582
UNIT FOUR
Factors That Infl
uence Arterial Blood
Pressure
Arterial pressure depends on a variety of factors. These
include heart action which includes stroke volume and heart
rate; blood volume; resistance to fl
ow; and blood viscosity
(f
g. 15.34)
.
Heart Action
Each ventricular contraction determines the volume of
blood that enters the arterial system, called the
stroke vol-
ume.
It equals about 70 milliliters in an average-weight
male at rest. The volume discharged from the ventricle per
minute is called the
cardiac output.
It is calculated by mul-
tiplying the stroke volume by the heart rate in beats per
minute. (Cardiac output = stroke volume
×
heart rate.) For
example, if the stroke volume is 70 milliliters and the heart
rate is 72 beats per minute, the cardiac output is 5,040 mil-
liliters per minute.
but the pressure begins to drop almost immediately as the
contraction ends, and the arterial walls recoil. This alternate
expanding and recoiling of the arterial wall can be felt as a
pulse
in an artery that runs close to the surface.
Figure 15.33
shows several sites where a pulse can be detected. The radial
artery, for example, courses near the surface at the wrist and
is commonly used to sense a person’s radial pulse.
The radial pulse rate is equal to the rate at which the
left ventricle contracts, and for this reason, it can be used to
determine heart rate. A pulse can also refl
ect blood pressure,
because an elevated pressure produces a pulse that feels
strong and full, whereas a low pressure produces a pulse
that is weak and easily compressed. Clinical Application 15.3
describes how to measure arterial blood pressure.
PRACTICE
42
Distinguish between systolic and diastolic blood pressure.
43
Which cardiac event causes systolic pressure? Diastolic pressure?
44
What causes a pulse in an artery?
S
ystemic arterial blood pressure usually
is measured using an instrument called
a sphygmomanometer (sfig
mo-mah-
nom
e
˘-ter) (fig. 15G). This device consists of an
inF
atable cu±
connected by tubing to a compress-
ible bulb and a pressure gauge. The bulb is used to
pump air into the cu±
, and a rise in pressure is indi-
cated on the pressure gauge. The pressure in the
cuff is expressed in millimeters of mercury (mm
Hg) based on older equipment that used a glass
tube containing a column of mercury in place of a
pressure gauge. The older devices have been dis-
continued because of the danger of mercury.
To measure arterial blood pressure, the cuff
of the sphygmomanometer is usually wrapped
around the arm so that it surrounds the brachial
artery. Air is pumped into the cuff until the cuff
pressure exceeds the pressure in that artery.
The vessel is squeezed closed and its blood flow
stopped. At this moment, if the diaphragm of a
stethoscope is placed over the brachial artery
at the distal border of the cu±
, no sounds can be
heard from the vessel because the blood flow is
interrupted. As air is slowly released from the cu±
,
the air pressure inside it decreases. When the cu±
pressure is approximately equal to the systolic
blood pressure in the brachial artery, the artery
opens enough for a small amount of blood to
spurt through. This movement produces a sharp
Aneroid gauge
Rubber cuff
Air control valve
Bulb
15.3
CLINICAL APPLICATION
Measurement of Arterial Blood Pressure
FIGURE 15G
A sphygmomanometer is used to measure arterial blood pressure. The use of
the column of mercury is the most accurate measurement, but due to environmental concerns,
it has been replaced by alternative gauges and digital readouts.
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