1002
UNIT 4
Maintenance of the Body
26
in the tubules is precisely tailored to need, and Na
1
is
never
secreted into the filtrate.
In contrast, the proximal tubules reabsorb about 60–80% of
the filtered K
1
, and the thick ascending limb of the nephron
Regulatory Site: The Collecting Duct
Like Na
1
balance, K
1
balance is maintained chiefly by renal
mechanisms. However, there are important differences in the
way this balance is achieved. Te amount of Na
1
reabsorbed
Stretch in afferent
arterioles
Angiotensinogen
(from liver)
Na
+
(and H
2
O)
reabsorption
Granular cells of kidneys
Renin
Posterior pituitary
Systemic arterioles
Angiotensin I
Angiotensin II
Systemic arterioles
Vasoconstriction
Aldosterone
Blood volume
Blood pressure
Distal kidney tubules
Adrenal cortex
Vasoconstriction
Peripheral resistance
(+)
(+)
(+)
(+)
Peripheral resistance
H
2
O reabsorption
Inhibits baroreceptors
in blood vessels
Sympathetic
nervous system
ADH (antidiuretic
hormone)
Collecting ducts
of kidneys
Filtrate NaCl concentration in
ascending limb of nephron loop
Causes
Causes
Causes
Causes
Results in
Secretes
Results in
Targets
Results in
Releases
Release
Catalyzes conversion
Converting enzyme (in lungs)
(+)
(+)
(+)
(+)
(+)
(+)
(+)
stimulates
Renin-angiotensin-aldosterone
mechanism
Neural regulation (sympathetic
nervous system effects)
ADH release and effects
Systemic
blood pressure/volume
Figure 26.10
Mechanisms regulating sodium and water balance help maintain blood
pressure homeostasis.
The thirst mechanism, activated by angiotensin II and other factors, is
not shown (see Figure 26.5).
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