400
U N I T 5
Circulatory Function
sympathetic-mediated acceleration of heart rate and
blood vessel tone, are called the
vasomotor center
. The
third, which controls parasympathetic-mediated slow-
ing of heart rate, is called the
cardioinhibitory center
.
These brain stem centers receive information from many
areas of the nervous system, including the hypothala-
mus. The arterial baroreceptors and chemoreceptors
provide the medullary cardiovascular center with con-
tinuous information regarding changes in blood pres-
sure (see Chapter 18).
The sympathetic nervous system serves as the final
common pathway for controlling the smooth muscle
tone of the blood vessels. Most of the sympathetic pre-
ganglionic fibers that control vessel function originate in
the vasomotor center of the brain stem, travel down the
spinal cord, and exit in the thoracic and lumbar (T1 to
L2) segments. The sympathetic neurons that supply the
blood vessels maintain them in a state of tonic activity, so
that even under resting conditions, the blood vessels are
partially constricted. Vessel constriction and relaxation
are accomplished by altering this basal input. Increasing
sympathetic activity causes constriction of some vessels,
such as those of the skin, the gastrointestinal tract, and
the kidneys. Blood vessels in skeletal muscle are supplied
by both vasoconstrictor and vasodilator fibers. Activation
of sympathetic vasodilator fibers causes vessel relaxation
and provides the muscles with increased blood flow dur-
ing exercise. Although the parasympathetic nervous sys-
tem contributes to the regulation of heart function, it has
little or no control over blood vessels.
The actions of the ANS are mediated by chemical
neurotransmitters.
Acetylcholine
is the postganglionic
neurotransmitter for parasympathetic neurons and
norepinephrine
is the main postganglionic neurotrans-
mitter for sympathetic neurons. Sympathetic neurons
also respond to epinephrine, which is released into the
bloodstream by the adrenal medulla. The neurotrans-
mitter
dopamine
can also act as a neurotransmitter for
some sympathetic neurons. The synthesis, release, and
inactivation of the autonomic neurotransmitters are dis-
cussed in Chapter 34.
R E V I EW E X E R C I S E S
1.
In persons with atherosclerosis of the coronary
arteries, symptoms of myocardial ischemia do
not usually occur until the vessel has been 75%
occluded. Use Poiseuille law to explain.
2.
Once an arterial aneurysm has begun to form,
it will continue to enlarge as the result of the
increased tension in its wall.
A.
Explain the continued increase in size using the
law of Laplace.
B.
Using information related to cross-sectional area
and velocity of flow, explain why there is stasis
of blood flow with the tendency to form clots in
aneurysms with a large cross-sectional area.
SUMMARY CONCEPTS
■■
The systemic circulation consists of arteries and
arterioles, capillaries, and venules and veins.The
walls of the blood vessels, except the capillaries,
are composed of three layers: an outer layer,
the tunica externa, composed of large collagen
fibers that protect the vessel and anchor it to
the surrounding structures; a middle layer, the
tunica media, composed of smooth muscle
that constricts to regulate vessel diameter; and
an inner layer, the tunica intima, of flattened
endothelial cells that provide a smooth and
slippery surface for blood flow.
■■
The arterial system is a high-pressure system
that delivers blood to the tissues. It relies on
the intermittent ejection of blood from the left
ventricle and the generation of arterial pressure
pulsations that move blood toward the capillaries
where the exchange of gases, nutrients, and
wastes occur.
■■
The venous system is a low-pressure system that
collects blood from the capillaries. It relies on the
presence of valves in the veins of the extremities
to prevent retrograde flow and on the milking
action of the skeletal muscles that surround the
veins to return blood to the right heart.
■■
The arterioles, capillaries, and venules of the
microcirculation facilitate the exchange of
gases, nutrients, and metabolic waste-products
between body tissues and the circulatory
system. Local control of blood flow in the
microcirculation is governed largely by the
metabolic needs of the tissues and is regulated
by local tissue factors such as lack of oxygen and
the accumulation of metabolites, endothelial-
derived vasodilators and vasoconstrictors, and
humoral factors such as histamine, bradykinin,
and the prostaglandins.
■■
Collateral circulation, which involves the
development of collateral channels between
smaller arteries, is a mechanism for long-term
regulation of blood flow in areas where larger
vessels have become occluded.
■■
Neural control of the circulation is vested in
the autonomic nervous system, with both the
sympathetic and parasympathetic nervous
systems exerting control over heart rate and the
sympathetic nervous system controlling cardiac
contractility and blood vessel tone.
