hypertension, edema, or both. Volume
control centers on renal regulation of Na+ balance. When
the extracellular fluid volume decreases, less Na+ is ex-
creted; when it increases, more Na+ is lost. Na+ retention
leads to expansion of extracellular fluid volume, since Na+
is confined to this region and causes increased water re-
tention. Renal Na+ flux is controlled by the aldosterone-
angiotensin-renin system (Chapter 32) and atrial natri-
uretic peptide (discussed earlier).
39.4 Electrolyte Balance
The major electrolytes are Na+, K+, CD, and HCOj"
(HC0
3
is discussed below, under “Acid-Base Balance.”)
Sodium
The average Na+ content of the human body is 60 mEq/kg,
of which 50% is in extracellular fluid, 40% is in bone, and
10% is intracellular. The chief dietary source of sodium is
salt added in cooking. Excess sodium is largely excreted
in the urine, although some is lost in perspiration. Gas-
trointestinal losses are small except in diarrhea.
Sodium
balance
is
integrated
with
regulation
of
extracellular fluid volume.
Depletional hyponatremia
(sodium loss greater than water loss) may result from
