994
UNIT 4
Maintenance of the Body
26
Dry mouth
. When the blood’s osmotic pressure increases, the
salivary glands produce less saliva because the osmotic gra-
dient drawing water from the blood into the salivary ducts is
reduced.
A decrease in blood volume
(
or pressure
). A substantial de-
crease in blood volume or pressure (5–10%, as in hemor-
For the body to remain properly hydrated, water intake must
equal water output.
Water intake
varies widely from person to
person and is strongly influenced by habit, but is typically about
2500 ml a day in adults
(Figure 26.4)
. Most water enters the
body through ingested liquids and solid foods. Body water pro-
duced by cellular metabolism is called
metabolic water
or
wa-
ter of oxidation
.
Water output
occurs by several routes. Water that vaporizes
out of the lungs in expired air or diffuses directly through the
skin is called
insensible water loss
. Some is lost in obvious per-
spiration and in feces. Te kidneys excrete the rest (about 60%)
in urine.
Healthy people have a remarkable ability to maintain the os-
molality of their body fluids within very narrow limits (280–300
mOsm). A rise in plasma osmolality triggers (1) thirst, which
prompts us to drink water, and (2) release of antidiuretic hor-
mone (ADH), which causes the kidneys to conserve water and
excrete concentrated urine. On the other hand, a decline in os-
molality inhibits both thirst and ADH release, and the latter
prompts the kidneys to excrete large volumes of dilute urine.
In the body, water and Na
1
are closely tied together. In fact,
Na
1
acts as a powerful “water magnet.” However, the ADH and
thirst mechanisms controlling osmolality regulate water
inde-
pendently
of sodium. Later, we will describe mechanisms that
regulate Na
1
and water
together
for the purpose of maintaining
blood volume and pressure.
Regulation of Water Intake
Te
thirst mechanism
is the driving force for water intake. It is
governed by the hypothalamic
thirst center
, which is activated
by various stimuli
(Figure 26.5)
:
Osmoreceptors
. Hypothalamic
osmoreceptors
detect ECF os-
molality through changes in plasma membrane stretch that
result from gaining or losing water. An increase in osmolality
of only 1–2% activates these osmoreceptors.
Feces 4%
Sweat 8%
Insensible loss
via skin and
lungs 28%
Urine 60%
2500 ml
Average output
per day
Average intake
per day
Beverages 60%
Foods 30%
Metabolism 10%
1500 ml
700 ml
200 ml
100 ml
1500 ml
750 ml
250 ml
Figure 26.4
Major sources of water intake and output.
When
intake and output are in balance, the body is adequately hydrated.
Dry mouth
Renin-angiotensin-
aldosterone
mechanism
Hypothalamic
thirst center
Sensation of thirst;
person takes a
drink
Water absorbed
from GI tract
Angiotensin II
ECF osmolality
Blood pressure
Water moistens
mouth, throat;
stretches stomach,
intestine
ECF osmolality
Plasma volume
Initial stimulus
Result
Reduces, inhibits
Increases, stimulates
Physiological response
Plasma volume
( 5 –10%)
Granular cells
in kidney
Saliva
Osmoreceptors
in hypothalamus
Figure 26.5
The thirst mechanism for regulating water
intake.
Only large volume changes (5–10% of plasma volume)
activate the thirst mechanism. Osmolality changes are the normal
stimuli.
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