Cardiovascular Physiology
387
averaged over the entire cardiac cycle. We can say mean “arte-
rial” pressure without specifying which artery we are referring
to because the aorta and other large arteries have such large
diameters that they offer only negligible resistance to fl ow,
and the mean pressures are therefore similar everywhere in the
large arteries.
One additional important point should be made: We have
stated that arterial compliance is an important determinant of
pulse pressure, but for complex reasons, compliance does not sig-
nifi cantly infl uence the mean arterial pressure. Thus, for example,
a person with a low arterial compliance (due to arteriosclerosis)
but an otherwise normal cardiovascular system will have a large
pulse pressure but a normal mean arterial pressure. The determi-
nants of mean arterial pressure are described in Section D. The
method for measuring blood pressure is described next.
Measurement of Systemic Arterial Pressure
Both systolic and diastolic blood pressure are readily measured
in human beings with the use of a device called a sphygmo-
manometer. An infl atable cuff containing a pressure gauge is
wrapped around the upper arm, and a stethoscope is placed
in a spot on the arm just below the cuff where the brachial
artery lies.
The cuff is then infl ated with air to a pressure greater
than systolic blood pressure (
Figure 12–32
). The high pres-
sure in the cuff is transmitted through the tissue of the arm
and completely compresses the artery under the cuff, thereby
preventing blood fl ow through the artery. The air in the cuff
is then slowly released, causing the pressure in the cuff and
on the artery to drop. When cuff pressure has fallen to a value
just below the systolic pressure, the artery opens slightly and
allows blood fl ow for a brief time at the peak of systole. During
this interval, the blood fl ow through the partially compressed
artery occurs at a very high velocity because of the small open-
ing and the large pressure difference across the opening. The
high-velocity blood fl
ow is turbulent and, therefore, produces
vibrations (called
Korotkoff’s sounds
) that can be heard
through the stethoscope. Thus, the pressure at which sounds
are fi rst heard as the cuff pressure decreases is identifi ed as the
systolic blood pressure.
As the pressure in the cuff decreases further, the dura-
tion of blood fl ow through the artery in each cycle becomes
longer. When the cuff pressure reaches the diastolic blood
pressure, all sound stops because fl ow is now continuous and
nonturbulent through the open artery. Thus, diastolic pressure
is identifi ed as the cuff pressure at which sounds disappear.
No sound; cuff
pressure above
systolic pressure;
artery completely
occluded during
cycle
Cuff pressure
just below
systolic pressure;
first sounds heard;
soft, tapping, and
intermittent
Sounds loud,
tapping, and
intermittent
Low muffled
sound lasting
continuously
Cuff pressure
below diastolic
pressure; thus
vessel is always
open; no turbulence,
no sound
Time
120
80
Arterial pressure (mmHg)
Cuff pressure (mmHg)
(c)
(d)
(e)
(b)
(a)
(a)
Cuff pressure
Sound
Arterial pressure
Period of turbulent
flow through
constricted vessel
120
80
100
(b)
(c)
(d)
(e)
Figure 12–32
Sounds heard through a
stethoscope as the cuff pressure of
a sphygmomanometer is gradually
lowered. Sounds are fi rst heard at
systolic pressure, and they disappear
at diastolic pressure.
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