815
CHAPTER TWENTY-ONE
Water, Electrolyte, and Acid-Base Balance
21.4
ELECTROLYTE BALANCE
An
electrolyte balance
(e-lek
tro-lı¯t bal
ans) exists when
the quantities of electrolytes (molecules that release ions in
water) the body gains equal those lost
(f g. 21.6)
.
Electrolyte Intake
The electrolytes of greatest importance to cellular functions
release sodium, potassium, calcium, magnesium, chloride,
sulfate, phosphate, bicarbonate, and hydrogen ions. These
electrolytes are primarily obtained from foods, but they
may also be found in drinking water and other beverages.
In addition, some electrolytes are by-products of metabolic
reactions.
Regulation of Electrolyte Intake
Ordinarily, a person obtains sufficient electrolytes by
responding to hunger and thirst. However, a severe electro-
lyte deF ciency may cause
salt craving,
a strong desire to eat
salty foods.
Electrolyte Output
The body loses some electrolytes by perspiring (sweat
has about half the solute concentration of plasma). The
quantities of electrolytes leaving vary with the amount of
Regulation of Water Output
The distal convoluted tubules and collecting ducts of the
nephrons regulate the volume of water excreted in the urine.
The epithelial linings of these segments of the renal tubule
remain relatively impermeable to water unless antidiuretic
hormone (ADH) is present.
Recall from chapter 13 (p. 498) that osmoreceptors in
the hypothalamus help control release of ADH. If the blood
plasma becomes more concentrated because of excessive
water loss, the osmoreceptors lose water by osmosis and
shrink. This change triggers impulses that signal the poste-
rior pituitary gland to release ADH. The ADH released into
the bloodstream reaches the kidneys, where it increases the
permeability of the distal convoluted tubules and collecting
ducts. Consequently, water reabsorption increases, conserv-
ing water. This action resists further osmotic change in the
plasma. The
osmoreceptor
-
ADH mechanism
can reduce a
normal urine production of 1,500 milliliters per day to about
500 milliliters per day when the body is dehydrated.
If a person drinks too much water, the plasma becomes
less concentrated, and the osmoreceptors swell as they
receive extra water by osmosis. In this instance, ADH release
is inhibited, and the distal tubules and collecting ducts
remain impermeable to water. Consequently, less water is
reabsorbed and more urine produced.
Table 21.2
summa-
rizes this mechanism. Clinical Application 21.1 discusses
disorders resulting from water imbalance.
Diuretics
are chemicals that promote urine production. They act in
dif
erent ways. Alcohol and certain narcotic drugs promote urine For-
mation by inhibiting ADH release. Caf
eine inhibits the reabsorption
oF sodium ions or other solutes in parts oF the renal tubules. As a con-
sequence, the osmotic pressure oF the tubular ±
uid increases, reduc-
ing osmotic reabsorption oF water and increasing urine volume.
PRACTICE
10
By what routes does the body lose water?
11
What is the primary regulator oF water loss?
12
What types oF water loss are unavoidable?
13
How does the hypothalamus regulate water balance?
FIGURE 21.6
Electrolyte balance exists when the intake oF
electrolytes From all sources equals the output oF electrolytes.
TABLE
21.2
|
Events in Regulation of Water Output
Dehydration
1. Extracellular ±
uid becomes osmotically more concentrated.
2. Osmoreceptors in the hypothalamus are stimulated by the increase in
the osmotic pressure oF body ±
uids.
3. The hypothalamus signals the posterior pituitary gland to release ADH
into the blood.
4. Blood carries ADH to the kidneys.
5. ADH causes the distal convoluted tubules and collecting ducts to
increase water reabsorption.
6. Urine output decreases, and Further water loss is minimized.
Excess Water Intake
1. Extracellular ±
uid becomes osmotically less concentrated.
2. This change stimulates osmoreceptors in the hypothalamus.
3. The posterior pituitary gland decreases ADH release.
4. Renal tubules decrease water reabsorption.
5. Urine output increases, and excess water is excreted.
Urine
Metabolic
reactions
Electrolyte intake
Fluids
Feces
Electrolyte output
Foods
Perspiration
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