Chapter 25
The Urinary System
967
25
to the detriment of the kidneys. When the volume of the
extracellular fluid is normal and the sympathetic nervous
system is at rest, the renal blood vessels are dilated and
renal autoregulation mechanisms prevail. However, when
the extracellular fluid volume is extremely low (as in hypo-
volemic shock during severe hemorrhage) it is necessary
to shunt blood to vital organs and neural controls may
override autoregulatory mechanisms. Tis could reduce
renal blood flow to the point of damaging the kidneys.
When blood pressure falls, norepinephrine released
by sympathetic nerve fibers (and epinephrine released by
the adrenal medulla) causes vascular smooth muscle to
do not cause large changes in water and solute excretion.
However, the intrinsic controls cannot handle extremely low
systemic blood pressure, such as might result from serious hem-
orrhage (
hypovolemic shock
). Once the mean arterial pressure
drops below 80 mm Hg, autoregulation ceases and extrinsic
controls take over.
Extrinsic Controls: Neural and Hormonal Mechanisms
Te
purpose of the extrinsic controls regulating the GFR is to main-
tain systemic blood pressure (Figure 25.12, right side).
1.
Sympathetic nervous system controls.
Neural renal con-
trols serve the needs of the body as a whole—sometimes
Stretch of smooth
muscle in walls of
afferent arterioles
Granular cells of
juxtaglomerular
complex of kidney
Inhibits baroreceptors
in blood vessels of
systemic circulation
Sympathetic
nervous system
Macula densa cells
of juxtaglomerular
complex of kidney
Intrinsic mechanisms
directly regulate GFR despite
moderate changes in blood pressure (between 80
and 180 mm Hg mean arterial pressure).
Extrinsic mechanisms
indirectly regulate GFR
by maintaining systemic blood pressure, which
drives filtration in the kidneys.
Angiotensin II
Aldosterone
secretion by
adrenal cortex
Na
+
reabsorption
by kidney tubules;
water follows
Blood volume
Systemic
blood pressure
Vasoconstriction of
systemic arterioles;
peripheral resistance
Vasodilation of
afferent arterioles
Blood pressure in
afferent arterioles;
GFR
Myogenic mechanism
of autoregulation
Tubuloglomerular
mechanism of
autoregulation
Hormonal (renin-angiotensin-aldosterone)
mechanism
Neural controls
Renin
Filtrate flow and
NaCl in ascending
limb of nephron loop
Vasodilation of
afferent arterioles
GFR
GFR
Targets
Release of vasoactive
chemicals inhibited
Release
Catalyzes cascade
resulting in
formation of
SYSTEMIC BLOOD PRESSURE
Figure 25.12
Physiological mechanisms regulating glomerular filtration rate (GFR) in the
kidneys.
(Note that while the extrinsic controls are aimed at maintaining blood pressure, they also
maintain GFR since bringing blood pressure back up allows the kidneys to maintain GFR.)
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