412
Chapter 12
SECTION D SUMMARY
I. Mean arterial pressure, the primary regulated variable in the
cardiovascular system, equals the product of cardiac output
and total peripheral resistance.
II. The factors that determine cardiac output and total peripheral
resistance are summarized in Figure 12–51.
Baroreceptor Refl
exes
I. The primary baroreceptors are the arterial baroreceptors,
including the two carotid sinuses and the aortic arch.
Nonarterial baroreceptors are located in the systemic veins,
pulmonary vessels, and walls of the heart.
II. The fi ring rates of the arterial baroreceptors are proportional
to mean arterial pressure and to pulse pressure.
III. An increase in fi ring of the arterial baroreceptors due to
an increase in pressure causes, by way of the medullary
cardiovascular center, an increase in parasympathetic outfl ow
to the heart and a decrease in sympathetic outfl ow to the
heart, arterioles, and veins. The result is a decrease in cardiac
output and total peripheral resistance and, thus, a decrease in
mean arterial pressure. The opposite occurs when the initial
change is a decrease in arterial pressure.
Blood Volume and Long-Term Regulation
of Arterial Pressure
I. The baroreceptor refl exes are short-term regulators of arterial
pressure but adapt to a maintained change in pressure.
II. The most important long-term regulator of arterial pressure is
the blood volume.
Other Cardiovascular Refl
exes and Responses
I. Blood pressure can be infl uenced by many factors other than
baroreceptors, including arterial blood gas concentrations,
pain, emotions and sexual activity.
Additional Clinical Examples
I. Cushing’s phenomenon is a clinical condition in which
traumatic head injury leads to increased intracranial pressure,
decreased brain blood fl ow, and a large increase in arterial
blood pressure.
SECTION D KEY TERMS
aortic arch baroreceptor
408
arterial baroreceptors
408
baroreceptors
408
carotid sinus
408
medullary cardiovascular
center
409
total peripheral resistance
(TPR)
405
SECTION D CLINICAL TERMS
Cushing’s phenomenon
412
SECTION D REVIEW QUESTIONS
1. Write the equation relating mean arterial pressure to cardiac
output and total peripheral resistance.
2. What variable accounts for the fact that mean pulmonary
arterial pressure is lower than mean systemic arterial
pressure?
3. Draw a fl ow diagram illustrating the factors that determine
mean arterial pressure.
4. Identify the receptors, afferent pathways, integrating center,
efferent pathways, and effectors in the arterial baroreceptor
refl ex.
5. When the arterial baroreceptors decrease or increase their
rate of fi ring, what changes in autonomic outfl ow and
cardiovascular function occur?
6. Describe the role of blood volume in the long-term regulation
of arterial pressure.
Elevated Intracranial Pressure
A number of different circumstances can cause elevated
pressure in the brain, including the presence of a rapidly
growing cancerous tumor or a traumatic head injury that
triggers internal hemorrhage or edema. What distinguishes
these situations from similar problems elsewhere in the body
is the fact that the enclosed bony cranium does not allow
physical swelling toward the outside; consequently, pressure
is directed inward. This inward pressure exerts a collapsing
force on intracranial vasculature, and the reduction in radius
greatly increases the resistance to blood fl ow (recall that
resistance increases as the fourth power of a decrease in
radius). Blood fl ow is thus reduced below the level needed
to satisfy metabolic requirements, and brain oxygen levels
ADDITIONAL CLINICAL EXAMPLES
fall, while carbon dioxide and other metabolic wastes
increase. Accumulated metabolites in the brain interstitial
fl uid powerfully stimulate sympathetic neurons controlling
systemic arterioles, resulting in a large increase in TPR and,
consequently, a major elevation in MAP (MAP = CO
×
TPR).
In principle, this elevated systemic pressure is adaptive, in
that it can overcome the collapsing pressures and force blood
to fl ow through the brain once again. In practice, removal
of the tumor or accumulated fl uid is the only way to restore
brain blood fl ow at a normal mean arterial pressure. This
association of a rise in intracranial pressure with dramatically
increased arterial blood pressure is known as
Cushing’s
phenomenon
(not to be confused with Cushing’s syndrome or
disease, which are described in Chapter 11).
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