816
UNIT FIVE
Recall from chapter 2 (p. 57) that water molecules are
polar. Molecules that have polar regions, such as carbohy-
drates and proteins, dissolve in water but remain intact. In
contrast, ionically bonded molecules, such as electrolytes,
dissociate in water to release ions.
The total solute concentration of a body fl uid determines
its
osmolarity.
One molecule of glucose yields one dissolved
perspiration. More electrolytes are lost in sweat on warmer
days and during strenuous exercise. Varying amounts of
electrolytes are lost in the feces. The greatest electrolyte out-
put occurs as a result of kidney function and urine produc-
tion. The kidneys alter renal electrolyte losses to maintain
the proper composition of body fl uids, thereby promoting
homeostasis.
(fig. 21A). Dehydration may also accompany ill-
nesses in which prolonged vomiting or diarrhea
depletes body F
uids.
During dehydration, the skin and mucous
membranes of the mouth feel dry, and body
weight drops. Severe hyperthermia may develop
as the body temperature regulating mechanism
falters due to lack of water for sweat. In severe
dehydration, as waste products accumulate in
the extracellular F
uid, symptoms of cerebral dis-
turbances, including mental confusion, delirium,
and coma, may develop. Dehydration is respon-
sible for some of the symptoms of heatstroke, dis-
cussed in this chapter’s opening vignette (p. 811).
Infants are more likely to become dehy-
drated because their kidneys are less e±
cient at
conserving water than those of adults. Elderly
people are also especially susceptible to develop-
ing water imbalances because the sensitivity of
their thirst mechanisms decreases with age, and
physical disabilities may make it di±
cult to obtain
adequate F
uids.
A
mong the more common disorders
involving an imbalance in the water of
body F
uids are dehydration, water intox-
ication, and edema.
Dehydration
In 1994, thousands of starving people died in the
African nation of Rwanda. It wasn’t lack of food
that killed most of these people, but cholera, a
bacterial infection that cripples the ability of intes-
tinal mucosal cells to reabsorb water. The severe
diarrhea that develops can kill in days, sometimes
even hours. The dehydration it causes is deadly.
Dehydration
is a deficiency condition that
occurs when output of water exceeds intake. It
is a great problem for athletes, military person-
nel, and certain industrial workers. Dehydration
may develop following excessive sweating or as a
result of prolonged water deprivation accompa-
nied by continued water output. In either case, as
water is lost, the extracellular F
uid becomes more
concentrated, and water leaves cells by osmosis
The treatment for dehydration is to replace
the lost water and electrolytes. If only water is
replaced, the extracellular F
uid will become more
dilute than normal. This may produce a condition
called water intoxication.
Water Intoxication
Until recently, runners were advised to drink as
much fluid as they could, particularly in long
events. But the death of a young woman in the
2002 Boston Marathon from low blood sodium
(
hyponatremia,
“water intoxication”) due to
excessive F
uid intake, inspired further study and
a reevaluation of this advice. Researchers from
Harvard Medical School studied 488 runners from
the race, and found that 13% of them developed
hyponatremia. Tendency to develop the condi-
tion was associated with longer race time, high or
low body mass index, and signi²
cant weight gain
during the race. Drinking sports drinks instead of
water does not make a di³
erence because these
beverages are mostly water.
In recognition of the possibility of hypona-
tremia, USA Track and ´ield, the national govern-
ing body for the sport, offers on their website
(www.usatf.org) instructions for runners to deter-
mine exactly how much to consume during a one-
hour training run. The goal is to replace exactly
what is lost.
Edema
Edema
is an abnormal accumulation of extra-
cellular fluid in the interstitial spaces (fig. 21B).
Causes include a decrease in the plasma protein
concentration (
hypoproteinemia
), obstructions
in lymphatic vessels, increased venous pressure,
and increased capillary permeability.
Hypoproteinemia
may result from failure of
the liver to synthesize plasma proteins; kidney
21.1
CLINICAL APPLICATION
Water Balance Disorders
Water is lost from
extracellular fluid
compartment
Solute
concentration
increases in
extracellular
fluid compartment
Water leaves
intracellular fluid
compartment
by osmosis
Cell
membrane
Nucleus
1
3
2
FIGURE 21A
If excess extracellular F
uids are lost, cells dehydrate by osmosis.
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