Cardiovascular Physiology
409
systolic and diastolic pressure). At a particular steady pressure,
for example, 100 mmHg, there is a certain rate of discharge
by the neuron. This rate can be increased by raising the arte-
rial pressure, or it can be decreased by lowering the pressure.
Thus, the rate of discharge of the carotid sinus is directly pro-
portional to the mean arterial pressure.
If the experiment is repeated using the same mean pres-
sures as before but allowing pressure pulsations, it is found
that at any given mean pressure, the larger the pulse pressure,
the faster the rate of fi ring by the carotid sinus. This respon-
siveness to pulse pressure adds a further element of infor-
mation to blood pressure regulation, because small changes
in factors such as blood volume may cause changes in arte-
rial pulse pressure with little or no change in mean arterial
pressure.
The Medullary Cardiovascular Center
The primary integrating center for the baroreceptor refl exes
is a diffuse network of highly interconnected neurons called
the
medullary cardiovascular center,
located in the medulla
oblongata. The neurons in this center receive input from
the various baroreceptors. This input determines the action
potential frequency from the cardiovascular center along neu-
ral pathways that terminate upon the cell bodies and dendrites
of the vagus (parasympathetic) neurons to the heart and the
sympathetic neurons to the heart, arterioles, and veins. When
the arterial baroreceptors increase their rate of discharge, the
result is a decrease in sympathetic outfl
ow to the heart, arteri-
oles, and veins, and an increase in parasympathetic outfl ow to
the heart (
Figure 12–55
). A decrease in baroreceptor fi ring
rate results in the opposite pattern.
As parts of the baroreceptor refl exes, angiotensin II
generation and vasopressin secretion are also altered to help
restore blood pressure. Decreased arterial pressure elicits
increased plasma concentrations of both these hormones,
which raise arterial pressure by constricting arterioles. Chapter
14 will further describe the roles of angiotensin II and vaso-
pressin in terms of their effects on salt and water balance via
the kidneys.
Operation of the Arterial Baroreceptor Refl ex
Our description of the arterial baroreceptor refl ex is now com-
plete. If arterial pressure decreases, as during a hemorrhage
(
Figure 12–56
), the discharge rate of the arterial baroreceptors
also decreases. Fewer impulses travel up the afferent nerves to the
medullary cardiovascular center, and this induces (1) increased
heart rate because of increased sympathetic activity to the heart
and decreased parasympathetic activity, (2) increased ventricu-
lar contractility because of increased sympathetic activity to the
ventricular myocardium, (3) arteriolar constriction because of
increased sympathetic activity to the arterioles (and increased
plasma concentrations of angiotensin II and vasopressin), and
(4) increased venous constriction because of increased sympa-
thetic activity to the veins. The net result is an increased car-
diac output (increased heart rate and stroke volume), increased
total peripheral resistance (arteriolar constriction), and return
of blood pressure toward normal.
Conversely, an increase in arterial blood pressure for any
reason causes increased fi ring of the arterial baroreceptors,
which refl exly induces a compensatory decrease in cardiac out-
put and total peripheral resistance.
Having emphasized the great importance of the arterial
baroreceptor refl ex, we must now add an equally important
qualifi cation. The baroreceptor refl ex functions primarily as
a short-term regulator of arterial blood pressure. It is acti-
vated instantly by any blood pressure change and functions
to restore blood pressure rapidly toward normal. Yet, if arte-
rial pressure deviates from its normal set point for more than
a few days, the arterial baroreceptors adapt to this new pres-
sure; that is, they have a decreased frequency of action poten-
tial fi ring at any given pressure. Thus, in patients who have
chronically elevated blood pressure, the arterial baroreceptors
continue to oppose minute-to-minute changes in blood pres-
sure, but at the higher set point.
0
80
120
160
Normal
resting value
Baroreceptor action
potential frequency
40
Mean arterial pressure (mmHg)
Figure 12–54
Effect of changing mean arterial pressure (MAP) on the fi ring of
action potentials by afferent neurons from the carotid sinus. This
experiment is done by pumping blood in a nonpulsatile manner
through an isolated carotid sinus so as to be able to set the pressure
inside it at any value desired.
Sympathetic
outflow to
heart, arterioles,
veins
Parasympathetic
outflow to heart
Arterial baroreceptors
Firing
Reflex via medullary
cardiovascular center
Arterial pressure
Figure 12–55
Neural components of the arterial baroreceptor refl ex. If the initial
change were a decrease in arterial pressure, all the arrows in the
boxes would be reversed.
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