CHAPTER 7 • ORGAN BLOOD FLOW
173
capillary pressure. Dilation of the afferent
arteriole (panel A) increases distal pressures
(glomerular capillaries, efferent arteriole, and
peritubular capillaries), while increasing total
flow
(assuming
constant
aortic
pressure);
this causes increased glomerular filtration.
If the afferent arteriole constricts (panel B),
distal pressures, glomerular filtration, and
blood flow are reduced. If the efferent arteri-
ole dilates (panel C), this increases total flow
but reduces glomerular capillary pressure and
filtration, while increasing peritubular capil-
lary pressure. Efferent arteriole constriction
increases glomerular capillary pressure and
glomerular filtration while reducing flow and
peritubular capillary pressure (panel D).
REGULATION OF RENAL BLOOD FLOW
The renal circulation exhibits strong autoreg-
ulation between arterial pressures of about
80 to 180 mm Hg. Autoregulation of blood
flow is accompanied by autoregulation of glo-
merular filtration so that filtration remains
essentially unchanged over a wide range of
arterial pressures. For this to occur, glomeru-
lar capillary pressure must remain unchanged
when arterial pressure changes. This takes
place because the principal site for autoregula-
tion is the afferent arteriole. If arterial pressure
falls, the afferent arteriole dilates, which helps
to maintain the glomerular capillary pressure
and flow despite the fall in arterial pressure.
Two
mechanisms
have
been
proposed
to
explain
renal
autoregulation: myogenic
mechanisms and tubuloglomerular feedback.
Myogenic mechanisms were described earlier
in this chapter. Briefly, a reduction in afferent
arteriole pressure is sensed by the vascular
smooth muscle, which responds by relaxing;
an increase in pressure induces smooth mus-
cle contraction. The tubuloglomerular feed-
back mechanism is poorly understood, and
the actual mediators have not been identified.
It is believed, however, that changes in perfu-
sion pressure alter glomerular filtration and
therefore tubular flow and sodium delivery to
the macula densa of the juxtaglomerular appa-
ratus, which then signals the afferent arteriole
to constrict or dilate. The macula densa of
the juxtaglomerular apparatus is a group of
specialized cells of the distal tubule that lie
adjacent to the afferent arteriole as the distal
tubule loops up back toward the glomerulus.
These cells sense solute osmolarity, particu-
larly sodium chloride. Some investigators have
proposed that adenosine (which is a vasocon-
strictor in the kidney), locally produced angi-
otensin II (a vasoconstrictor), or vasodilators
such as nitric oxide, PGE2, and prostacyclin
are involved in tubuloglomerular feedback
and autoregulation. Locally produced angio-
tensin II strongly influences efferent arteriole
tone. Thus, inhibition of angiotensin II forma-
tion by an ACE inhibitor dilates the efferent
arteriole, which decreases glomerular capil-
lary pressure and reduces glomerular filtration
under some conditions (e.g., renal artery ste-
nosis). Drugs that inhibit prostaglandin and
prostacyclin
biosynthesis
(cyclooxygenase
inhibitors such as aspirin or ibuprofen) alter
renal hemodynamics and may impair renal
function, particularly with long-term use.
The renal circulation responds strongly
to
sympathetic
adrenergic
stimulation.
Under
normal
conditions,
relatively
little
sympathetic tone on the renal vasculature
occurs; however, with strenuous exercise or
in response to severe hemorrhage, increased
renal sympathetic nerve activity can virtually
shut down renal blood flow. Because renal
blood flow receives a relatively large fraction
of cardiac output and therefore contributes
significantly to systemic vascular resistance,
renal vasoconstriction can serve an important
role in maintaining arterial pressure under
these conditions; however, intense renal vaso-
constriction seriously impairs renal perfusion
and function, and it can lead to renal failure.
Pulmonary Circulation
Two separate circulations perfusing respiratory
structures exist: the pulmonary circulation,
which is derived from the pulmonary artery
and supplies blood flow to the alveoli for gas
exchange, and the bronchial circulation, which
is derived from the thoracic aorta and sup-
plies nutrient flow to the trachea and bronchial
structures. The pulmonary circulation receives
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