M echanism s th a t m aintain b lo o d p ressu re and
increase b lo o d flow during exercise • Figure 11.2 0
Several factors can adjust blood pressure to maintain homoeostasis.
Muscles
Heart
Working muscles require increased blood flow to meet oxygen
demands.
Your muscles, brain, glands, heart, sympathetic nervous system, and
systemic circulation work together to deliver the energy needed for
exercise and to maintain blood pressure.
Brain
Sympathetic
nervous system
Glands
Systemic
circulation
Working muscles
secrete metabolites
that increase local
vasodilation, which
decreases blood
pressure.
Baroreceptors in the
heart sense the
decrease in blood
pressure caused by
vasodilation and
send a message to
the brain.
Higher brain centers
stimulate the ANS,
including the CV
center.
Increased sympathetic
outflow to the cardiac
accelerator nerve
increases heart rate and
contractility. Sympathetic
outflow increases to the
adrenal medullae and to
vasomotor nerves
supplying the blood
vessels.
In response to
increased
sympathetic
outflow to the
adrenal medulla,
the adrenal
gland secretes
epinephrine,
which causes
vasoconstriction.
RESULTS:
Cardiac output is increased in response to increased heart rate and increased blood volume.
Blood flow to working muscles increases dramatically.
Blood pressure remains the same or is elevated slightly.
Vasoconstriction
caused by increased
vasomotor nerve
activity and
increased epineph-
rine secretion
results in increased
blood volume in
veins and venules,
increased resistance
in arteries and
arterioles, and the
diversion of blood
flow from the
digestive tract and
skin to working
muscles.
Several hormones can be used to help maintain blood
pressure, including epinephrine, norepinephrine, angio-
tensin II (AII), antidiuretic hormone (ADH), aldosterone,
and atrial natriuretic peptide (ANP). Epinephrine, nor-
epinephrine, and AII cause vasoconstriction and increase
blood pressure. Aldosterone acts on the kidney to reabsorb
sodium and water, thereby increasing the blood volume,
cardiac output, and blood pressure. ADH impacts the kid-
neys to directly increase water reabsorption, leading to in-
creased blood volume. In response to high blood pressure,
the heart itself secretes ANP, which causes vasodilation,
reduces blood volume, and lowers blood pressure.
To see how blood pressure is maintained under physi-
ological stress, consider exercise (Figure 11.20). When
you exercise, the working muscles use energy and gen-
erate metabolites that dilate the local arterioles (auto-
regulation). Local vasodilation increases tissue blood
flow and decreases blood pressure, which causes the
baroreceptor reflex to kick in to restore blood pressure.
At the same time, the proprioceptors and higher brain
centers signal the cardiac and vasomotor centers to in-
crease sympathetic outflow to the heart, blood vessels,
and adrenal medulla (which secretes epinephrine). All
of these systems combine to increase heart rate, contrac-
336 CHAPTER 11
The Cardiovascular System: Heart, Blood Vessels, and Circulation
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