BIOC63H3 Lecture Notes - Lecture 7: Carotid Sinus, Baroreflex, Cardiac Output

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Published on 14 Oct 2011
Biological Sciences
The overall function of the cardiovascular system is to deliver blood to the tissues so that O2 and
nutrients can be provided and waste products carried away. Blood flow to the tissues is driven by
the difference in pressure between the arterial and venous sides of the circulation. Mean arterial
pressure (Pa) is the driving force for blood flow, and it must be maintained at a high, constant
level of approximately 100 mm Hg. Because of the parallel arrangement of arteries off the aorta,
the pressure in the major artery serving each organ is equal to Pa. (The blood flow to each organ
is then independently regulated by changing the resistance of its arterioles through local control
The mechanisms that help to maintain Pa at a constant value are discussed in this section. The
basis for this regulation can be appreciated by examining the equation for Pa:
Pa = Cardiac output x TPR
Pa = Mean arterial pressure (mm Hg)
Cardiac output = Cardiac output (ml/min)
TPR = Total peripheral resistance (mm Hg/ml/min)
Inspection of this equation reveals that Pa can be changed by altering the cardiac output (or any
of its parameters), altering the TPR (or any of its parameters), or altering both cardiac output and
The mechanisms responsible for maintaining a constant value for arterial pressure, closely
monitor Pa and compare it with the set-point value of approximately 100 mm Hg. If Pa, increases
above the set point or decreases below the set point, the cardiovascular system makes
adjustments in cardiac output, in TPR, or in both, attempting to return Pa, to the set-point value.
Pa is regulated by two major systems. The first system is neurally mediated and known as the
baroreceptor reflex (Figure 1). The baroreceptor reflex attempts to restore Pa to its set-point
value in a matter of seconds. The second system is hormonally mediated and includes the renin-
angiotensin-aldosterone system, which regulates Pa more slowly, primarily by its effect on
blood volume.
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Baroreceptor Reflex
The baroreceptor mechanisms are fast, neurally mediated reflexes that attempt to keep arterial
pressure constant via changes in the output of the sympathetic and parasympathetic nervous
systems to the heart and blood vessels. Pressure sensors, the baroreceptors, are located within
the walls of the carotid sinus and the aortic arch (Figure 1) and relay information about blood
pressure to cardiovascular vasomotor centers in the brain stem. The vasomotor centers, in turn,
coordinate a change in output of the autonomic nervous system to effect the desired change in Pa.
Thus, the reflex arc consists of sensors for blood pressure; afferent neurons, which carry the
information to the brain stem; brain stem centers, which process the information and coordinate
an appropriate response; and efferent neurons, which direct changes in the heart and blood
vessels (Figure2)
Figure 1
The baroreceptors are located in the walls of the carotid sinus, where the common carotid artery
bifurcates into the internal and external carotid arteries, and in the aortic arch (Figure 1). The
carotid sinus baroreceptors are responsive to increases or decreases in arterial pressure, whereas
the aortic arch baroreceptors are primarily responsive to increases in arterial pressure.
The baroreceptors are mechanoreceptors, which are sensitive to pressure or stretch. Thus,
changes in arterial pressure cause more or less stretch on the mechanoreceptors, resulting in a
change in their membrane potential. Such a change in membrane potential is a receptor potential,
which increases or decreases the likelihood that action potentials will be fired in the afferent
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nerves that travel from the baroreceptors to the brain stem. (If the receptor potential is
depolarizing, then action potential frequency increases; if the receptor potential is
hyperpolarizing, then action potential frequency decreases.)
Figure 2
Increases in arterial pressure cause increased stretch on the baroreceptors and increased firing
rate in the afferent nerves. Decreases in arterial pressure cause decreased stretch on the
baroreceptors and decreased firing rate in the afferent nerves.
Although the baroreceptors are sensitive to the absolute level of pressure, they are even more
sensitive to changes in pressure and the rate of change of pressure. The strongest stimulus for
the baroreceptors is a rapid change in arterial pressure!
The sensitivity of the baroreceptors can be altered by disease. For example, in chronic
hypertension (elevated blood pressure), the baroreceptors do not "see" the elevated blood
pressure as abnormal. In such cases, the hypertension will be maintained, rather than corrected,
by the baroreceptor reflex. The mechanism of this defect is either decreased sensitivity of the
baroreceptors to increases in arterial pressure or an increase in the blood pressure set point of the
brain stem centers.
Information from the carotid sinus baroreceptors is carried to the brain stem on the carotid sinus
nerve, which joins the glossopharyngeal nerve (cranial nerve [CN] IX). Information from the
aortic arch baroreceptors is carried to the brain stem on the vagus nerve (CN X).
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