Chapter 19
The Cardiovascular System: Blood Vessels
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19
MAP and pulse pressure both decline with increasing dis-
tance from the heart. Te MAP loses ground to the never-
ending friction between the blood and the vessel walls, and the
pulse pressure is gradually phased out in the less elastic muscu-
lar arteries, where elastic rebound of the vessels ceases to occur.
At the end of the arterial tree, blood flow is steady and the pulse
pressure has disappeared.
Capillary Blood Pressure
As Figure 19.6 shows, by the time blood reaches the capillaries,
blood pressure has dropped to approximately 35 mm Hg and
by the end of the capillary beds is only around 17 mm Hg. Such
low capillary pressures are desirable because (1) capillaries are
fragile and high pressures would rupture them, and (2) most
capillaries are extremely permeable and thus even the low capil-
lary pressure can force solute-containing fluids (filtrate) out of
the bloodstream into the interstitial space.
As we describe later in this chapter, these fluid flows are im-
portant for continuously refreshing the interstitial fluid.
Venous Blood Pressure
Unlike arterial pressure, which pulsates with each contraction
of the le± ventricle, venous blood pressure is steady and changes
very little during the cardiac cycle. Te pressure gradient in the
veins, from venules to the termini of the venae cavae, is only
about 15 mm Hg (that from the aorta to the ends of the arteri-
oles is about 60 mm Hg).
Te difference in pressure between an artery and a vein be-
comes very clear when the vessels are cut. If a vein is cut, the blood
flows evenly from the wound, but a lacerated artery spurts blood.
Te very low pressure in the venous system reflects the cumula-
tive effects of peripheral resistance, which dissipates most of the
energy of blood pressure (as heat) during each circuit.
resistance to blood flow. However, as long as a pressure gradi-
ent exists, no matter how small, blood continues to flow until it
completes the circuit back to the heart.
Arterial Blood Pressure
Arterial blood pressure reflects two factors: (1) how much the
elastic arteries close to the heart can stretch (their
compliance
or
distensibility
) and (2) the volume of blood forced into them at
any time. If the amounts of blood entering and leaving the elas-
tic arteries in a given period were equal, arterial pressure would
be constant. Instead, as Figure 19.6 reveals, blood pressure is
pulsatile
—it rises and falls in a regular fashion—in the elastic
arteries near the heart.
As the le± ventricle contracts and expels blood into the aorta,
it imparts kinetic energy to the blood, which stretches the elas-
tic aorta as aortic pressure reaches its peak. Indeed, if the aorta
were opened during this period, blood would spurt upward 5 or
6 feet! Tis pressure peak generated by ventricular contraction
is called the
systolic pressure
(sis-tah
9
lik) and averages 120 mm
Hg in healthy adults. Blood moves forward into the arterial bed
because the pressure in the aorta is higher than the pressure in
the more distal vessels.
During diastole, the aortic valve closes, preventing blood
from flowing back into the heart. Te walls of the aorta (and
other elastic arteries) recoil, maintaining sufficient pressure to
keep the blood flowing forward into the smaller vessels. During
this time, aortic pressure drops to its lowest level (approximately
70 to 80 mm Hg in healthy adults), called the
diastolic pressure
(di-as-tah
9
lik). You can picture the elastic arteries as pressure
reservoirs that operate as auxiliary pumps to keep blood cir-
culating throughout the period of diastole, when the heart is
relaxing. Essentially, the volume and energy of blood stored in
the elastic arteries during systole are given back during diastole.
Te difference between the systolic and diastolic pressures is
called the
pulse pressure
. It is felt as a throbbing pulsation in an
artery (a
pulse
) during systole, as ventricular contraction forces
blood into the elastic arteries and expands them. Increased
stroke volume and faster blood ejection from the heart (a re-
sult of increased contractility) raise pulse pressure
temporarily
.
Atherosclerosis chronically increases pulse pressure because the
elastic arteries become less stretchy.
Because aortic pressure fluctuates up and down with each
heartbeat, the important pressure figure to consider is the
mean
arterial pressure (MAP)
—the pressure that propels the blood
to the tissues. Diastole usually lasts longer than systole, so MAP
is not simply the value halfway between systolic and diastolic
pressures. Instead, it is roughly equal to the diastolic pressure
plus one-third of the pulse pressure.
For a person with a systolic blood pressure of 120 mm Hg
and a diastolic pressure of 80 mm Hg:
Aorta
Arteries
Arterioles
Capillaries
Venules
Veins
Venae cavae
Blood pressure (mm Hg)
Systolic pressure
Mean pressure
Diastolic
pressure
0
20
40
60
80
100
120
Figure 19.6
Blood pressure in various blood vessels of the
systemic circulation.
MAP
5
diastolic pressure
1
pulse pressure
3
MAP
5
80 mm Hg
1
40 mm Hg
5
93 mm Hg
3
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