512
Chapter 14
A Summary Example:
The Response to Sweating
Figure 14–26
shows the factors that control renal sodium and
water excretion in response to severe sweating. You may notice
the salty taste of sweat on your upper lip when you exercise.
Sweat does contain sodium and chloride in addition to water,
but is actually hypoosmotic compared to the body fl
uids from
which it is derived. Therefore, sweating causes both a decrease
in extracellular volume and an increase in body fl
uid osmolar-
ity. The renal retention of water and sodium minimizes the
deviations from normal caused by the loss of water and salt in
the sweat.
Thirst and Salt Appetite
Defi cits of salt and water must eventually be compensated for
by ingestion of these substances, because the kidneys cannot
create new sodium ions or water. The kidneys can only mini-
mize their excretion until ingestion replaces the losses.
The subjective feeling of thirst is stimulated by an increase
in plasma osmolarity and by a decrease in extracellular fl
uid
volume (
Figure 14–27
). Plasma osmolarity is the single most
important stimulus under normal physiological conditions.
These are precisely the same two changes that stimulate vaso-
pressin production, and the osmoreceptors and baroreceptors
that control vasopressin secretion are the same as those for
thirst. The brain centers that receive input from these recep-
tors and that mediate thirst are located in the hypothalamus,
very close to those areas that synthesize vasopressin.
Another infl uencing factor may be angiotensin II, which
stimulates thirst by a direct effect on the brain, at least in exper-
imental animals. Thus, the renin-angiotensin system may help
regulate not only sodium balance but water balance as well, and
constitutes one of the pathways by which thirst is stimulated
when extracellular volume is decreased.
There are still other pathways controlling thirst. For
example, dryness of the mouth and throat causes thirst, which
is relieved by merely moistening them. Some kind of “meter-
ing” of water intake by other parts of the gastrointestinal tract
also occurs. For example, a thirsty person given access to water
stops drinking after replacing the lost water. This occurs well
before most of the water has been absorbed from the gas-
trointestinal tract and has a chance to eliminate the stimula-
tory inputs to the systemic baroreceptors and osmoreceptors.
This is probably mediated by afferent sensory nerves from the
mouth, throat, and gastrointestinal tract, and probably pre-
vents overhydration.
Salt appetite
is an important part of sodium homeo-
stasis and consists of two components: “hedonistic” appetite
and “regulatory” appetite. Most mammals “like” salt and
eat it whenever they can, regardless of whether they are salt-
defi cient. Human beings have a strong hedonistic appetite
for salt, as manifested by almost universally large intakes of
salt whenever it is cheap and readily available. For example,
the average American consumes 10–15 g/day despite the fact
that human beings can survive quite normally on less than
0.5 g/day
. However
, humans have re
lative
ly little regula-
tory salt appetite, at least until a bodily salt defi cit becomes
extremely large.
Potassium Regulation
Potassium is the most abundant intracellular ion. Although
only 2 percent of total-body potassium is in the extracellular
fl uid, the potassium concentration in this fl uid is extremely
important for the function of excitable tissues, notably nerve
Sodium excretion
H
2
O excretion
Plasma vasopressin
Plasma
aldosterone
GFR
Plasma volume
Loss of hypoosmotic
salt solution
Severe sweating
Plasma osmolarity
(
H
2
O concentration)
Reflexes
Reflexes
Begin
Figure 14–26
Pathways by which sodium and water
excretion decrease in response to severe
sweating. This fi gure is an amalgamation
of Figures 14–20, 14–22, 14–24, and the
reverse of 14–25.
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