1000
UNIT 4
Maintenance of the Body
26
released by certain cells of the heart atria when they are stretched
by the effects of elevated blood pressure, ANP has diuretic and
natriuretic (salt-excreting) effects. It promotes excretion of Na
1
and water by the kidneys by inhibiting the ability of the collecting
ducts to reabsorb Na
1
and by suppressing the release of ADH,
renin, and aldosterone. Additionally, ANP acts both directly and
indirectly (by inhibiting renin-induced generation of angiotensin
II) to relax vascular smooth muscle, and in this way it causes va-
sodilation. Collectively, these effects reduce blood pressure.
Influence of Other Hormones
Female Sex Hormones
Te
estrogens
are chemically similar
to aldosterone and, like aldosterone, enhance NaCl reabsorp-
tion by the renal tubules. Because water follows, many women
retain fluid as their estrogen levels rise during the menstrual
cycle. Estrogens are also largely responsible for the edema expe-
rienced by many pregnant women.
Progesterone
appears to decrease Na
1
reabsorption by
blocking the effect aldosterone has on the renal tubules. Tus,
progesterone has a diuretic-like effect and promotes Na
1
and
water loss.
Glucocorticoids
Glucocorticoids
, such as cortisol and hydro-
cortisol, enhance tubular reabsorption of Na
1
, but they also
promote an increased glomerular filtration rate that may mask
their effects on the tubules. However, when their plasma levels
are high, the glucocorticoids exhibit potent aldosterone-like ef-
fects and promote edema.
Cardiovascular Baroreceptors
Blood volume is carefully monitored and regulated to maintain
blood pressure and cardiovascular function. Because Na
1
con-
tent determines fluid volume and fluid volume determines blood
pressure, the baroreceptors indirectly monitor Na
1
content.
As blood volume (and with it, pressure) rises, baroreceptors
in the heart and the large vessels of the neck and thorax (carotid
arteries and aorta) alert the cardiovascular centers in the brain
stem. Shortly aFer, sympathetic nervous system impulses to the
kidneys decline, allowing the afferent arterioles to dilate. As the
glomerular filtration rate rises, Na
1
output and water output
increase. Tis phenomenon, part of the baroreceptor reflex de-
scribed in Chapter 19 (pp. 706–707), reduces blood volume and
blood pressure.
Drops in systemic blood pressure lead to reflex constriction
of systemic arterioles including the afferent arterioles, which
reduces filtrate formation and urinary output and increases sys-
temic blood pressure
(Figure 26.10)
. Te baroreceptors pro-
vide information on the “fullness” or volume of the circulation
that is critical for maintaining cardiovascular homeostasis.
Regulation of Potassium Balance
Potassium, the chief intracellular cation, is required for normal
neuromuscular functioning as well as for several essential meta-
bolic activities. Even slight changes in K
1
concentration in the
EC± have profound and potentially life-threatening effects on
prods the adrenal cortex to release aldosterone, and also directly
increases Na
1
reabsorption by kidney tubules. In addition, it has
a number of other actions described in Chapter 19 (pp. 709–710)
that are all aimed at raising blood volume and blood pressure.
Homeostatic Imbalance
26.2
People with Addison’s disease (hypoaldosteronism) lose tre-
mendous amounts of NaCl and water to urine. Tey are per-
petually teetering on the brink of hypovolemia, but as long as
they ingest adequate amounts of salt and fluids, they can avoid
problems with Na
1
balance.
Influence of Atrial Natriuretic Peptide (ANP)
We can summarize the influence of
atrial natriuretic peptide
(ANP)
in one sentence: It reduces blood pressure and blood vol-
ume by inhibiting nearly all events that promote vasoconstriction
and Na
1
and water retention
(Figure 26.9)
. A hormone that is
Stimulates
Releases
Targets
Effects
Restores
K
+
concentration
in the ECF
Homeostatic plasma
levels of Na
+
and K
+
Body Na
+
content
triggers renin release,
increasing angiotensin II
Adrenal cortex
Kidney tubules
Na
+
reabsorption
K
+
secretion
Aldosterone
Figure 26.8
Mechanisms and consequences of aldosterone
release.
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