CHAPTER 7 • ORGAN BLOOD FLOW
155
If the perfusion pressure to an organ is
increased and decreased over a wide range of
pressures and the steady-state autoregulatory
flow response is measured, then the relation-
ship between steady-state flow and perfusion
pressure can be plotted as shown in the right
panel of Figure 7.3. There is a range of pres-
sures
(autoregulatory
range)
over
which
flow changes relatively little despite a large
change in perfusion pressure. The “flatness”
of the autoregulation curve varies considerably
among organs; the flatter the relationship, the
better the autoregulation. Coronary, cerebral,
and renal circulations show a high degree of
autoregulation, whereas skeletal muscle and
gastrointestinal circulations show only a mod-
erate degree of autoregulation. The cutaneous
circulation displays virtually no autoregulation.
Autoregulation has limits even in organs
that display a high degree of autoregulation.
When the perfusion pressure falls below 60
to 70 mm Hg in the cerebral and coronary
circulations, the resistance vessels become
maximally dilated and their ability to autoreg-
ulate is lost. Furthermore, at very high per-
fusion pressures (-170 mm Hg in Fig. 7.3),
the upper limit of the autoregulatory range
is reached and the vessels undergo no fur-
ther constriction with increases in perfusion
pressure; therefore, flow increases as pressure
increases. The autoregulatory response can
be modulated by neurohumoral influences
and disease states. For example, sympathetic
stimulation and chronic hypertension can
shift the cerebral autoregulatory range to the
right as described later in this chapter.
Autoregulation may involve both meta-
bolic and myogenic mechanisms. If the per-
fusion pressure to an organ is reduced, the
initial fall in blood flow leads to a fall in tis-
sue P02 and the accumulation of vasodila-
tor metabolites. These changes cause the
resistance vessels to dilate in an attempt to
restore normal flow. A reduction in perfusion
pressure may also be sensed by the smooth
muscle in resistance vessels, which responds
by relaxing (myogenic response), leading to
an increase in flow.
Under what conditions does autoregu-
lation occur, and why is it important? In
hypotension caused by blood loss, despite
baroreceptor reflexes that lead to constriction
of much of the systemic vasculature, blood
flow to the brain and myocardium will not
decline appreciably (unless the arterial pres-
sure falls below the autoregulatory range).
This is because of the strong capacity of these
organs to autoregulate and their ability to
escape
sympathetic
vasoconstrictor
influ-
ences. The autoregulatory response helps to
ensure that these critical organs have an ade-
quate blood flow and oxygen delivery even in
the presence of systemic hypotension.
Other situations occur in which systemic
arterial pressure does not change, but in
which autoregulation is very important nev-
ertheless. Autoregulation can occur when
a distributing artery to an organ (e.g., coro-
nary artery) becomes partially occluded. This
arterial stenosis increases resistance and the
pressure drop along the vessel length. This
reduces pressure in small distal arteries and
arterioles, which are the primary vessels for
regulating blood flow within an organ. These
resistance vessels dilate in response to the
reduced pressure and blood flow caused by
the upstream stenosis. This autoregulatory
response helps to maintain normal blood flow
in the presence of upstream stenosis, and it
is particularly important in organs such as
the brain and heart that depend on a steady
delivery of oxygen to maintain normal organ
function.
PROBLEM 7-1
An experiment was conducted using
an isolated perfused organ (e.g.,
intestinal segment) in which arterial
and venous pressures were controlled
while blood flow was measured. When
venous pressure was suddenly raised
from 0 to 15 mm Hg while arterial
pressure was maintained at 100 mm
Hg, flow decreased by 25%. Calculate
the percentage change that occurred
in vascular resistance in response to
venous pressure elevation. Discuss the
involvement of metabolic and myogenic
mechanisms in this response.
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