Chapter 16
The Endocrine System
613
16
In this discussion we are focusing on the major roles of blood-
borne aldosterone produced by the adrenal cortex, but aldosterone
is also secreted by cardiovascular organs. Tere it is a paracrine
and plays a completely different role in cardiac regulation.
Decreasing blood volume and blood pressure, and ris-
ing blood levels of K
1
, stimulate aldosterone secretion. Te
reverse conditions inhibit its secretion. Four mechanisms
regulate aldosterone secretion, but two of them—the renin-
angiotensin-aldosterone mechanism and plasma concentrations
of potassium—are by far the most important (
Figure 16.15
and
±able 16.3).
The Renin-Angiotensin-Aldosterone Mechanism
Te renin-
angiotensin-aldosterone mechanism (re
9
nin an
0
je-o-ten
9
sin)
influences both blood volume and blood pressure by regulating
the release of aldosterone and therefore Na
1
and water reab-
sorption by the kidneys.
(al-dos
9
ter-ōn), the most potent mineralocorticoid. Aldosterone
accounts for more than 95% of the mineralocorticoids produced.
Aldosterone reduces excretion of Na
1
from the body. Its pri-
mary target is the distal parts of the kidney tubules, where it
stimulates Na
1
reabsorption and water retention accompanied
by elimination of K
1
and, in some instances, alterations in the
acid-base balance of the blood (by H
1
excretion). Aldosterone
also enhances Na
1
reabsorption from perspiration, saliva, and
gastric juice.
Aldosterone’s regulatory effects are brief (lasting approxi-
mately 20 minutes), allowing plasma electrolyte balance to be
precisely controlled and continuously modified. Te mecha-
nism of aldosterone activity involves the synthesis and activa-
tion of proteins required for Na
1
transport such as Na
1
-K
1
A±Pase, the pump that exchanges Na
1
for K
1
.
Primary regulators
Other factors
Blood volume
and/or blood
pressure
Angiotensin II
Blood pressure
and/or blood
volume
K
+
in
blood
Direct
stimulating
effect
Renin
Initiates
cascade
that
produces
Kidney
Hypo-
thalamus
Heart
CRH
Anterior
pituitary
Zona glomerulosa
of adrenal cortex
Enhanced
secretion
of aldosterone
Targets
kidney tubules
Absorption of Na
+
and
water; increased K
+
excretion
Blood volume
and/or blood pressure
Inhibitory
effect
Stress
ACTH
Atrial natriuretic
peptide (ANP)
Figure 16.15
Major mechanisms controlling aldosterone release from the
adrenal cortex.
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