Chapter 19
The Cardiovascular System: Blood Vessels
705
19
Role of the Cardiovascular Center
Several clusters of neurons in the medulla oblongata act to-
gether to integrate blood pressure control by altering cardiac
output and blood vessel diameter. Tis
cardiovascular center
consists of the
cardiac centers
(the cardioacceleratory and car-
dioinhibitory centers discussed in Chapter 18), and the
vaso-
motor center
that controls the diameter of blood vessels.
Te vasomotor center transmits impulses at a fairly steady
rate along sympathetic efferents called
vasomotor fibers
. Tese
fibers exit from the ±
1
through L
2
levels of the spinal cord and
innervate the smooth muscle of blood vessels, mainly arterioles.
As a result, the arterioles are almost always in a state of moder-
ate constriction, called
vasomotor tone
.
Te degree of vasomotor tone varies from organ to organ. Gen-
erally, arterioles of the skin and digestive viscera receive vasomotor
impulses more frequently and tend to be more strongly constricted
than those of skeletal muscles. Any increase in sympathetic activ-
ity produces generalized vasoconstriction and raises blood pres-
sure. Decreased sympathetic activity allows the vascular muscle to
relax somewhat and lowers blood pressure to basal levels.
Cardiovascular center activity is modified by inputs from (1)
baroreceptors (pressure-sensitive mechanoreceptors that respond
to changes in arterial pressure and stretch), (2) chemoreceptors (re-
ceptors that respond to changes in blood levels of carbon dioxide,
H
1
, and oxygen), and (3) higher brain centers. Let’s take a look.
Short-Term Regulation: Neural Controls
Neural controls alter both cardiac output and peripheral resis-
tance. Neural controls of peripheral resistance are directed at
two main goals:
Maintaining adequate MAP by altering blood vessel diam-
eter on a moment-to-moment basis. (Remember, very small
changes in blood vessel diameter cause substantial changes
in peripheral resistance, and hence in systemic blood pres-
sure.) Under conditions of low blood volume, all vessels ex-
cept those supplying the heart and brain are constricted to
allow as much blood as possible to flow to those two vital
organs.
Altering blood distribution to respond to specific demands
of various organs. For example, during exercise blood is
shunted temporarily from the digestive organs to the skeletal
muscles.
Most neural controls operate via reflex arcs involving
barore-
ceptors
and associated afferent fibers. Tese reflexes are inte-
grated in the cardiovascular center of the medulla, and their
output travels via autonomic fibers to the heart and vascular
smooth muscle. Occasionally, inputs from
chemoreceptors
and higher brain centers also influence the neural control
mechanism.
Venous return
Exercise
Contractility of cardiac muscle
Sympathetic activity
Parasympathetic activity
Epinephrine in blood
EDV
ESV
Stroke volume (SV)
Heart rate (HR)
Cardiac output (CO = SV
×
HR)
Activity of respiratory pump
(ventral body cavity pressure)
Activity of muscular pump
(skeletal muscles)
Sympathetic venoconstriction
BP activates cardiac centers in medulla
Initial stimulus
Result
Physiological response
Figure 19.8
Major factors enhancing cardiac output.
BP
5
blood pressure,
EDV
5
end diastolic volume, ESV
5
end systolic volume.
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