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U N I T 5
Circulatory Function
U N D E R S T A N D I N G
Determinants of
The arterial blood pressure, which is the force that moves blood through the arterial
system, reflects the intermittent contraction and relaxation of the left ventricle. It is
determined by (1) the properties of the arterial system and the factors that maintain
(2) the systolic and (3) the diastolic components of the blood pressure.These factors
include the blood volume, elastic properties of the blood vessels, cardiac output,
and peripheral vascular resistance.
Arterial Pressure Regulation.
The
arterial blood pressure represents the force
that distributes blood throughout the cap-
illaries of the body. The aorta and large
arteries have large amounts of elastin in
their walls that allow them to stretch and
store energy during ventricular systole
and recoil during diastole. The arterioles,
which are the terminal components of the
arterial system, serve as resistance vessels
that regulate blood pressure by control-
ling the distribution of blood to the cap-
illary beds. The elastic properties of the
aorta and large arteries, coupled with the
high resistance properties of the arterioles,
allow the arterial system to act as a filter
that converts the intermittent flow gener-
ated by the heart to virtually steady flow
in the capillaries. The low-pressure venous
system collects blood from the capillaries
and returns it to the heart as a means of
maintaining the cardiac output needed to
sustain the arterial pressure.
Systolic Pressure.
The systolic blood
pressure reflects the amount of blood
(stroke volume) that is ejected from the
heart with each beat, the rate and force
with which it is ejected, and the elastic-
ity or compliance of the aorta and large
arteries. The blood that is ejected from the
heart during systole does not move directly
through the circulation. Instead, a substan-
tial fraction of the stroke volume is stored
in large arteries. Because the walls of these
vessels are elastic, they can be stretched
to accommodate a large volume of blood
without an appreciable change in pressure.
The systolic pressure often increases with
aging as the aorta and large arteries lose
their elasticity and become more rigid.
Arterial pressure (mm Hg)
Systolic
Diastolic
120
80
40
0
Aorta
Arteries
Arterioles
Capillaries
Venules
Veins
Vena cava
Systolic
Peripheral
resistance
240
220
200
180
160
140
120
100
80
60
40
20
250
230
210
170
150
130
110
90
70
50
30
10
Aorta
Left
atrium
Left
ventricle
1
2
