154
CARDIOVASCULAR PHYSIOLOGY CONCEPTS
vascular transmural pressures (see Fig. 5.16)
and thereby have substantial effects on pul-
monary vascular resistance. Excessive disten-
sion of the gastrointestinal tract, as occurs
during intestinal obstruction, can increase
vascular resistance in the wall of the intestine
to a point where tissues become ischemic.
Blood vessels in organs such as the brain or
kidneys, which are surrounded by a rigid cra-
nium or capsule, are particularly susceptible
to increases in extravascular pressure that
occur with edema, vascular hemorrhage (e.g.,
cerebral stroke), or the growth of a tumor.
Autoregulation of Blood Flow
Autoregulation is the intrinsic ability of an organ
to maintain a constant blood flow despite changes
in perfusion pressure
.
For example, if perfusion
pressure is decreased to an organ by partial
occlusion of the artery supplying the organ,
blood flow will initially fall, then return toward
normal levels over the next few minutes. This
autoregulatory response occurs in isolated,
perfused organs, which are not subject to neu-
ral or humoral influences. Therefore, it is a
local or intrinsic response of the organ.
When perfusion pressure (arterial - venous
pressure; PA
- PV) initially decreases, blood flow
(F) falls because of the following relationship
between pressure, flow, and resistance (R):
f _ (Pa-Pv)
R
If resistance remains unchanged, the reduction
in flow will be proportionate to the reduction
in perfusion pressure; however, in most organs
of the body, resistance does not remain constant
when perfusion pressure is decreased. The reduc-
tions in flow and perfusion pressure are thought
to activate metabolic and myogenic mechanisms
that cause arteriolar vasodilation and a fall in
resistance (R). As resistance decreases, blood
flow increases despite the presence of a reduced
perfusion pressure. This autoregulatory response
is shown in the left panel of Figure 7.3. For exam-
ple, if perfusion pressure is reduced from 100 to
70 mm Hg, it causes flow to decrease initially by
approximately 30%. Over the next few minutes,
however, flow begins to increase back toward
control as the organ blood flow is autoregulated
(red lines). Blood flow increases because vascu-
lar resistance falls as the resistance vessels dilate.
■ FIGURE 7.3 Autoregulation of blood flow. The le ft panel shows that decreasing perfusion pressure from
100 to 70 mm Hg at point
A
results in a transient decrease in flow. If no autoregulation occurs, resistance
remains unchanged and flow remains decreased. W ith autoregulation
(red line),
the initial fall in pressure
and flow are followed by a decrease in vascular resistance, which causes flow to increase to a new steady-
state level despite the reduced perfusion pressure (point B). The rig ht panel shows steady-state, autoreg-
ulatory flows plotted against different perfusion pressures. Points
A
and
B
represent the control flow and
autoregulatory steady-state flow, respectively, from the le ft panel. The autoregulatory range is the range
of pressures over which flow shows little change. Below or above the autoregulatory range, flow changes
are approxim ately proportional to the changes in perfusion pressure. The autoregulatory range as well as
the flatness of the autoregulatory response curve varies among organs.
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