Nervous Regulation of the Circulation and Rapid Control of Arterial Pressure pdf - The Circulation

pumping, as we shall see later in this and subsequent chapters.

the volume of these vessels. This can

veins,

The innervation of the large vessels, particularly of the

flow through the tissues.

The innervation of the

the capillaries, precapillary sphincters, and metar-

of sympathetic nerve fibers to the blood vessels, demonstrating that in most

Figure 18–2 shows distribution

Sympathetic Innervation of the Blood Vessels.

The precise pathways of these fibers in the spinal cord and in the sympathetic

distributed to the vasculature of the peripheral areas.

on the right side of Figure 18–1, and (2) almost immediately into peripheral por-

innervate mainly the vasculature of the internal viscera and the heart, as shown

by two routes to the circulation: (1) through specific

one of which lies on each side of the vertebral column. Next, they pass

chain,

or two lumbar spinal nerves. They then pass immediately into a

nervous control of the circulation. Sympathetic vasomotor nerve fibers leave

Figure 18–1 shows the anatomy of sympathetic

Sympathetic Nervous System.

see later in the chapter.

also contributes specifically to regulation of heart function, as we shall

. The

teristics, as follows.

sion, we need to present still other specific anatomical and functional charac-

60, and this subject was also introduced in Chapter 17. For our present discus-

. The total function of this system is presented in Chapter

The nervous system controls the circulation almost entirely through the

arterial pressure.

activity by the heart, and, especially, providing very rapid control of systemic

ing blood flow to different areas of the body, increasing or decreasing pumping

tion has more global functions, such as redistribut-

tissue blood flow control mechanisms. We shall see

As discussed in Chapter 17, adjustment of blood

Circulation

Nervous Regulation of the

of Arterial Pressure

Circulation, and Rapid Control

204

Nervous Regulation of the

flow tissue by tissue is mainly the function of local

in this chapter that nervous control of the circula-

auto-

nomic nervous system

Autonomic Nervous System

By far the most important part of the autonomic nervous system for regulating
the circulation is the sympathetic nervous system

parasympathetic nervous

system

the spinal cord through all the thoracic spinal nerves and through the first one

sympathetic

sympathetic nerves that

tions of the spinal nerves

chains are discussed more fully in Chapter 60.

tissues all the vessels except
terioles are innervated.

small arteries and arterioles allows sympathetic stim-

ulation to increase resistance to blood flow and thereby to decrease rate of blood

makes it possi-

ble for sympathetic stimulation to decrease
push blood into the heart and thereby play a major role in regulation of heart

The effects of parasympathetic stimulation on

shown in Figure 18–1 by the dashed red line

nerves,

regulation of the circulation. Its most important circu-

trointestinal actions, it plays only a minor role in

functions of the body, such as control of multiple gas-

and volume of pumping.

markedly increases the activity of the heart, both

shown in Figure 18–1 and also discussed in Chapter 9.

sympathetic fibers also go directly to the heart, as

sympathetic nerve fibers supplying the blood vessels,

Nervous Regulation of the Circulation, and Rapid Control of Arterial Pressure

Chapter 18

205

Sympathetic Nerve Fibers to the Heart.

In addition to

It should be recalled that sympathetic stimulation

increasing the heart rate and enhancing its strength

Parasympathetic Control of Heart Function, Especially Heart
Rate.

Although the parasympathetic nervous system is

exceedingly important for many other autonomic

latory effect is to control heart rate by way of parasym-
pathetic nerve fibers to the heart in the vagus

from the brain medulla directly to the heart.

heart function were discussed in detail in Chapter 9.

Vasoconstrictor

Sympathetic chain

Cardioinhibitor

Vasodilator

Blood
vessels

Vasomotor center

Blood
vessels

Heart

Vagus

to the heart.

shown by the red dashed line is a

of the circulation. Also

control

Figure 18–1

Anatomy of sympathetic nervous

vagus nerve that carries parasym-
pathetic signals

Arteries

Sympathetic

vasoconstriction

Arterioles

Capillaries

Venules

Veins

Sympathetic innervation of the systemic circulation.

Figure 18–2

need to decrease heart pumping, the

heart rate and contractility. Conversely, when there is

activity. The

amount of vascular constriction, it also controls heart

At the

Control of Heart Activity by the Vasomotor Center.

1 to 3 minutes, until the norepinephrine was destroyed.

vessels once again became constricted, and the arterial

was transported in the blood to all blood vessels, the

fibers throughout the body). As this injected hormone

throughout the body. A few minutes later, a small

result, the arterial pressure fell from 100 to 50 mm Hg,

impulses from the spinal cord to the periphery. As a

This blocked all transmission of sympathetic nerve

constrictor tone. In the experiment of this figure,

Figure 18–4 demonstrates the significance of vaso-

partial state of contraction in the blood vessels, called

. These impulses normally maintain a

second. This continual firing is called

entire body, causing continuous slow firing of these

normal conditions, the vasoconstrictor area of the

mally Caused by Sympathetic Vasoconstrictor Tone.

Continuous Partial Constriction of the Blood Vessels Is Nor-

pressure, which we describe later in this chapter.

control of many circulatory functions. An example

the vasomotor center, thus providing “reflex”

, and output signals from

medulla and lower pons. The neurons of this area

3. A

medulla. The fibers from these neurons project

2. A

fibers to all levels of the spinal cord, where they

anterolateral portions of the upper medulla. The

1. A

center, as follows:

center is still unclear, experiments have made it

body.

to virtually all arteries, arterioles, and veins of the

. This center transmits

of the pons, shown in Figures 18–1 and 18–3, is an area

Vasomotor Center in the Brain and Its Control of the Vasocon-

intestines, spleen, and skin but much less potent in

strictor effect is especially powerful in the kidneys,

some tissues than others. This sympathetic vasocon-

essentially all segments of the circulation, but more to

fibers. The vasoconstrictor fibers are distributed to

The sympathetic nerves carry tremendous numbers of

Its Control by the Central Nervous System

Sympathetic Vasoconstrictor System and

heart muscle contractility.

Principally, parasympathetic stimulation causes a

206

Unit IV

The Circulation

marked decrease in heart rate and a slight decrease in

vasoconstrictor nerve fibers and only a few vasodilator

skeletal muscle and the brain.

strictor System.

Located bilaterally mainly in the retic-

ular substance of the medulla and of the lower third

called the vasomotor center
parasympathetic impulses through the vagus nerves to
the heart and transmits sympathetic impulses through
the spinal cord and peripheral sympathetic nerves

Although the total organization of the vasomotor

possible to identify certain important areas in this

vasoconstrictor area located bilaterally in the

neurons originating in this area distribute their

excite preganglionic vasoconstrictor neurons of the
sympathetic nervous system.

vasodilator area located bilaterally in the

anterolateral portions of the lower half of the

upward to the vasoconstrictor area just described;
they inhibit the vasoconstrictor activity of this area,
thus causing vasodilation.

sensory area located bilaterally in the tractus

solitarius in the posterolateral portions of the

receive sensory nerve signals from the circulatory
system mainly through the vagus and
glossopharyngeal nerves
this sensory area then help to control activities of
both the vasoconstrictor and vasodilator areas of

is the baroreceptor reflex for controlling arterial

Under

vasomotor center transmits signals continuously to
the sympathetic vasoconstrictor nerve fibers over the

fibers at a rate of about one half to two impulses per

sympathetic vaso-

constrictor tone

vasomotor tone.

total spinal anesthesia was administered to an animal.

demonstrating the effect of losing vasoconstrictor tone

amount of the hormone norepinephrine was injected
into the blood (norepinephrine is the principal
vasoconstrictor hormonal substance secreted at the
endings of the sympathetic vasoconstrictor nerve

pressure rose to a level even greater than normal for

same time that the vasomotor center is controlling the

lateral portions of the vasomotor center

transmit excitatory impulses through the sympathetic
nerve fibers to the heart when there is need to increase

medial portion of

Reticular

substance

Motor

Cingulate

Orbital

Temporal

Pons

Medulla

VASOMOTOR
CENTER

VASOCONSTRICTOR

VASODILATOR

Mesencephalon

tion of the circulation. The dashed lines represent inhibitory

Areas of the brain that play important roles in the nervous regula-

Figure 18–3

pathways.

at their endings, although in primates, the vasodilator

not norepinephrine,

acetylcholine,

dilator fibers release

constrictor fibers. In lower animals such as the cat, these

The sympathetic nerves to skeletal

Nervous System.

Vasodilator

dilates rather than constricts certain vessels, as dis-

a “beta” adrenergic receptor stimulatory effect, which

to cause vasoconstriction, but in an occasional tissue

where they act directly on all blood vessels, usually

carried in the blood stream to all parts of the body,

. These two hormones are

are transmitted to the blood vessels. They cause the

Adrenal Medullae and Their Relation to the Sympathetic Vaso-

vasoconstriction, as discussed in Chapter 60.

alpha adren-

rine. Norepinephrine acts directly on the

The substance secreted at the endings of the

Norepinephrine—The Sympathetic Vasoconstrictor Transmitter

stimulus. Thus, widespread basal areas of the brain can

motor center, depending on the precise portions of

, and the

, the

Also, stimulation of the

hypothalamus and thence to the vasomotor center.

for instance, excites the vasomotor center

inhibit the vasomotor center. Stimulation of the

thalamus cause mainly excitation, whereas the

motor center. The

The

of the reticular substance cause excitation, whereas

in Figure 18–3 by the rose-colored area. In general, the

vasomotor center. This reticular substance is shown

Control of the Vasomotor Center by Higher Nervous Centers.

either increase or decrease heart activity. Heart rate

contractility. Therefore, the vasomotor center can

, which then

Nervous Regulation of the Circulation, and Rapid Control of Arterial Pressure

Chapter 18

207

the vasomotor center sends signals to the adjacent
dorsal motor nuclei of the vagus nerves
transmit parasympathetic impulses through the vagus
nerves to the heart to decrease heart rate and heart

and strength of heart contraction ordinarily increase
when vasoconstriction occurs and ordinarily decrease
when vasoconstriction is inhibited.

Large numbers of small neurons located throughout
the reticular substance of the pons, mesencephalon,
and diencephalon can either excite or inhibit the

neurons in the more lateral and superior portions

the more medial and inferior portions cause inhibition.

hypothalamus plays a special role in controlling

the vasoconstrictor system because it can exert either
powerful excitatory or inhibitory effects on the vaso-

posterolateral portions of the hypo-

anterior

portion can cause either mild excitation or inhibition,
depending on the precise part of the anterior hypo-
thalamus stimulated.

Many parts of the cerebral cortex can also excite or

motor

cortex,
because of impulses transmitted downward into the

anterior temporal lobe

orbital areas of the frontal cortex, the anterior part of

the cingulate gyrus

amygdala

septum

hippocampus can all either excite or inhibit the vaso-

these areas that are stimulated and on the intensity of

have profound effects on cardiovascular function.

Substance.

vasoconstrictor nerves is almost entirely norepineph-

ergic receptors of the vascular smooth muscle to cause

constrictor System.

Sympathetic impulses are transmit-

ted to the adrenal medullae at the same time that they

medullae to secrete both epinephrine and norepineph-
rine into the circulating blood

epinephrine causes vasodilation because it also has

cussed in Chapter 60.

Sympathetic

System and its Control by the Central

muscles carry sympathetic vasodilator fibers as well as

Arterial pressure (mm Hg)

Seconds

150

125

100

Total spinal

anesthesia

Injection of norepinephrine

resulting from loss of “vasomotor

marked decrease in pressure

the arterial pressure, showing

Effect of total spinal anesthesia on

Figure 18–4

tone.”

from danger.

, and it

few seconds. This is called the

instance, during extreme fright, the arterial pressure

cise, a similar rise in pressure can also occur. For

activity.

center. These increase the arterial pressure instanta-

ing system of the brain stem is also activated, which

vated to cause exercise, most of the reticular activat-

results mainly from the following effect: At the same

The increase in arterial pressure during exercise

rises about 30 to 40 per cent, which increases blood

exercise. In most heavy exercise, the arterial pressure

in Chapter 17. Additional increase results from simul-

increased metabolism of the muscle cells, as explained

blood flow. Part of this increase results from local

heavy exercise, the muscles require greatly increased

in pressure that occurs during muscle exercise. During

Types of Stress

Increase in Arterial Pressure

one half normal within 10 to 40 seconds. Therefore,

two times normal within 5 to 10 seconds. Conversely,

arterial pressure is its rapidity of response, beginning

Rapidity of Nervous Control of Arterial Pressure.

to the acute rise in arterial pressure.

under normal conditions. This contributes still more

blood. During strong sympathetic stimulation, the

force of the heart muscle, this, too, increasing the

addition, sympathetic nervous signals have a

increasing to as great as three times normal. In

increase in the heart rate, the rate sometimes

cardiac pumping.

autonomic nervous system, further enhancing

the heart itself is directly stimulated by the

3. Finally,

pressure.

quantities of blood. This, too, increases the arterial

volume of blood in the heart chambers. The stretch

blood vessels toward the heart, thus increasing the

This displaces blood out of the large peripheral

pressure.

peripheral resistance, thereby increasing the arterial

This greatly increases the total

each of which helps to increase arterial pressure. They

heart. Thus, three major changes occur simultaneously,

together. At the same time, there is reciprocal inhibi-

increases in arterial pressure. For this purpose, the

Arterial Pressure

in Rapid Control of

Role of the Nervous System

nerves of the muscles.

nerves, and also through the spinal cord to the

centers of the medulla, to the heart through the vagus

The pathway probably then goes to the vasodilatory

vasovagal syncope.

causes the person to lose consciousness. This overall

falls rapidly, which reduces blood flow to the brain and

to slow the heart rate markedly. The arterial pressure

becomes activated, and at the same time, the vagal car-

fainting. In this case, the muscle vasodilator system

Emotional Fainting—Vasovagal Syncope.

before the muscles require increased nutrients.

anticipatory increase in blood flow

that at the onset of exercise, the sympathetic vasodila-

response to their needs. Yet some experiments suggest

Possible Unimportance of the Sympathetic Vasodilator System.

anterior hypothalamus.

Figure 18–3. The principal area of the brain controlling

The pathway for central nervous system control of

vasculature.

208

Unit IV

The Circulation

effect is believed to be caused by epinephrine exciting
specific beta adrenergic receptors in the muscle

the vasodilator system is shown by the dashed lines in

this system is the

It is doubtful that the sympathetic vasodilator system
plays an important role in the control of the circulation
in the human being because complete block of the sym-
pathetic nerves to the muscles hardly affects the ability
of these muscles to control their own blood flow in

tor system might cause initial vasodilation in skeletal
muscles to allow

even

A particularly

interesting vasodilatory reaction occurs in people who
experience intense emotional disturbances that cause

dioinhibitory center transmits strong signals to the heart

effect is called

Emotional fainting

begins with disturbing thoughts in the cerebral cortex.

center of the anterior hypothalamus next to the vagal

sympa-

thetic vasodilator

One of the most important functions of nervous
control of the circulation is its capability to cause rapid

entire vasoconstrictor and cardioaccelerator functions
of the sympathetic nervous system are stimulated

tion of parasympathetic vagal inhibitory signals to the

are as follows:

1. Almost all arterioles of the systemic circulation

are constricted.

2. The veins especially (but the other large vessels of

the circulation as well) are strongly constricted.

of the heart then causes the heart to beat with far
greater force and therefore to pump increased

Much of this is caused by an

significant direct effect to increase contractile

capability of the heart to pump larger volumes of

heart can pump about two times as much blood as

An espe-

cially important characteristic of nervous control of

within seconds and often increasing the pressure to

sudden inhibition of nervous cardiovascular stimula-
tion can decrease the arterial pressure to as little as

nervous control of arterial pressure is by far the most
rapid of all our mechanisms for pressure control.

During Muscle Exercise and Other

An important example of the ability of the nervous
system to increase the arterial pressure is the increase

vasodilation of the muscle vasculature caused by

taneous elevation of arterial pressure caused by sym-
pathetic stimulation of the overall circulation during

flow almost an additional twofold.

time that the motor areas of the brain become acti-

includes greatly increased stimulation of the vasocon-
strictor and cardioacceleratory areas of the vasomotor

neously to keep pace with the increase in muscle

In many other types of stress besides muscle exer-

sometimes rises to as high as double normal within a

alarm reaction

provides an excess of arterial pressure that can imme-
diately supply blood to any or all muscles of the body
that might need to respond instantly to cause flight

toward normal. Thus, the baroreceptor feedback

of arterial pressure, around 100 mm Hg, even a slight

about 30 mm Hg higher.

except that they operate, in general, at pressure levels

about 180 mm Hg. The responses of the aortic barore-

50 to 60 mm Hg, but above these levels, they respond

sinus nerve. Note that the carotid sinus baroreceptors

the rate of impulse transmission in a Hering’s carotid

Figure 18–6

“aortic baroreceptors” in the arch of the aorta are

medullary area of the brain stem. Signals from the

the high neck, and then to the

Hering’s nerves

baroreceptors” are transmitted through very small

Figure 18–5 shows that signals from the “carotid

carotid sinus,

tion, an area known as the

regions; but, as shown in Figure 18–5, baroreceptors

stretched. A few baroreceptors are located in the wall

lie in the walls of the arteries; they are stimulated when

Physiologic Anatomy of the Baroreceptors and Their Innerva-

the central nervous system. “Feedback” signals are

arteries. A rise in arterial pressure stretches the

, located

Basically, this reflex is initiated by stretch receptors,

System—Baroreceptor Reflexes

The Baroreceptor Arterial Pressure Control

which we explain in the following sections.

negative feedback reflex mechanisms

tain the arterial pressure at or near normal. Almost

sure, there are multiple subconscious special nervous

Normal Arterial Pressure

Nervous Regulation of the Circulation, and Rapid Control of Arterial Pressure

Chapter 18

209

Reflex Mechanisms for Maintaining

Aside from the exercise and stress functions of the
autonomic nervous system to increase arterial pres-

control mechanisms that operate all the time to main-

all of these are

By far the best known of the nervous mechanisms for
arterial pressure control is the baroreceptor reflex.

called either baroreceptors or pressoreceptors
at specific points in the walls of several large systemic

baroreceptors and causes them to transmit signals into

then sent back through the autonomic nervous system
to the circulation to reduce arterial pressure down-
ward toward the normal level.

tion.

Baroreceptors are spray-type nerve endings that

of almost every large artery of the thoracic and neck

are extremely abundant in (1) the wall of each inter-
nal carotid artery slightly above the carotid bifurca-

and (2) the

wall of the aortic arch.

to the glossopharyngeal nerves in

tractus solitarius in the

transmitted through the vagus nerves also to the same
tractus solitarius of the medulla.

Response of the Baroreceptors to Pressure.

shows the effect of different arterial pressure levels on

are not stimulated at all by pressures between 0 and

progressively more rapidly and reach a maximum at

ceptors are similar to those of the carotid receptors

Note especially that in the normal operating range

change in pressure causes a strong change in the
baroreflex signal to readjust arterial pressure back

mechanism functions most effectively in the pressure
range where it is most needed.

Glossopharyngeal nerve

Hering’s nerve

Vagus nerve

Aortic baroreceptors

Carotid body

Carotid sinus

The baroreceptor system for controlling arterial pressure.

Figure 18–5

160

244

Number of impulses from carotid

sinus nerves per second

= maximum

Arterial blood pressure (mm Hg)

P, change in arterial blood pressure in mm Hg.

I, change in carotid sinus nerve impulses per second;

pressure.

Activation of the baroreceptors at different levels of arterial

Figure 18–6

blood pressure regulation has been controversial. One

arterial pressure, their importance in long-term

Are the Baroreceptors Important in Long-Term Regulation of

In summary, a primary purpose of the arterial

Hg. Thus, one can see the extreme variability of pres-

falling to as low as 50 mm Hg or rising to over 160 mm

that the pressure range increased 2.5-fold, frequently

became the broad, low curve of the figure, showing

the baroreceptors, the frequency distribution curve

exactly 100 mm Hg. Conversely, after denervation of

115 mm Hg—indeed, during most of the day at almost

the day within a narrow range between 85 and

both the normal dog and the denervated dog. Note

Figure 18–9 shows the frequency distributions of the

ment, eating, defecation, and noises.

events of the day, such as lying down, standing, excite-

aorta had been removed. Note the extreme variability

from a normal dog, and the lower record shows an

function of the baroreceptors. The upper record in this

Figure 18–8 shows the importance of this buffer

, and the nerves

sure, it is called a

Function of the Baroreceptor

the head and upper body.

the body. This minimizes the decrease in pressure in

cause loss of consciousness. However, the falling pres-

to fall, and marked reduction of this pressure could

been lying down. Immediately on standing, the arterial

The ability of the baroreceptors to maintain

another minute.

the pressure in the carotid sinuses to rise, and the

carotids are occluded. Removal of the occlusion allows

than usual, causing the aortic arterial pressure to rise

their inhibitory effect on the vasomotor center. The

result, the baroreceptors become inactive and lose

arteries. This reduces the carotid sinus pressure; as a

Figure 18–7 shows a typical reflex change in arterial

sure has opposite effects, reflexly causing the pressure

and a decrease in cardiac output. Conversely, low pres-

Therefore, exci-

The net effects are (1)

parasympathetic center.

ius of the medulla, secondary signals

Circulatory Reflex Initiated by the Baroreceptors.

when the pressure is stationary at 150 mm Hg.

Hg but at that moment is rising rapidly, the rate of

sure. That is, if the mean arterial pressure is 150 mm

rapidly changing pressure

respond much more to a

more, the baroreceptors

systole and decreases again during diastole. Further-

changes in arterial pressure; in fact, the rate of impulse

The baroreceptors respond extremely rapidly to

210

Unit IV

The Circulation

firing increases in the fraction of a second during each

than to a stationary pres-

impulse transmission may be as much as twice that

After the

baroreceptor signals have entered the tractus solitar-

inhibit the vaso-

constrictor center of the medulla and excite the vagal

vasodi-

lation of the veins and arterioles throughout the
peripheral circulatory system and (2) decreased heart
rate and strength of heart contraction.
tation of the baroreceptors by high pressure in the
arteries reflexly causes the arterial pressure to decrease
because of both a decrease in peripheral resistance

to rise back toward normal.

pressure caused by occluding the two common carotid

vasomotor center then becomes much more active

and remain elevated during the 10 minutes that the

carotid sinus reflex now causes the aortic pressure to
fall immediately to slightly below normal as a momen-
tary overcompensation and then return to normal in

Function of the Baroreceptors During Changes in Body
Posture.

relatively constant arterial pressure in the upper body
is important when a person stands up after having

pressure in the head and upper part of the body tends

sure at the baroreceptors elicits an immediate reflex,
resulting in strong sympathetic discharge throughout

Pressure “Buffer”
Control System.

Because the baroreceptor system

opposes either increases or decreases in arterial pres-

pressure buffer system

from the baroreceptors are called buffer nerves.

figure shows an arterial pressure recording for 2 hours

arterial pressure recording from a dog whose barore-
ceptor nerves from both the carotid sinuses and the

of pressure in the denervated dog caused by simple

mean arterial pressures recorded for a 24-hour day in

that when the baroreceptors were functioning nor-
mally the mean arterial pressure remained throughout

sure in the absence of the arterial baroreceptor
system.

baroreceptor system is to reduce the minute by minute
variation in arterial pressure to about one third that
which would occur were the baroreceptor system not
present.

Arterial Pressure?

Although the arterial baroreceptors

provide powerful moment-to-moment control of

reason that the baroreceptors have been considered

150

Both common

carotids clamped

Carotids released

100

Arterial pressure (mm Hg)

Minutes

caused by clamping both common carotids (after the two vagus

Typical carotid sinus reflex effect on aortic arterial pressure

Figure 18–7

nerves have been cut).

receptors excite nerve fibers that, along with the

adjacent to the aorta). The chemo-

each common carotid artery, and usually one to three

carotid bodies,

chemoreceptor organs

gen ion excess. They are located in several small

tive to oxygen lack, carbon dioxide excess, and hydro-

chemoreceptors

The

ate the response.

that chemoreceptors, instead of stretch receptors, initi-

chemoreceptor reflex

cussed in Chapters 19 and 29.

its associated nervous and hormonal mechanisms), dis-

interaction with additional systems, principally the

toward normal. Thus, long-term regulation of mean

blood volume, which helps to restore arterial pressure

kidneys. This, in turn, causes a gradual decrease in

sure, the baroreceptor reflexes may mediate decreases

For example, with prolonged increases in arterial pres-

influencing sympathetic nerve activity of the kidneys.

long-term blood pressure regulation, especially by

however, have suggested that the baroreceptors do

longer than a few days at a time. Experimental studies,

their potency as a control system for correcting dis-

This “resetting” of the baroreceptors may attenuate

impulses, but gradually, over 1 to 2 days, the rate of

low level, the baroreceptors at first transmit no

Conversely, when the arterial pressure falls to a very

the mean arterial pressure still remains at 160 mm Hg.

days, at the end of which time the rate of firing will

the rate of firing diminishes considerably; then it

are at first transmitted. During the next few minutes,

160 mm Hg, a very high rate of baroreceptor impulses

pressure rises from the normal value of 100 mm Hg to

level to which they are exposed. That is, if the arterial

Nervous Regulation of the Circulation, and Rapid Control of Arterial Pressure

Chapter 18

211

by some physiologists to be relatively unimportant in
chronic regulation of arterial pressure chronically is
that they tend to reset in 1 to 2 days to the pressure

diminishes much more slowly during the next 1 to 2

have returned to nearly normal despite the fact that

baroreceptor firing returns toward the control level.

turbances that tend to change arterial pressure for

not completely reset and may therefore contribute to

in renal sympathetic nerve activity that promote
increased excretion of sodium and water by the

arterial pressure by the baroreceptors requires

renal–body fluid–pressure control system (along with

Control of Arterial Pressure by the Carotid and Aortic
Chemoreceptors—Effect of Oxygen Lack on Arterial Pressure.

Closely associated with the baroreceptor pressure
control system is a

that operates in

much the same way as the baroreceptor reflex except

are chemosensitive cells sensi-

about 2 millimeters in size (two

one of which lies in the bifurcation of

aortic bodies

NORMAL

DENERVATED

Time (min)

Arterial pressure (mm Hg)

200

100

200

100

1973. By permission of the American Heart Association, Inc.)

blood pressure and other variables in dogs. Circ Res 32:564,

Guyton AC: Role of baroreceptor reflex in daily control of arterial

had been denervated. (Redrawn from Cowley AW Jr, Liard JF,

several weeks after the baroreceptors

Two-hour records of arterial pressure in a normal dog

Figure 18–8

(above) and

in the same dog (below)

100

150

200

Percentage of occurrence

250

Mean arterial pressure (mm Hg)

Normal

Denervated

tion, Inc.)

Res 32:564, 1973. By permission of the American Heart Associa-

control of arterial blood pressure and other variables in dogs. Circ

AW Jr, Liard JP, Guyton AC: Role of baroreceptor reflex in daily

the baroreceptors had been denervated. (Redrawn from Cowley

period in a normal dog and in the same dog several weeks after

Frequency distribution curves of the arterial pressure for a 24-hour

Figure 18–9

The kidneys, for instance, often

tion caused by intense cerebral ischemia is often so great

as 250 mm Hg.

activity is tremendous: it can elevate the mean arterial

The magnitude of the ischemic effect on vasomotor

ischemic response.

central nervous system ischemic response,

elevation in arterial pressure. This arterial pressure ele-

center, also contribute to the marked stimulation and

It is possible that other factors, such as buildup of

pathetic vasomotor nervous control areas in the brain’s

blood flow to the vasomotor center, the local con-

from the brain stem vasomotor center: at low levels of

pump. This effect is believed to be caused by failure of

excited. When this occurs, the systemic arterial pressure

cerebral ischemia—

severely enough to cause nutritional deficiency—that is,

the brain. However, when blood flow to the vasomotor

chemoreceptors, and the low-pressure receptors, all of

by reflexes that originate in the baroreceptors, the

s Vasomotor

Control of Arterial

prevent damming of blood in the veins, atria, and pul-

strength of heart contraction. Thus, this reflex helps

Then efferent signals are transmitted back through

The stretch receptors of the atria that elicit

as 15 per cent. An additional 40 to 60 per cent increase

the sinus node: it was pointed out in Chapter 10 that

as 75 per cent. A small part of this increase is caused by

heart rate, sometimes increasing the heart rate as much

is discussed again in Chapter 29, along with other mech-

greater arterial pressure. This volume reflex mechanism

heart to greater cardiac output and leads, therefore, to

normal. (We will also see in Chapter 19 that atrial

from the tubules. Combination of these two effects—

fluid into the kidney tubules. The diminution of antidi-

pressure to rise, with resultant increase in filtration of

antidiuretic hormone. The decreased afferent arteriolar

of the afferent arterioles in the kidneys. And still

Atrial Reflexes That Activate the Kidneys—The “Volume Reflex.”

potent for control of arterial pressure.

volume, and they elicit reflexes parallel to the barore-

cannot detect the systemic arterial pressure, they do

Thus, one can see that even though the low-pressure

sure rises about 100 mm Hg.

also are denervated, the pres-

low-pressure receptors

denervated, the pressure rises about 40 mm Hg. If

rises only about 15 mm Hg. With the

into a dog with allreceptors intact, the arterial pressure

example, if 300 milliliters of blood suddenly are infused

in response to changes in blood volume. To give an

role, especially in minimizing arterial pressure changes

arteries. These low-pressure receptors play an important

They are similar to

low-pressure receptors.

Pressure and Other Circulatory Factors.

Atrial and Pulmonary Artery Reflexes That Help Regulate Arterial

The chemoreceptors are discussed in much more

pressure.

Therefore, it is at the lower pressures that this reflex

troller until the arterial pressure falls below 80 mm Hg.

pressure back toward normal. However, this chemo-

the vasomotor center, and this elevates the arterial

The signals transmitted from the chemoreceptors

below a critical level, the chemoreceptors become stim-

with arterial blood. Whenever the arterial pressure falls

abundant blood flow through a small nutrient artery, so

baroreceptor fibers, pass through Hering’s nerves and

212

Unit IV

The Circulation

the vagus nerves into the vasomotor center of the brain
stem.

Each carotid or aortic body is supplied with an

that the chemoreceptors are always in close contact

ulated because diminished blood flow causes decreased
oxygen as well as excess buildup of carbon dioxide and
hydrogen ions that are not removed by the slowly
flowing blood.

excite

receptor reflex is not a powerful arterial pressure con-

becomes important to help prevent still further fall in

detail in Chapter 41 in relation to respiratory control,
in which they play a far more important role than in
pressure control.

Both the atria and

the pulmonary arteries have in their walls stretch recep-
tors called
the baroreceptor stretch receptors of the large systemic

arterial barorecep-

tors
the

receptors in the pulmonary artery and in the atria

detect simultaneous increases in pressure in the low-
pressure areas of the circulation caused by increase in

ceptor reflexes to make the total reflex system more

Stretch of the atria also causes significant reflex dilation

other signals are transmitted simultaneously from the
atria to the hypothalamus to decrease secretion of

resistance in the kidneys causes the glomerular capillary

uretic hormone diminishes the reabsorption of water

increase in glomerular filtration and decrease in reab-
sorption of the fluid—increases fluid loss by the kidneys
and reduces an increased blood volume back toward

stretch caused by increased blood volume also elicits a
hormonal effect on the kidneys—release of atrial natri-
uretic peptide that adds still further to the excretion of
fluid in the urine and return of blood volume toward
normal.)

All these mechanisms that tend to return the blood

volume back toward normal after a volume overload act
indirectly as pressure controllers as well as blood
volume controllers because excess volume drives the

anisms of blood volume control.

Atrial Reflex Control of Heart Rate (the Bainbridge Reflex).

increase in atrial pressure also causes an increase in

a direct effect of the increased atrial volume to stretch

such direct stretch can increase the heart rate as much

in rate is caused by a nervous reflex called the Bain-
bridge reflex.
the Bainbridge reflex transmit their afferent signals
through the vagus nerves to the medulla of the brain.

vagal and sympathetic nerves to increase heart rate and

monary circulation.

Central Nervous System Ischemic
Response—
Pressure by the Brain’
Center in Response to Diminished
Brain Blood Flow

Most nervous control of blood pressure is achieved

which are located in the peripheral circulation outside

center in the lower brain stem becomes decreased

to cause

the vasoconstrictor and

cardioaccelerator neurons in the vasomotor center
respond directly to the ischemia and become strongly

often rises to a level as high as the heart can possibly

the slowly flowing blood to carry carbon dioxide away

centration of carbon dioxide increases greatly and
has an extremely potent effect in stimulating the sym-

medulla.

lactic acid and other acidic substances in the vasomotor

vation in response to cerebral ischemia is known as the

or simply CNS

pressure for as long as 10 minutes sometimes to as high

The degree of sympathetic vasoconstric-

that some of the peripheral vessels become totally or
almost totally occluded.

the vessels in the muscles and in the abdomen. The

exercise tightens the muscles, thereby compressing

blood vessels throughout the body. Even anticipation of

skeletal muscles contract during exercise, they compress

When the

normal skeletal muscles.

The abdominal compression reflex is probably much

an increase in both cardiac output and arterial pressure.

. The resulting effect on

for the heart to pump. This overall response is called the

result, increased quantities of blood are made available

abdominal vascular reservoirs toward the heart. As a

the abdomen, helping to translocate blood out of the

muscles. This compresses all the venous reservoirs of

tal muscles of the body, particularly to the abdominal

chemoreceptor reflex is elicited, nerve signals are trans-

When a baroreceptor or

responses are the following.

system, at least two conditions in which the skeletal

Cardiac Output and

Control of Arterial Pressure

Special Features of Nervous

rises high enough to compress the cerebral arteries.

into the cranial vault around the brain. The Cushing

typical Cushing reaction is shown in Figure 18–10,

ing blood to begin again to flow through the brain. A

higher than the cerebrospinal fluid pressure, thus allow-

the brain to relieve the brain ischemia. Ordinarily, the

pressure, blood will flow once again into the vessels of

the arterial pressure to rise. When the arterial pressure

brain. This initiates a CNS ischemic response that causes

pressure, it compresses the whole brain as well as the

the brain in the cranial vault. For instance, when the

The so-called

the “last ditch stand” pressure control mechanism.

pressure whenever blood flow to the brain decreases dan-

very powerfully to prevent further decrease in arterial

arterial pressure. Instead, it operates principally as an

stimulation at a pressure of 15 to 20 mm Hg. Therefore,

60 mm Hg and below, reaching its greatest degree of

the arterial pressure falls far below normal, down to

ischemic response, it does not become significant until

of the most powerful of all the activators of the sympa-

the CNS ischemic response is one

discharge. Therefore,

Nervous Regulation of the Circulation, and Rapid Control of Arterial Pressure

Chapter 18

213

entirely cease their production of urine because of renal
arteriolar constriction in response to the sympathetic

thetic vasoconstrictor system.

Importance of the CNS Ischemic Response as a Regulator of Arte-
rial Pressure.

Despite the powerful nature of the CNS

it is not one of the normal mechanisms for regulating

emergency pressure control system that acts rapidly and

gerously close to the lethal level. It is sometimes called

Cushing Reaction.

Cushing reaction is a

special type of CNS ischemic response that results from
increased pressure of the cerebrospinal fluid around

cerebrospinal fluid pressure rises to equal the arterial

arteries in the brain and cuts off the blood supply to the

has risen to a level higher than the cerebrospinal fluid

blood pressure comes to a new equilibrium level slightly

caused in this instance by pumping fluid under pressure

reaction helps protect the vital centers of the brain from
loss of nutrition if ever the cerebrospinal fluid pressure

Role of the Skeletal Nerves and
Skeletal Muscles in Increasing

Arterial Pressure

Although most rapidly acting nervous control of the cir-
culation is effected through the autonomic nervous

nerves and muscles also play major roles in circulatory

Abdominal Compression Reflex.

mitted simultaneously through skeletal nerves to skele-

abdominal compression reflex
the circulation is the same as that caused by sympathetic
vasoconstrictor impulses when they constrict the veins:

more important than has been realized in the past
because it is well known that people whose skeletal
muscles have been paralyzed are considerably more
prone to hypotensive episodes than are people with

Increased Cardiac Output and Arterial Pressure Caused by
Skeletal Muscle Contraction During Exercise.

Connector to

subarachnoid

space

Pressure

bottle

Arterial

pressure

transducer

Pen

recorder

CSF pressure

reduced

CSF pressure

raised

Zero

pressure

Moving paper

Arterial

pressure

brospinal fluid (CSF) pressure.

resulting from increased cere-

rapid rise in arterial pressure

Cushing reaction,” showing a

Figure 18–10

“

side instead of following a straight course.

guiding mechanism, the plane will oscillate from side to

systems. For instance, if the feedback “gain” is too great

The vasomotor waves are of considerable theoretical

sure receptor and the subsequent pressure response.

oscillate if the intensity of “feedback” is strong enough

Thus, any reflex pressure control mechanism can

repeated itself cyclically as long as the cerebrospinal

ischemia was relieved and again the pressure fell. This

then initiated another rise in pressure. Again the

value, causing brain ischemia once again. The ischemia

pathetic nervous system became inactive. As a result,

high value, the brain ischemia was relieved and the sym-

200 mm Hg. When the arterial pressure rose to such a

160 mm Hg, which compressed the cerebral vessels and

ment, the cerebrospinal fluid pressure was raised to

ischemic pressure control mechanism. In this experi-

Figure 18–11

The record in

control becomes weaker.

circulation becomes powerful, whereas baroreceptor

because in this low range, chemoreceptor control of the

arterial pressure is in the range of 40 to 80 mm Hg

taneously with the baroreceptor reflex. It probably plays

same type of waves. This reflex usually oscillates simul-

chemoreceptor reflex

The

another cycle, and the oscillation continues on and on.

few seconds later. This high pressure then initiates

response is not instantaneous, and it is delayed until a

again, elevating the pressure to a high value. The

lowers the pressure a few seconds later. The decreased

That is, a high pressure excites the baroreceptors;

much less intense than shown in the figure. They are

in experimental pressure recordings, although usually

The vasomotor waves of Figure 18–11

Oscillation of the Baroreceptor and Chemoreceptor Reflexes.

some of which are the following.

of one or more nervous pressure control mechanisms,

The cause of vasomotor waves is “reflex oscillation”

showing the cyclical rise and fall in arterial pressure.

waves.” Such records are demonstrated in Figure 18–11,

These waves are called

each cycle varies from 26 seconds in the anesthetized

more slowly than the respiratory waves. The duration of

great as 10 to 40 mm Hg at times—that rise and fall

piration, some much larger waves are also noted—as

Waves

Vasomotor

20 mm Hg with each respiratory cycle.

ration, the blood pressure can rise and fall as much as

remainder of the respiratory cycle. During deep respi-

waves, the net result during normal respiration is usually

receptors.

3. The pressure changes caused in the thoracic vessels

decreases the cardiac output and arterial pressure.

This reduces the quantity of blood returning to the

2. Every time a person inspires, the pressure in the

the vasomotor center with each respiratory cycle.

respiratory center of the medulla “spill over” into

1. Many of the “breathing signals” that arise in the

some of which are reflex in nature, as follows:

sure. The waves result from several different effects,

manner, causing

usually rises and falls 4 to 6 mm Hg in a wavelike

With each cycle of respiration, the arterial pressure

Respiratory Waves in the

100 mm Hg up to 130 to 160 mm Hg.

exercise, an increase usually from a normal mean of

The increase in cardiac output in turn is an essential

cardiac output that sometimes occurs in heavy exercise.

increase the cardiac output. This is an essential effect in

eral vessels into the heart and lungs and, therefore, to

214

Unit IV

The Circulation

resulting effect is to translocate blood from the periph-

helping to cause the fivefold to sevenfold increase in

ingredient in increasing the arterial pressure during

Arterial Pressure

respiratory waves in the arterial pres-

thoracic cavity becomes more negative than usual,
causing the blood vessels in the chest to expand.

left side of the heart and thereby momentarily

by respiration can excite vascular and atrial stretch

Although it is difficult to analyze the exact relations

of all these factors in causing the respiratory pressure

an increase in arterial pressure during the early part
of expiration and a decrease in pressure during the

Arterial Pressure “

”

—Oscillation of Pressure

Reflex Control Systems

Often while recording arterial pressure from an animal,
in addition to the small pressure waves caused by res-

dog to 7 to 10 seconds in the unanesthetized human.

vasomotor waves or “Mayer

B are often seen

caused mainly by oscillation of the baroreceptor reflex.

this then inhibits the sympathetic nervous system and

pressure in turn reduces the baroreceptor stimulation
and allows the vasomotor center to become active once

can also oscillate to give the

the major role in causing vasomotor waves when the

Oscillation of the CNS Ischemic Response.

A resulted from oscillation of the CNS

initiated a CNS ischemic pressure response up to

the arterial pressure fell rapidly back to a much lower

fluid pressure remained elevated.

and if there is a delay between excitation of the pres-

importance because they show that the nervous reflexes
that control arterial pressure obey the same principles
as those applicable to mechanical and electrical control

in the guiding mechanism of an automatic pilot for an
airplane and there is also delay in response time of the

Pressure (mm Hg)

200
160
120

80
40

100

Vasomotor waves caused by baroreceptor reflex

response.

Vasomotor waves caused by oscillation of the CNS ischemic

Figure 18–11

oscillation.

N Y Acad Sci 940:431, 2001.

angiotensin II, nitric oxide, and exercise training. Ann

of sympathetic outflow in heart failure. The roles of

Zucker IH, Wang W, Pliquett RU, et al: The regulation

Physiol 282:R1044, 2002.

rogenic hypertension. Am J Physiol Regul Integr Comp

Thrasher TN: Unloading arterial baroreceptors causes neu-

Hypertens Rep 5:262, 2003 .

tension: role of the rostral ventrolateral medulla. Curr

Sved AF, Ito S, Sved JC: Brainstem mechanisms of hyper-

Am J Physiol Regul Integr Comp Physiol 281:R683, 2001.

Morrison SF: Differential control of sympathetic outflow.

News Physiol Sci 16:266, 2001.

Mifflin SW:What does the brain know about blood pressure?

Am J Physiol Regul Integr Comp Physiol 286:R1, 2004.

nerve activity: is there a role for arterial baroreceptors?

Malpas SC: What sets the long-term level of sympathetic

term blood pressure regulation. Am J Hypertens 14:147S,

Lohmeier TE: The sympathetic nervous system and long-

hypertension. Hypertension 39:550, 2002.

Lohmeier TE, Lohmeier JR, Warren S, et al: Sustained acti-

system. Ann N Y Acad Sci 940:338, 2001.

Krieger EM, Da Silva GJ, Negrao CE: Effects of exercise

Circulation 105:2518, 2002.

tension, orthostatic tachycardia, and malignant vagotonia.

of baroreflex failure: hypertensive crisis, volatile hyper-

Ketch T, Biaggioni I, Robertson R, Robertson D: Four faces

tens 14:103S, 2001.

of leptin and sympathetic nervous system. Am J Hyper-

Hall JE, Hildebrandt DA, Kuo J: Obesity hypertension: role

phia: WB Saunders Co, 1980.

Guyton AC: Arterial Pressure and Hypertension. Philadel-

Intern Med 137:753, 2002.

clinical disorders of the autonomic nervous system. Ann

Goldstein DS, Robertson D, Esler M, et al: Dysautonomias:

Physiol 284:R259, 2003.

brain: new perspectives. Am J Physiol Regul Integr Comp

Felder RB, Francis J, Zhang ZH, et al: Heart failure and the

Ann N Y Acad Sci 940:500, 2001.

control of sympathetic outflow in human heart failure.

Floras JS: Arterial baroreceptor and cardiopulmonary reflex

practice. Acta Physiol Scand 177:275, 2003.

humans: translation from pathophysiology into clinical

Esler M, Lambert G, Brunner-La Rocca HP, et al: Sympa-

and arterial pressure. Ann N Y Acad Sci 940:324, 2001.

DiCarlo SE, Bishop VS: Central baroreflex resetting as a

940:395, 2001.

nerves in control of renal function. Ann N Y Acad Sci

DiBona GF: Peripheral and central interactions between

50:61, 1974.

tion in angiotensin II–induced hypertension. Circulation

Cowley AW Jr, Guyton AC: Baroreceptor reflex contribu-

medial hypothalamus. Neuroscience 126:229, 2004.

Cao WH, Fan W, Morrison SF: Medullary pathways mediat-

84: 169, 2004.

roendocrine control of body fluid metabolism. Physiol Rev

Antunes-Rodrigues J, De Castro M, Elias LLK, et al: Neu-

Nervous Regulation of the Circulation, and Rapid Control of Arterial Pressure

Chapter 18

215

References

ing specific sympathetic responses to activation of dorso-

the renin-angiotensin system and the renal sympathetic

means of increasing and decreasing sympathetic outflow

thetic nerve activity and neurotransmitter release in

training on baroreflex control of the cardiovascular

vation of the central baroreceptor pathway in angiotensin

2001.