814
UNIT FIVE
Water Output
Water normally enters the body only through the mouth, but
it can be lost by a variety of routes. These include obvious
losses in urine, feces, and sweat (sensible perspiration), as
well as evaporation of water from the skin (insensible per-
spiration) and from the lungs during breathing.
If an average adult takes in 2,500 milliliters of water each
day, then 2,500 milliliters must be eliminated to maintain
water balance. Of this volume, perhaps 60% will be lost in
urine, 6% in feces, and 6% in sweat. About 28% will be lost
by evaporation from the skin and lungs (F
g. 21.5
b
). These
percentages vary with such environmental factors as temper-
ature and relative humidity and with physical exercise.
If water intake is insufficient, water output must be
reduced to maintain balance. Water lost in sweat is a nec-
essary part of the body’s temperature control mechanism;
water lost in feces accompanies the elimination of undi-
gested food materials; and water lost by evaporation is
largely unavoidable. Therefore, the primary means of regu-
lating water output is control of urine production.
Proteins called
aquaporins
form water-selective membrane chan-
nels that enable certain cells, including red blood cells and cells
in the proximal convoluted tubules and descending limbs of the
nephron loops, to admit water. A mutation in one aquaporin gene
(which instructs cells to manufacture a type of aquaporin protein)
causes a form of
diabetes insipidus,
in which the renal tubules fail
to reabsorb water. Rare individuals who lack other aquaporin genes
apparently have no symptoms. This suggests that cells have more
than one way to admit water.
in response, the hypothalamus causes the person to feel
thirsty and to seek water. A thirsty person usually has a dry
mouth, caused by loss of extracellular water and the result-
ing decreased fl
ow of saliva.
The thirst mechanism is normally triggered whenever
the total body water decreases by as little as 1%. The act
of drinking and the resulting distension of the stomach
wall trigger nerve impulses that inhibit the thirst mecha-
nism. Thus, drinking stops long before the swallowed water
is absorbed. This inhibition helps prevent the person from
drinking more than is required to replace the volume lost,
avoiding development of an imbalance.
Table 21.1
summa-
rizes this mechanism.
PRACTICE
7
What is water balance?
8
Where is the thirst center?
9
What stimulates F
uid intake? What inhibits it?
Water of
metabolism
(250 mL or 10%)
Water in
moist food
(750 mL or 30%)
Water in
beverages
(1,500 mL or 60%)
Total intake
(2,500 mL)
Average daily intake of water
Water lost in feces
(150 mL or 6%)
Water lost in sweat
(150 mL or 6%)
Water lost through
skin and lungs
(700 mL or 28%)
Water lost in urine
(1,500 mL or 60%)
Total output
(2,500 mL)
Average daily output of water
(a)
(b)
FIGURE 21.5
Water balance. (
a
) Major sources of body water. (
b
) Routes by which the body loses water. Urine production is most important in the
regulation of water balance.
TABLE
21.1
|
Regulation of Water Intake
1. The body loses as little as 1% of its water.
2. An increase in the osmotic pressure of extracellular F
uid due to
water loss stimulates osmoreceptors in the thirst center.
3. Activity in the hypothalamus causes the person to feel thirsty and to
seek water.
4. Drinking and the resulting distension of the stomach by water
stimulate nerve impulses that inhibit the thirst center.
5. Water is absorbed through the walls of the stomach and small
intestine.
6. The osmotic pressure of extracellular F
uid returns to normal.
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