408
Chapter 12
information is new—all of it was presented in previous fi
gures.
A change in only a single variable will produce a change in
mean systemic arterial pressure by altering either cardiac out-
put or total peripheral resistance. For example,
Figure 12–52
illustrates how the decrease in blood volume during hemor-
rhage leads to a decrease in mean arterial pressure.
Conversely, any deviation in arterial pressure, such as
that occurring during hemorrhage, will elicit homeostatic
refl
exes so that cardiac output and/or total peripheral resis-
tance will change in the direction required to minimize the
initial change in arterial pressure.
In the short term—seconds to hours—these homeo-
static adjustments to mean arterial pressure are brought about
by refl exes called the baroreceptor refl exes. They utilize mainly
changes in the activity of the autonomic nerves supplying the
heart and blood vessels, as well as changes in the secretion of the
hormones (epinephrine, angiotensin II, and vasopressin) that
infl uence these structures. Over longer time spans, the barore-
ceptor refl
exes become less important, and factors controlling
blood volume play a dominant role in determining blood pres-
sure. The next two sections describe these phenomena.
Baroreceptor Refl exes
Arterial Baroreceptors
It is only logical that the refl
exes that homeostatically regulate
arterial pressure originate primarily with arterial receptors
that respond to changes in pressure. High in the neck, each
of the two major vessels supplying the head, the common
carotid arteries, divides into two smaller arteries (
Figure
12–53
). At this division, the wall of the artery is thinner
than usual and contains a large number of branching, vine-
like nerve endings. This portion of the artery is called the
carotid sinus
(the term
sinus
denotes a recess, space, or
dilated channel). Its nerve endings are highly sensitive to
stretch or distortion. The degree of wall stretching is directly
related to the pressure within the artery. Thus, the carotid
sinuses serve as pressure receptors, or
baroreceptors.
An
area functionally similar to the carotid sinuses is found in the
arch of the aorta and is termed the
aortic arch barorecep-
tor.
The two carotid sinuses and the aortic arch barorecep-
tor constitute the
arterial baroreceptors.
Afferent neurons
travel from them to the brainstem and provide input to the
neurons of cardiovascular control centers there.
Action potentials recorded in single afferent fi bers from
the carotid sinus demonstrate the pattern of baroreceptor
response (
Figure 12–54
). In this experiment, the pressure in
the carotid sinus is artifi cially controlled so that the pressure
is steady, and not pulsatile (i.e., not varying as usual between
Figure 12–52
Sequence of events by which a decrease in blood volume leads to a
decrease in mean arterial pressure.
Cardiac muscle
Stroke volume
Cardiac output
Ventricular end–diastolic
volume
Atrial pressure
Venous pressure
Venous return
Blood
volume
Arterial blood pressure
Hemorrhage
(blood loss)
Internal carotid
artery
Afferent neurons
to brainstem
cardiovascular
control centers
Carotid sinus
baroreceptor
Common carotid
arteries
Aortic arch
baroreceptor
Figure 12–53
Locations of arterial baroreceptors.
Figure 12–53
physiological
inquiry
When you fi rst stand up after getting out of bed, how does the
pressure detected by these baroreceptors change?
Answer can be found at end of chapter.
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