386
Chapter 12
peak ventricular ejection is called
systolic pressure (SP).
The
minimum arterial pressure occurs just before ventricular ejec-
tion begins and is called
diastolic pressure (DP).
Arterial pres-
sure is generally recorded as systolic/diastolic—that is, 120/80
mmHg in our example (see
Figure 12–31b
for average values
at different ages in the population of the United States).
The difference between systolic pressure and diastolic
pressure (120 – 80 = 40 mmHg in the example) is called the
pulse pressure.
It can be felt as a pulsation or throb in the
arteries of the wrist or neck with each heartbeat. During dias-
tole, nothing is felt over the artery, but the rapid rise in pres-
sure at the next systole pushes out the artery wall, and it is this
expansion of the vessel that produces the detectable throb.
The most important factors determining the magni-
tude of the pulse pressure are (1) stroke volume, (2) speed
of ejection of the stroke volume, and (3) arterial compliance.
Specifi cally, the pulse pressure a ventricular ejection produces
is greater if the volume of blood ejected increases, if the speed
at which it is ejected increases, or if the arteries are less com-
pliant. This last phenomenon occurs in
arteriosclerosis,
the
stiffening of the arteries that progresses with age and accounts
for the increasing pulse pressure seen so often in older people
(see Figure 12–31b).
It is evident from Figure 12–31a that arterial pressure
is continuously changing throughout the cardiac cycle. The
average, or
mean, arterial pressure (MAP)
in the cycle is
not merely the value halfway between systolic pressure and
diastolic pressure, because diastole lasts longer than systole.
The true mean arterial pressure can be obtained by complex
methods, but at a typical resting heart rate, it is approximately
equal to the diastolic pressure plus one-third of the pulse pres-
sure (SP – DP), largely because diastole lasts about twice as
long as systole:
MAP = DP +
1
3
(SP – DP)
Thus, in our example:
MAP = 80 +
1
3
(40) = 93 mmHg
The MAP is the most important of the pressures described
because it is the pressure driving blood into the tissues
Aortic or pulmonary valve
Entry
from
heart
Arteries
Exit via
arterioles
Systole
Diastole
Figure 12–31
(a) Typical arterial pressure fl uctuations during the cardiac cycle.
(b) Changes in arterial pressure with age in the U.S. population.
Adapted from National Institutes of Health Publication #04-5230, August 2004.
Figure 12–31
physiological
inquiry
At an elevated heart rate, the amount of time spent in diastole
is reduced more than the amount of time spent in systole. How
would you estimate the mean arterial blood pressure at a heart
rate elevated to the point at which the times spent in systole and
diastole are roughly equal?
Answer can be found at end of chapter.
Figure 12–30
Movement of blood into and out of the arteries during the cardiac
cycle. The lengths of the arrows denote relative quantities fl owing
into and out of the arteries and remaining in the arteries.
Time
120
80
Pressure (mmHg)
(a)
200
Pressure (mmHg)
(b)
150
100
50
20
40
60
80
Age (years)
0
0
Diastolic
pressure
Systolic pressure
Aortic valve
closure
Mean pressure
Systolic pressure
Mean pressure
Diastolic pressure
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