813
CHAPTER TWENTY-ONE
Water, Electrolyte, and Acid-Base Balance
PRACTICE
5
Which factors control the movement of water and electrolytes
from one F
uid compartment to another?
6
How does the sodium ion concentration in body F
uids a±
ect the
net movement of water between the compartments?
21.3
WATER BALANCE
Water balance
exists when water intake equals water out-
put. Homeostasis requires control of both water intake and
water output. Ultimately, maintenance of the internal envi-
ronment depends on thirst centers in the brain to vary water
intake and on the kidneys’ ability to vary water output.
Water Intake
The volume of water gained each day varies among individu-
als. An average adult living in a moderate environment takes
in about 2,500 milliliters daily. Probably 60% is obtained
from drinking water or beverages, and another 30% comes
from moist foods. The remaining 10% is a by-product of the
oxidative metabolism of nutrients, called
water of metabo-
lism
(f
g. 21.5
a
)
.
Regulation of Water Intake
The primary regulator of water intake is thirst. The intense
feeling of thirst derives from the osmotic pressure of extra-
cellular fl uids and a
thirst center
in the hypothalamus of the
brain.
As the body loses water, the osmotic pressure of the
extracellular fluids increases. Such a change stimulates
osmoreceptors
(oz
mo-re-sep
torz) in the thirst center, and
ion concentration increases, cells shrink as they lose
water. Although the solute composition of body fl
uids var-
ies between intracellular and extracellular compartments,
water will “follow salt” and distribute by osmosis such that
the water concentration (and total solute concentration) is
essentially equal inside and outside cells.
Di±
erent substances may be distributed to di±
erent compartments.
²or example, an infusion of 1 liter of isotonic sodium chloride solu-
tion is restricted largely to the extracellular F
uid because of the active
transport sodium pumps in cell membranes. In contrast, a liter of iso-
tonic glucose solution may be given intravenously without damaging
red blood cells, but as the glucose is metabolized aerobically, it reacts
to release carbon dioxide and water. Thus, the liter of isotonic glucose
yields a liter of water that can be distributed throughout intracellular
and extracellular compartments.
FIGURE 21.4
Net movements of F
uid between compartments
result from di±
erences in hydrostatic and osmotic pressures.
Ion concentration (m Eq/L)
20
30
50
60
70
80
90
100
110
120
130
140
Relative concentrations and ratios of ions in extracellular and intracellular fluids
150
40
10
0
14:1
Ratio
(Extracellular: intracellular)
Na
+
1:28
K
+
5:1
Ca
+2
1:19
Mg
+2
26:1
Cl
3:1
HCO
3
1:19
PO
4
–3
1:2
SO
4
–2
Extracellular fluid
Intracellular fluid
Interstitial fluid
Intracellular
fluid
Capillary wall
Cell
membrane
Serous
membrane
Transcellular
fluid
Lymph
Fluid leaves plasma
at arteriolar end of
capillaries because
outward force of
hydrostatic pressure
predominates
Hydrostatic pressure
within interstitial
spaces forces fluid
into lymph capillaries
Fluid returns to
plasma at venular
ends of capillaries
because inward force
of colloid osmotic
pressure predominates
Interstitial fluid is
in equilibrium with
transcellular and
intracellular fluids
Plasma
Lymph
vessel
FIGURE 21.3
³Extracellular³F
uids have relatively high concentrations
of sodium (Na
+
), calcium (Ca
+2
), chloride (Cl
), and bicarbonate (HCO
3
)
ions. Intracellular F
uid has relatively high concentrations of potassium
(K
+
), magnesium (Mg
+2
), phosphate (PO
4
–3
), and sulfate (SO
4
–2
) ions.
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