The many factors that can contribute to increasing blood
pressure are summarized in Figure 11.19. Addressing
many of these factors can result in a decrease in blood
pressure. Now that you understand the factors that can
influence blood pressure, let’s look at how it is regulated.
Blood Pressure Is Closely Regulated
To keep blood flowing at a constant rate, blood pressure
must be kept at a steady level. This is especially impor-
tant for the blood flowing against gravity to the brain. Your
body has a neural mechanism and several hormonal mech-
anisms that regulate blood pressure. The neural mecha-
nism involves the cardiac and vasomotor centers in the
medulla oblongata. These centers receive and integrate
input from several areas:
Baroreceptors
.
Located
in
the
carotid
arteries,
right
atrium,
and
aortic
arch;
sense the stretch of these arteries due to
the pressure of blood in the vessels and
send a steady stream of nerve impulses to
the cardiac and vasomotor centers. The
frequency
of nerve
impulses
is
directly
proportional to the pressure in these vessels.
Proprioceptors.
Located
in
limbs
and joints;
sense
movements (such as those related to exercise).
Chem oreceptors
. Located in the carotid arteries and
aortic arch; sense chemicals in blood, such as the pH of
blood and its levels of oxygen and carbon dioxide.
H ig h e r
brain
centers.
Cortex,
limbic
system,
hypo-
thalamus; also integrate information from various
sensory neurons and send commands to direct the
cardiac center to alter heart rate and/or contractility
and the vasomotor center to adjust vasoconstriction or
vasodilation.
The cardiac and vasomotor centers integrate this informa-
tion and send appropriate signals through the vagus nerve
(parasympathetic nerve), the cardiac accelerator nerve
to the heart (sympathetic nerve), or vasomotor nerves to
the blood vessels (sympathetic nerves). Parasympathetic
stimulation of the heart decreases heart rate, while sym-
pathetic outflow causes increases in both heart rate and
contractility. Sympathetic stimulation of the blood vessels
causes vasoconstriction. So, parasympathetic
stimulation tends to decrease cardiac output
and blood pressure, and sympathetic stimula-
tion has the opposite effect.
The neural regulatory system is known as
the baroreceptor reflex. For example, when
you stand up, gravity pulls blood away from the
head and causes a drop in blood pressure. The baroreceptor
reflex kicks in and restores blood pressure by increasing the
heart rate, contractility, and blood pressure within a heart-
baroreceptors
(bar'-o-re-SEP-tors)
Neurons capable of re-
sponding to changes
in blood, air, or fluid
pressure.
Sum m ary o f facto rs th a t increase b lo o d p ressu re • Figure 11.19
Changes within green boxes increase cardiac output; changes within blue boxes increase
systemic vascular resistance. Opposite changes lead to decreased blood pressure.
Increased blood
Skeletal muscle pump
Respiratory
Venoconstriction
volume
pump
of veins
t 1
>
f
f
Decreased
Increased sympathetic
Increased
Increased number
Increased
parasympathetic
impulses and hormones
venous
of red blood cells,
body size, as
impulses
from adrenal medulla
return
as in polycythemia
in obesity
Increased heart
Increased stroke
Increased
Increased total
Decreased blood vessel
rate (HR)
volume (SV)
blood viscosity
blood vessel length
radius (vasoconstriction)
\
___
__
J
V
______
I
Increased cardiac
Increased vascular
output (CO)
resistance
___
J
Increased mean arterial
pressure (MAP)
334 CHAPTER 11
The Cardiovascular System: Heart, Blood Vessels, and Circulation
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