Vascular
Biology,
Structure and Function
39
than when it is steady and that the carotid sinus baroreceptors are most
sensitive to frequencies
of
pressure oscillation between
1
and
10
Hz.
An increase in blood pressure sensed by baroreceptors is signaled to
the medullary cardiovascular center, which in turn, through autonomic
motor neurons, initiates a reflex reduction in both cardiac output and
peripheral vascular resistance. The reduction in cardiac output results
from both
a
decreased heart rate and
a
reduced force
of
cardiac
contraction results in
a
decrease in arterial blood pressure. Thus the
baroreceptor reflex of the carotid sinus
is
a negative feedback loop that
tends to stabilize arterial blood pressure at a particular set point. This set
point concept has been instrumental in understanding many physiological
control mechanisms.
2.4.3
Arterial Chemoreceptors
Chemoreceptors are located in the carotid and aortic bodies. These
chemoreceptors respond with an increase in discharge frequency to an
increase in C02 or to decreases in
02
and reduced pH of the blood
perfusing the carotid and aortic bodies. Because C02 and
02
are
intimately related to the ventilation-perfusion process, chemoreceptors
are particularly important in regulating ventilation.
An increase in discharge frequency of the chemorecptor results in
peripheral
vasoconstriction
and
a
slowing of the
heart
rate.
Vasoconstriction can result in an increase in blood pressure, which in
turn can stimulate the baroreceptor to cause a reflex decrease in blood
pressure. Thus, chmoreceptors have a direct effect on heart rate and an
indirect effect on blood pressure.
Smooth muscle can exert influence on large vessels such
as
the aorta.
Its activity in smaller arteries is greater, because of the increased wall
thickness to radius ratio. With varied vasoactivity, arterial lumen can be
modulated to regulate perfusion. Fig.
2.4.3
illustrates this. Considerable
variations in the constituent wall components, collagen, elastin and
smooth muscle can be observed.
Geometric change, such as the
increased wall thickness to radius ratio is clearly visible.
