The Urinary System
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-
). Once the mean arterial pressure
drops below 80 mm Hg, autoregulation ceases and extrinsic
controls take over.
Extrinsic Controls: Neural and Hormonal Mechanisms
purpose of the extrinsic controls regulating the GFR is to main-
tain systemic blood pressure (Figure 25.12, right side).
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
Granular cells of
complex of kidney
in blood vessels of
Macula densa cells
complex of kidney
directly regulate GFR despite
moderate changes in blood pressure (between 80
and 180 mm Hg mean arterial pressure).
indirectly regulate GFR
by maintaining systemic blood pressure, which
drives filtration in the kidneys.
by kidney tubules;
Blood pressure in
Filtrate flow and
NaCl in ascending
limb of nephron loop
Release of vasoactive
SYSTEMIC BLOOD PRESSURE
Physiological mechanisms regulating glomerular ﬁltration rate (GFR) in the
(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.)