Vascular Distensibility and Functions of the Arterial and Venous Systems pdf - The Circulation

In hemodynamic studies, it usually is much more important to know the

Vascular Compliance (or Vascular Capacitance)

tensibility of systemic arteries.

and their distensibilities are correspondingly greater, about six times the dis-

to those of the systemic circulation. But, the pulmonary arteries normally

In the pulmonary circulation, the pulmonary vein distensibilities are similar

in a vein as in an artery of comparable size.

ies, on average, are about eight times less distensible than the veins. That is, a

of the arteries are far stronger than those of the veins. Consequently, the arter-

Anatomically, the walls

Difference in Distensibility of the Arteries and the Veins.

mm Hg, or 10 per cent per mm Hg.

blood to increase its volume by 1 milliliter, the distensibility would be 0.1 per

That is, if 1 mm Hg causes a vessel that originally contained 10 millimeters of

fractional increase in volume for each millimeter of mercury rise in pressure, in

Vascular distensibility normally is expressed as the

Units of Vascular Distensibility.

blood. Therefore, the veins provide a

The most distensible by far of all the vessels are the veins. Even slight

of the tissues.

provides smooth, continuous flow of blood through the very small blood vessels

the pulsatile output of the heart and to average out the pressure pulsations. This

For example, the distensible nature of the arteries allows them to accommodate

Vascular distensibility also plays other important roles in circulatory function.

because of increased pressure but also because of decreased resistance, usually

ance. The result is increased blood flow not only

sure in blood vessels is increased, this dilates the

one example of this in Chapter 14: When the pres-

. We have seen

Vascular Distensibility

Venous Systems

Functions of the Arterial and

Vascular Distensibility and

171

A valuable characteristic of the vascular system is
that all blood vessels are distensible

blood vessels and therefore decreases their resist-

giving at least twice as much flow increase for each increase in pressure as one
might expect.

increases in venous pressure cause the veins to store 0.5 to 1.0 liter of extra

reservoir function for storing large quan-

tities of extra blood that can be called into use whenever required elsewhere in
the circulation.

accordance with the following formula:

given increase in pressure causes about eight times as much increase in blood

operate under pressures about one sixth of those in the systemic arterial system,

total

quantity of blood that can be stored in a given portion of the circulation for
each millimeter of mercury pressure rise than to know the distensibilities of the

Increase in pressure

Original volume

Vascular distensibility

Increase in volume

liters of blood, the pressure falls to zero.

100 mm Hg, but when it is

milliliters of blood, the mean arterial pressure is

small arteries, and arterioles) is

average adult person (including all the large arteries,

system, showing that when the arterial system of the

represent, respectively, the volume-pressure curves

The red and blue solid curves in Figure 15

curve.

Arterial and Venous Circulations

Volume-Pressure Curves of the

The compliance of a systemic vein is about 24 times

equal to distensibility times volume.

Compliance and distensibility are quite different. A

of the respective vascular bed; that is,

individual vessels. This value is called the

172

Unit IV

The Circulation

compliance

or capacitance

highly distensible vessel that has a slight volume may
have far less compliance than a much less distensible
vessel that has a large volume because compliance is

that of its corresponding artery because it is about 8
times as distensible and it has a volume about 3 times
as great (8

¥ 3 = 24).

A convenient method for expressing the relation of
pressure to volume in a vessel or in any portion of the
circulation is to use the so-called volume-pressure

–1

of the normal systemic arterial system and venous

filled with about 700

filled with only 400 milli-

Vascular compliance

Increase in volume

Increase in pressure

ished blood volume after serious hemorrhage.

sion. Delayed compliance in the reverse direction is one

blood when necessary, such as after too large a transfu-

spondingly decrease. This effect is a characteristic of all

to longer lengths, and their tensions corre-

vein, but then the smooth muscle

after several minutes. In other words, the volume of

decrease immediately and approaches about 9 mm Hg

removed after it is injected, the pressure begins to

5 to 12 mm Hg. Even though none of the blood is

vein that is occluded at both ends. An extra volume of

gure, the pressure is recorded in a small segment of a

2. In this

to hours. This effect is shown in Figure 15

increase in pressure, but progressive delayed stretching

The term

(Stress-Relaxation) of Vessels

sympathetic tone, especially to the veins, reduces the

highly important during hemorrhage. Enhancement of

that the body uses to increase heart pumping.

into the heart, which is one of the principal methods

transferring blood to other segments. For instance, an

dimensions of one segment of the circulation, thus

inhibition decreases the pressure at each volume.

volume of the arteries or veins, whereas sympathetic

has on the volume-pressure curves. It is evident that

Also shown in Figure 15

the Volume-Pressure Relations of the Arterial and Venous

5 mm Hg. This mainly explains why as much as one

normally ranges from 2000 to 3500 milliliters, and a

In the entire systemic venous system, the volume

change of several hundred millimeters in this volume
is required to change the venous pressure only 3 to

half liter of blood can be transfused into a healthy
person in only a few minutes without greatly altering
function of the circulation.

Effect of Sympathetic Stimulation or Sympathetic Inhibition on

Systems.

–1 are the effects that

exciting or inhibiting the vascular sympathetic nerves

increase in vascular smooth muscle tone caused by
sympathetic stimulation increases the pressure at each

Control of the vessels in this manner by the sympa-
thetics is a valuable means for diminishing the

increase in vascular tone throughout the systemic cir-
culation often causes large volumes of blood to shift

Sympathetic control of vascular capacitance is also

vessel sizes enough that the circulation continues to
operate almost normally even when as much as 25 per
cent of the total blood volume has been lost.

Delayed Compliance

“delayed compliance” means that a vessel

exposed to increased volume at first exhibits a large

of smooth muscle in the vessel wall allows the pressure
to return back toward normal over a period of minutes

–

blood is suddenly injected until the pressure rises from

blood injected causes immediate elastic distention of the

fibers of the vein begin

to “creep”

smooth muscle tissue and is called stress-relaxation,
which was explained in Chapter 8.

Delayed compliance is a valuable mechanism by

which the circulation can accommodate much extra

of the ways in which the circulation automatically
adjusts itself over a period of minutes or hours to dimin-

500

1000 1500 2000 2500 3000 3500

Pressure (mm Hg)

100

120

140

Volume (ml)

Sympathetic stimulation

Sympathetic inhibition

Arterial system

Venous system

Normal volume

pathetic nerves to the circulatory system.

systems, showing the effects of stimulation or inhibition of the sym-

Volume-pressure curves” of the systemic arterial and venous

Figure 15–1

“

affects the pulse pressure.

. Any condition of the cir-

In effect, then, pulse pressure is determined ap-

rises to as much as twice normal, because the arteries

4, the pulse pressure in old age sometimes

Figure 15

instance, as demonstrated by the middle top curves in

stroke volume of blood pumped into the arteries. For

rial system, the greater the rise in pressure for a given

sure. Conversely, the less the compliance of the arte-

systole and diastole, thus causing a greater pulse pres-

therefore, the greater the pressure rise and fall during

dated in the arterial tree with each heartbeat, and,

In general, the greater the stroke volume output, the

the heart during systole.

of the arterial tree. A third,

Two major factors affect the pulse pressure: (1) the

pulse pressure.

these two pressures, about 40 mm Hg, is called the

it is about 80 mm Hg. The difference between

pressure,

At the lowest point of each pulse, called the

is about 120 mm Hg.

systolic pressure,

pulse, called the

healthy young adult, the pressure at the top of each

3. In the

root of the aorta is shown in Figure 15

reaches the capillaries; therefore, tissue blood

occur during diastole. However, normally the compli-

neously, only during cardiac systole, and no

rial system, all of this new blood would have to

the arteries. Were it not for distensibility of the arte-

With each beat of the heart a new surge of blood

Arterial Pressure Pulsations

Vascular Distensibility and Functions of the Arterial and Venous Systems

Chapter 15

173

fills

flow

through the peripheral blood vessels almost instanta-

flow would

ance of the arterial tree reduces the pressure pulsa-
tions to almost no pulsations by the time the blood

flow is

mainly continuous with very little pulsation.

A typical record of the pressure pulsations at the

–

diastolic

stroke volume output of the heart and (2) the compli-
ance (total distensibility)
less important factor is the character of ejection from

greater the amount of blood that must be accommo-

–

have become hardened with arteriosclerosis and there-
fore are relatively noncompliant.

proximately by the ratio of stroke volume output to
compliance of the arterial tree
culation that affects either of these two factors also

Pressure (mm Hg)

Minutes

Increased

volume

Decreased

volume

Delayed

compliance

Delayed

compliance

into a venous segment and later removing the excess blood,

Effect on the intravascular pressure of injecting a volume of blood

Figure 15–2

demonstrating the principle of delayed compliance.

– 80

+20

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

Pressure (mm Hg)

Seconds

Slow rise

to peak

Sharp

incisura

Exponential diastolic decline

(may be distorted by

reflected wave)

Sharp

upstroke

(Redrawn from Opdyke DF: Fed Proc 11:734, 1952.)

Pressure pulse contour recorded from the ascending aorta.

Figure 15–3

Arteriosclerosis Aortic stenosis

160

120

160

120

Normal

Patent ductus

arteriosus

Aortic

regurgitation

patent ductus arteriosus, and aortic regurgitation.

Aortic pressure pulse contours in arteriosclerosis, aortic stenosis,

Figure 15–4

observed in the capillaries.

the arterioles, and, especially, the capillaries. In fact,

becomes progressively less in the smaller arteries,

travels into the peripheral vessels. Note especially in

Figure 15

Damping of the Pressure Pulses in the Smaller Arteries, Arteri-

forward total movement of blood volume.

small distal arteries. In the aorta, the velocity of trans-

each vascular segment, the slower the velocity, which

35 m/sec. In general, the greater the compliance of

branches, 7 to 10 m/sec; and in the small arteries, 15 to

normal aorta is 3 to 5 m/sec; in the large arterial

The velocity of pressure pulse transmission in the

in the arteries.

5. This is called

the aorta, as shown in Figure 15

mal aorta rapidly overcomes this inertia, and the wave

periphery. However, the rising pressure in the proxi-

systole, at

When the heart ejects blood into the aorta during

to the Peripheral Arteries

Transmission of Pressure Pulses

valve to close.

to zero between heartbeats. Also, there is no incisura

cle. As a result, the aortic pressure can fall all the way

beat, the blood that has just been pumped into the

will not close completely. Therefore, after each heart-

, the aortic valve is absent or

into the pulmonary artery and lung blood vessels, thus

, one half or more of the

ow outward through the stenotic valve.

cantly, and the aortic pres-

, the diameter of the aortic valve

Figure 15

sus, and aortic regurgitation, each of which is shown in

among these are aortic stenosis, patent ductus arterio-

to altering the pulse pressure. Especially distinctive

Abnormal Pressure Pulse Contours

174

Unit IV

The Circulation

Some conditions of the circulation also cause abnor-
mal contours of the pressure pulse wave in addition

–4.

In aortic stenosis

opening is reduced signifi
sure pulse is decreased significantly because of dimin-
ished blood fl

In patent ductus arteriosus

blood pumped into the aorta by the left ventricle flows
immediately backward through the wide-open ductus

allowing the diastolic pressure to fall very low before
the next heartbeat.

In aortic regurgitation

aorta flows immediately backward into the left ventri-

in the aortic pulse contour because there is no aortic

first only the proximal portion of the aorta

becomes distended because the inertia of the blood
prevents sudden blood movement all the way to the

front of distention spreads farther and farther along

–

trans-

mission of the pressure pulse

explains the slow transmission in the aorta and the
much faster transmission in the much less compliant

mission of the pressure pulse is 15 or more times the
velocity of blood flow because the pressure pulse is
simply a moving wave of pressure that involves little

oles, and Capillaries.

–6 shows typical changes

in the contours of the pressure pulse as the pulse

the three lower curves that the intensity of pulsation

only when the aortic pulsations are extremely large or
the arterioles are greatly dilated can pulsations be

Wave fronts

aorta.

Progressive stages in transmission of the pressure pulse along the

Figure 15–5

Time (seconds)

Incisura

Radial artery

Arteriole

Capillary

Diastole

Proximal aorta

Femoral artery

Systole

toward the smaller vessels.

Changes in the pulse pressure contour as the pulse wave travels

Figure 15–6

The mean arterial pressure is the

pressure, as previously explained.

. The

hardening of the arteries, which itself is an end-stage

occurs beyond the age of 60 years. This results from

age, especially after the age of 50 years.

term regulation of arterial pressure; and it is well

control mechanisms. We shall see in Chapter 19 that

ages. The progressive increase in pressure with age

Figure 15

Normal Arterial Pressures as Measured by the Auscultatory

inside the arteries.

systolic and diastolic pressures is not entirely accurate,

pressure. The auscultatory method for determining

quality; this pressure is about equal to the diastolic

when the Korotkoff sounds change to the muf

in cuff pressure. One notes the manometer pressure

ed quality, then dis-

squeezed artery) is no longer present. Therefore, the

during diastole, which means that the basic factor

to equal diastolic pressure, the artery no longer closes

nally, when the pressure in the cuff falls

quality. Then,

Korotkoff sounds change in quality, having less of the

As the pressure in the cuff is lowered still more, the

about equal to the systolic pressure.

as these sounds begin to be heard, the pressure level

cubital artery in synchrony with the heartbeat. As soon

beneath the cuff during the peak of systolic pressure,

tolic pressure, blood begins to slip through the artery

Just as soon as the pressure in the cuff falls below sys-

artery. But then the cuff pressure gradually is reduced.

Therefore, no Korotkoff sounds are heard in the lower

lower artery during any part of the pressure cycle.

pressure is higher than systolic pressure, the brachial

above arterial systolic pressure. As long as this cuff

method, the pressure in the cuff is

stethoscope.

The jet causes turbulence in the vessel beyond the cuff,

debated, but they are believed to be caused mainly

The exact cause of Korotkoff sounds is still

Korotkoff sounds

pulsation. These sounds are called

arterial pressure cycle, a sound then is heard with each

with the stethoscope. However, when the cuff pressure

artery, no sounds are heard from the antecubital artery

upper arm. As long as the cuff continues to compress

pressures. A stethoscope is placed over the antecubital

Figure 15

Auscultatory Method.

systolic and diastolic pressures by indirect means,

studies are necessary. Instead, the clinician determines

routine pressure measurements in human patients,

Clinical Methods for Measuring

increase in pressure. Therefore, in effect,

the more compliant a vessel, the greater the quantity

occur. The compliance damps the pulsations because

greater the resistance, the more dif

front to distend the next segment of the vessel; the

The resistance damps the pulsations because a small

ment in the vessels and (2) compliance of the vessels.

cause of this is twofold: (1) resistance to blood move-

of the pressure pulses. The

This progressive diminution of the pulsations in the

Vascular Distensibility and Functions of the Arterial and Venous Systems

Chapter 15

175

periphery is called damping

amount of blood must flow forward at the pulse wave

ficult it is for this to

of blood required at the pulse wave front to cause an

the degree of

damping is almost directly proportional to the product
of resistance times compliance.

Systolic and Diastolic Pressures

It is not reasonable to use pressure recorders that
require needle insertion into an artery for making

although these are used on occasion when special

usually by the auscultatory method.

–7 shows the auscultatory

method for determining systolic and diastolic arterial

artery and a blood pressure cuff is inflated around the

the arm with too little pressure to close the brachial

is great enough to close the artery during part of the

by blood jetting through the partly occluded vessel.

and this sets up the vibrations heard through the

In determining blood pressure by the auscultatory

first elevated well

artery remains collapsed so that no blood jets into the

and one begins to hear tapping sounds from the ante-

indicated by the manometer connected to the cuff is

tapping quality and more of a rhythmical and harsher

causing the sounds (the jetting of blood through a

sounds suddenly change to a muffl
appear entirely after another 5- to 10-millimeter drop

fled

but it usually gives values within 10 per cent of those
determined by direct catheter measurement from

Method.

–8 shows the approximate normal

systolic and diastolic arterial pressures at different

results from the effects of aging on the blood pressure

the kidneys are primarily responsible for this long-

known that the kidneys exhibit definitive changes with

A slight extra increase in systolic pressure usually

result of atherosclerosis

final effect is a bounding

systolic pressure with considerable increase in pulse

Mean Arterial Pressure.

average of the arterial pressures measured millisecond

120

100

Sounds

150

mm Hg

100

pressures.

Auscultatory method for measuring systolic and diastolic arterial

Figure 15–7

and because of this, the pressure in the more

flow,

to an ovoid or slitlike state. For these reasons, the

sure, so that they usually are at least partially collapsed

neck causes these veins to collapse. Finally, veins

Also, the pressure in the neck veins often falls so low

rst rib.

points. For instance, the veins from the arms are com-

rounding tissues, so that blood

9, most of the large veins that enter

shown in Figure 15

zero and is of almost no importance. However, as

Venous Pressure

Venous Resistance and Peripheral

severe hemorrhage.

peripheral vessels is greatly depressed, such as after

rounds the heart. The right atrial pressure approaches

This is also the pressure in the chest cavity that sur-

5 mm Hg below atmospheric pressure.

The lower limit to the right atrial pressure is usually

from the peripheral vessels.

or (2) after massive transfusion of blood, which greatly

abnormal conditions, such as (1) serious heart failure

body. It can increase to 20 to 30 mm Hg under very

is about 0 mm Hg,

The

peripheral vessels. Therefore, we will discuss regula-

the tendency for blood to

The same factors that regulate right atrial pressure

ow of blood from the arteries into the veins.

venous pressures, and (3) dilatation of the arterioles,

blood volume, (2) increased large vessel tone through-

elevates the right atrial pressure. Some of the factors

pressure. Also, any effect that causes rapid in

versely, weakness of the heart elevates the right atrial

strongly, the right atrial pressure decreases. Con-

the tendency for blood to flow from the peripheral veins

between (1) the ability of the heart to pump blood out

Right atrial pressure is regulated by a balance

central venous pressure.

atrium of the heart; therefore, the pressure in the right

the veins and what determines the pressure.

To understand the various functions of the veins, it is

and Peripheral Venous Pressures

Pressure (Central Venous Pressure)

Venous Pressures—Right Atrial

, and they even help to regulate cardiac output,

der of the circulation. The peripheral veins can also

the circulation. Especially important, they are capable

For years, the veins were considered to be nothing

Veins and Their Functions

to the systolic pressure.

in Figure 15

pressure and 40 per cent by the systolic pressure. Note

of the cardiac cycle. Therefore, the mean arterial pres-

by millisecond over a period of time. It is not equal to

176

Unit IV

The Circulation

the average of systolic and diastolic pressure because
the arterial pressure remains nearer to diastolic pres-
sure than to systolic pressure during the greater part

sure is determined about 60 per cent by the diastolic

–8 that the mean pressure (solid green

line) at all ages is nearer to the diastolic pressure than

more than passageways for flow of blood to the heart,
but it has become apparent that they perform other
special functions that are necessary for operation of

of constricting and enlarging and thereby storing
either small or large quantities of blood and making
this blood available when it is required by the remain-

propel blood forward by means of a so-called venous
pump
an exceedingly important function that is described in
detail in Chapter 20.

first necessary to know something about pressure in

Blood from all the systemic veins flows into the right

atrium is called the

of the right atrium and ventricle into the lungs and (2)

into the right atrium. If the right heart is pumping

flow of

blood into the right atrium from the peripheral veins

that can increase this venous return (and thereby
increase the right atrial pressure) are (1) increased

out the body with resultant increased peripheral

which decreases the peripheral resistance and allows
rapid fl

also enter into the regulation of cardiac output
because the amount of blood pumped by the heart
depends on both the ability of the heart to pump and

flow into the heart from the

tion of right atrial pressure in much more depth in
Chapter 20 in connection with regulation of cardiac
output.

normal right atrial pressure

which is equal to the atmospheric pressure around the

increases the total blood volume and causes excessive
quantities of blood to attempt to flow into the heart

about

-3 to -

these low values when the heart pumps with excep-
tional vigor or when blood flow into the heart from the

Large veins have so little resistance to blood flow when
they are distended that the resistance then is almost

–

the thorax are compressed at many points by the sur-

flow is impeded at these

pressed by their sharp angulations over the fi

that the atmospheric pressure on the outside of the

coursing through the abdomen are often compressed
by different organs and by the intra-abdominal pres-

large veins do usually offer some resistance to blood

Pressure (mm Hg)

150

100

200

Age (years)

Systolic

Mean

Diastolic

age. The shaded areas show the approximate normal ranges.

Changes in systolic, diastolic, and mean arterial pressures with

Figure 15–8

rib. Thus, if the gravitational difference between the

of the subclavian vein as it passes over this rib. The

6 mm Hg because of compression

In the arm veins, the pressure at the level of the top

90 mm Hg.

the heart and the feet. The venous pressures at other

90 mm Hg simply because of the gravita-

, the pressure in the veins of the feet

mulate at this point. However, in an adult

remains about 0 mm Hg because the heart pumps into

a person is standing, the pressure in the right atrium

10. When

blood in the vessels, as shown in Figure 15

millimeters of distance below the surface. This pres-

pressure, but the pressure rises 1 mm Hg for each 13.6

In any body of water that is exposed to air, the pres-

Pressure

Effect of Gravitational Pressure on Venous

20 mm Hg.

20 mm Hg, the

ow from the legs to the heart. Thus,

above

abdominal pressure does rise, the pressure in the veins

) in the abdominal cavity. When the intra-

pregnancy, large tumors, or excessive

30 mm Hg as a result of

6 mm Hg,

The pressure in the abdominal cavity of a recum-

Effect of Intra-abdominal Pressure on Venous Pressures of the

of heart failure.

Hg, one often

6 mm

eral venous pressure in the limbs and elsewhere.

the right atrial pressure rises still further, the addi-

6 mm Hg. Then, as

the large veins. This enlarges the veins, and even the

normal value of 0 mm Hg, blood begins to back up in

When the right atrial pressure rises above its

Effect of High Right Atrial Pressure on Peripheral Venous Pres-

6 mm Hg greater than the right atrial pressure.

Vascular Distensibility and Functions of the Arterial and Venous Systems

Chapter 15

177

peripheral small veins in a person lying down is usually
+4 to +

sure.

collapse points in the veins open up when the right
atrial pressure rises above

+4 to +

tional increase causes a corresponding rise in periph-

Because the heart must be weakened greatly to cause
a rise in right atrial pressure as high as

+4 to +

finds that the peripheral venous pres-

sure is not noticeably elevated even in the early stages

Leg.

bent person normally averages about

but it can rise to

+15 to +

fluid (called

“ascites”

of the legs must rise

the abdominal pressure

before the abdominal veins will open and allow
the blood to fl
if the intra-abdominal pressure is

lowest possible pressure in the femoral veins is also
+

sure at the surface of the water is equal to atmospheric

sure results from the weight of the water and there-
fore is called gravitational pressure or hydrostatic
pressure.

Gravitational pressure also occurs in the vascular

system of the human being because of weight of the

–

the arteries any excess blood that attempts to accu-

who is stand-

ing absolutely still
is about

tional weight of the blood in the veins between

levels of the body are proportionately between 0 and

rib is usually about

gravitational pressure down the length of the arm then
is determined by the distance below the level of this

4 mm Hg

Atmospheric
pressure
collapse in neck

Axillary collapse

Abdominal
pressure
collapse

Intrathoracic
pressure =

Rib collapse

Compression points that tend to collapse the veins entering the

Figure 15–9

thorax.

9 0 mm

4 0 mm

3 5 mm

2 2 mm

8 mm

6 mm

0 mm

1 0 mm

Sagittal sinus

Effect of gravitational pressure on the venous pressures through-

Figure 15–10

out the body in the standing person.

ulcerates. The best treatment for such a condition is

cells, so that the muscles become painful and weak,

illaries causes constant edema in the legs. The edema

become very high, and leakage of

than a few minutes, the venous and capillary pressures

Whenever people with varicose veins stand for more

the entire leg, particularly the lower leg.

which are characterized by large,

varicose veins,

function of the valves entirely. Thus, the person devel-

because of failure of the venous pump; this further

no longer close completely. When this develops, the

not increase in size. Therefore, the lea

cross-sectional areas, but the lea

most of the time. Stretching the veins increases their

months, as occurs in pregnancy or when one stands

is especially true when the veins have been over-

or sometimes even are destroyed. This

The

Venous Valve Incompetence Causes “Varicose” Veins.

minutes of standing absolutely still, as often occurs

Indeed, 10 to 20 per cent of the blood volume can be

the legs swell, and the blood volume diminishes.

circulatory system into the tissue spaces. As a result,

ies also increase greatly, causing

Hg in about 30 seconds. The pressures in the capillar-

legs increase to the full gravitational value of 90 mm

does not work, and the venous pressures in the lower

If a person stands perfectly still, the venous pump

20 mm Hg.

circumstances, the venous pressure in the feet of a

pump,

blood is propelled toward the heart. This pumping

tenses the leg muscles, a certain amount of venous

quently, every time a person moves the legs or even

ow can be only toward the heart. Conse-

11, are arranged so that the direction of venous

veins. But the valves in the veins, shown in Figure

the muscles, and this squeezes the blood out of the

However, every time one moves the legs, one tightens

90 mm Hg in a standing adult.

Were it not for valves in the veins, the gravitational

Their Effects on Venous Pressure

Venous Valves and the “Venous Pump”:

vessels.

rial pressure is 100 mm Hg, this generally means that

190 mm Hg. Therefore, when one states that the arte-

has a mean arterial pressure of 100 mm Hg at the level

effects in the veins. For instance, a standing person who

the peripheral arteries and capillaries, in addition to its

The gravitational factor also affects pressures in

death can ensue.

downward to cause air embolism in the heart, and

ately into the venous system; the air may even pass

is opened during surgery, air can be sucked immedi-

and the base of the skull. Therefore, if the sagittal sinus

10 mm Hg because of

ing position, the venous pressure in the sagittal sinus

they cannot collapse. Consequently,

The veins inside the skull, on the other hand, are in

sure back to zero.

zero collapses the veins still more, which further

any tendency for the neck vein pressure to fall below

ow of the blood. Conversely,

that any tendency for the pressure to rise above this

at zero along their entire extent. The reason for this is

atmospheric pressure on the outside of the neck. This

The neck veins of a person standing upright collapse

35 mm Hg pressure in the veins of

caused by compression of the vein as it crosses the rib,

6 mm Hg pressure

29 mm Hg, this grav-

178

Unit IV

The Circulation

level of the rib and the hand is

itational pressure is added to the

making a total of

the hand.

almost completely all the way to the skull because of

collapse causes the pressure in these veins to remain

level opens the veins and allows the pressure to fall
back to zero because of fl

increases their resistance and again returns the pres-

a noncollapsible chamber (the skull cavity) so that

negative pressure

can exist in the dural sinuses of the head; in the stand-

at the top of the brain is about

the hydrostatic “suction” between the top of the skull

Effect of the Gravitational Factor on Arterial and Other Pres-
sures.

of the heart has an arterial pressure in the feet of about

this is the pressure at the gravitational level of the
heart but not necessarily elsewhere in the arterial

pressure effect would cause the venous pressure in the
feet always to be about

the muscles and compresses the veins in or adjacent to

15–
blood fl

system is known as the “venous pump” or “muscle

” and it is efficient enough that under ordinary

walking adult remains less than

fluid to leak from the

lost from the circulatory system within the 15 to 30

when a soldier is made to stand at rigid attention.

valves of the venous system frequently become
“incompetent”

stretched by excess venous pressure lasting weeks or

flets of the valves do

flets of the valves

pressure in the veins of the legs increases greatly

increases the sizes of the veins and finally destroys the

ops “

”

bulbous protrusions of the veins beneath the skin of

fluid from the cap-

in turn prevents adequate diffusion of nutritional
materials from the capillaries to the muscle and skin

and the skin frequently becomes gangrenous and

Valve

Deep vein

Perforating
vein

Superficial
vein

Venous valves of the leg.

Figure 15–11

The sinuses can swell the same as any other part

pulp.

areas for storing blood: the

Figure 15

decrease to low values.

milliliters of blood; the lungs can contribute another

heart, for instance, shrinks during sympathetic stimu-

system, must also be considered blood reservoirs. The

lungs,

several hundred milliliters. The

tribute as much as 300 milliliters; and (4) the

large abdominal veins,

tion; (3) the

liver,

of blood into other areas of the circulation; (2) the

These

c blood reservoirs.

Specific Blood Reservoirs.

variable reservoir function of the veins.

total blood volume has been lost, the circulatory

blood. Indeed, even after as much as 20 per cent of the

veins, causing them to constrict. This takes up much of

These in turn elicit nerve signals from the brain and

areas of the circulation, as discussed in Chapter 18.

pressure begins to fall, nervous signals are elicited

When blood is lost from the body and the arterial

are so compliant, it is said that the venous system

the veins. For this reason and also because the veins

As pointed out in Chapter 14, more than 60 per cent

Blood Reservoir Function of the Veins

zero pres-

chest thickness in front of the back. This is the

When a person is lying on his or her back, the tricus-

tricuspid valve.

the heart acts as a feedback regulator of pressure

level again rises to the normal value. In other words,

adequately, its pumping decreases, and blood dams up

versely, if the pressure falls, the right ventricle fails to

cuspid valve back toward the normal mean value. Con-

than usual, causing the heart to pump blood more

above normal, the right ventricle

The reason for lack of gravitational effects at the

discussed in this text are referred to this level, which is

12. Therefore, all circulatory pressure measurements

cuspid valve, as shown by the crossed axes in Figure

than 1 to 2 mm Hg. This is at or near the level of the tri-

pressure is referred. There is one point in the circula-

sure as being 100 mm Hg, but we have not stated the

right atrial pressure as being 0 mm Hg and arterial pres-

In discussions up to this point, we often have spoken of

Circulatory Pressures

Pressure Reference Level for Measuring Venous and Other

assessment of heart pumping ability.

right atrium. Pressures measured through such

to a pressure recorder. The only means by which

Venous pressure can also be measured with ease by

Direct Measurement of Venous Pressure and

15 mm Hg

veins of the neck begin to protrude; and at

10 mm Hg, the lower

person. However, when the right atrial pressure be-

neck veins. For instance, in the sitting position, the neck

The venous pressure

Clinical Estimation of Venous Pressure.

its sequelae.

as the heart. Tight binders on the legs also can be of

Vascular Distensibility and Functions of the Arterial and Venous Systems

Chapter 15

179

continual elevation of the legs to a level at least as high

considerable assistance in preventing the edema and

often can be estimated by simply observing the degree
of distention of the peripheral veins—especially of the

veins are never distended in the normal quietly resting

comes increased to as much as

atrial pressure essentially all the veins in the neck
become distended.

Right Atrial Pressure

inserting a needle directly into a vein and connecting it

right

atrial pressure can be measured accurately is by insert-
ing a catheter through the peripheral veins and into the

central

venous catheters are used almost routinely in some types
of hospitalized cardiac patients to provide constant

gravitational level in the circulatory system to which this

tory system at which gravitational pressure factors
caused by changes in body position of a healthy person
usually do not affect the pressure measurement by more

15–

called the reference level for pressure measurement.

tricuspid valve is that the heart automatically prevents

significant gravitational changes in pressure at this point
in the following way:

If the pressure at the tricuspid valve rises slightly

fills to a greater extent

rapidly and therefore to decrease the pressure at the tri-

fill

in the venous system until the pressure at the tricuspid

at the

pid valve is located at almost exactly 60 per cent of the

sure reference level for a person lying down.

of all the blood in the circulatory system is usually in

serves as a blood reservoir for the circulation.

from the carotid sinuses and other pressure-sensitive

spinal cord mainly through sympathetic nerves to the

the slack in the circulatory system caused by the lost

system often functions almost normally because of this

Certain portions of the circu-

latory system are so extensive and/or so compliant that
they are called “specifi

”

include (1) the spleen, which sometimes can decrease
in size sufficiently to release as much as 100 milliliters

the sinuses of which can release several hundred

milliliters of blood into the remainder of the circula-

which can con-

venous

plexus beneath the skin, which also can contribute

heart and the

although not parts of the systemic venous reservoir

lation and in this way can contribute some 50 to 100

100 to 200 milliliters when the pulmonary pressures

The Spleen as a Reservoir for Storing Red Blood Cells.

–13 shows that the spleen has two separate

venous sinuses and the

of the venous system and store whole blood.

Right ventricle

Right atrium

Natural reference
point

Reference point for circulatory pressure measurement (located

Figure 15–12

near the tricuspid valve).

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avidly.

Also, in many chronic infectious processes, the spleen

rapidly remove debris, bacteria, parasites, and so forth.

tious agents, the reticuloendothelial cells of the spleen

sinuses of the liver. When the blood is invaded by infec-

cleansing system for the blood, acting in concert with a

lined with similar cells. These cells function as part of a

reticuloendothelial cells, and the venous sinuses are

The pulp of the spleen contains many large phagocytic

nutrients, often for making new blood cells.

by the reticuloendothelial cells of the spleen, and the

nal demise in the spleen. After the cells rupture, the

would not withstand the trauma. For this reason, many

Therefore, it is to be expected that fragile red blood cells

entering the sinuses undergo thorough squeezing.

s immune system, described in Chapter 34.

manufactured in the lymph nodes. They are part of the

white pulp.

blood cells, which collectively are called the

to contract. As much as 50 milliliters of concentrated

These can then be expelled into the general

blood cells.

sequence, the red pulp of the spleen is a

sinuses and then into the general circulation. As a con-

trabeculae, while the plasma

The red cells are trapped by the

red pulp.

that whole blood, including the red blood cells, oozes

In the splenic pulp, the capillaries are so permeable

180

Unit IV

The Circulation

through the capillary walls into a trabecular mesh,
forming the

flows on into the venous

special reser-

voir that contains large quantities of concentrated red

circulation whenever the sympathetic nervous system
becomes excited and causes the spleen and its vessels

red blood cells can be released into the circulation,
raising the hematocrit 1 to 2 per cent.

In other areas of the splenic pulp are islands of white

Here lymphoid cells are manufactured similar to those

body’

Blood-Cleansing Function of the Spleen—Removal of Old Cells

Blood cells passing through the splenic pulp before

of the red blood cells destroyed in the body have their
fi
released hemoglobin and the cell stroma are digested

products of digestion are mainly reused by the body as

Reticuloendothelial Cells of the Spleen

similar system of reticuloendothelial cells in the venous

enlarges in the same manner that lymph nodes enlarge
and then performs its cleansing function even more

References

fluence of

cal signifi

“open”

“closed”

“set up”

O’

of Hypertension recommendations for conventional,

tives on arterial stiffness and pulse pressure in hyperten-

2003.

Pulp

Vein

Artery

Capillaries

Venous sinuses

Functional structures of the spleen. (Courtesy of Dr. Don W.

Figure 15–13

Fawcett, Montana.)