132
CARDIOVASCULAR PHYSIOLOGY CONCEPTS
states. In hypertension, for example, the curve
shifts to the right, thereby reducing the firing
rate at any given mean arterial pressure. This
resetting of the baroreceptor response can
occur at the level of the receptors themselves
as well as in the brainstem. In arteriosclerosis,
the carotid arteries at the region of the carotid
sinus become less compliant, and therefore
they stretch less in response to changes in
arterial blood pressure— this decreases their
sensitivity. During exercise, medullary and
hypothalamic control centers can modulate
autonomic efferent responses at a given level
of baroreceptor firing, thereby resetting arte-
rial pressure to a higher level.
Receptors located within the aortic arch
function similarly to carotid sinus receptors;
however, they have a higher threshold pres-
sure for firing and are less sensitive than the
carotid sinus receptors. Therefore, the aortic
arch baroreceptors serve as secondary baro-
receptors, with the carotid sinus receptors
normally being the dominant arterial barore-
ceptor.
To
understand
how
the
baroreceptor
reflex
operates,
consider
the
events
that
occur in response to a decrease in arterial
pressure (mean, pulse, or both) when a per-
son suddenly stands up (Fig. 6.8). When
upright posture is suddenly assumed from
the supine position, gravity causes venous
Decreased
Arterial
-----
Pressure
+
+
CO
SVR
+______+
i Parasympathetic
■ FIGURE 6.8 Baroreceptor feedback loop.
A sudden decrease in arterial pressure, as occurs
when a person suddenly stands up from a supine
position, decreases baroreceptor firing, activating
sym pathetic nerves and inhibiting parasympathetic
(vagal) nerves. This change in autonomic balance
increases (+) cardiac output (CO) and systemic vas-
cular resistance
(SVR),
which helps to restore nor-
mal arterial pressure.
CNS,
central nervous system.
T Sympathetic
CNS
Decreased
Receptor
Firing
blood pooling below the heart, particularly
in the legs (see Chapter 5). This decreases
venous
return,
central
venous
pressure,
and ventricular preload, leading to a fall
in cardiac output and arterial blood pres-
sure. Decreased stretching of baroreceptors
results in decreased baroreceptor firing and
decreased NTS activity. Nuclei within the
RVLM respond by increasing sympathetic
outflow, which increases systemic vascular
resistance
(vasoconstriction)
and
cardiac
output (increased heart rate and inotropy).
Decreased vagal outflow from the medulla
contributes to the elevation in heart rate.
Note that
baroreceptor firing normally exerts
a tonic inhibitory influence on sympathetic outflow
from the medulla
. Therefore, hypotension and
decreased baroreceptor firing disinhibits sym-
pathetic outflow (i.e., it increases sympathetic
activity) from the medulla. The combined
effects on systemic vascular resistance and
cardiac output increases arterial blood pres-
sure back toward its set point.
The carotid sinus reflex can be activated
by rubbing the neck over the carotid sinus
(i.e., carotid sinus massage). This mechani-
cal stimulation of the receptors increases their
firing, which leads to decreased sympathetic
and increased parasympathetic outflow from
the medulla. This action is sometimes used to
abort certain types of arrhythmias by activat-
ing the vagus efferents to the heart.
Another example of the operation of the
baroreceptor reflex is when a Valsalva maneu-
ver is performed, which is sometimes used to
assess autonomic reflex control of cardiovas-
cular function in humans. It is performed by
having the subject conduct a maximal, forced
expiration against a closed glottis and main-
taining this for at least 10 seconds. Contrac-
tion of the thoracic cage compresses the lungs
and causes a large increase in intrapleural
pressure
(the
pressure measured between
the lining of the thorax and the lungs—see
Fig.
5.16), which compresses the vessels
within
the
thoracic.
Aortic
compression
results in a transient rise in aortic pressure
(Phase I of Fig. 6.9). This results in a reflex
bradycardia caused by baroreceptor activa-
tion. Because the thoracic vena cava also
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