580
UNIT FOUR
ent throughout the vascular system, the term
blood pressure
most commonly refers to pressure in arteries supplied by
branches of the aorta (systemic arteries).
Arterial Blood Pressure
The arterial blood pressure rises and falls in a pattern corre-
sponding to the phases of the cardiac cycle. That is, when the
ventricles contract (ventricular systole), their walls squeeze
the blood inside their chambers and force it into the pul-
monary trunk and aorta. As a result, the pressures in these
arteries sharply increase. The maximum pressure achieved
during ventricular contraction is called the
systolic pressure.
When the ventricles relax (ventricular diastole), the arterial
pressure drops, and the lowest pressure that remains in the
arteries before the next ventricular contraction is termed the
diastolic pressure.
The surge of blood entering the arterial system during
ventricular systole distends the elastic walls of the arteries,
Veins also function as
blood reservoirs,
useful in times of
blood loss. For example, in hemorrhage accompanied by a
drop in arterial blood pressure, sympathetic nerve impulses
refl exly stimulate the muscular walls of the veins. The result-
ing venous constrictions help maintain blood pressure by
returning more blood to the heart. This mechanism ensures
a nearly normal blood fl
ow even when as much as 25% of
the blood volume is lost.
Figure 15.32
illustrates the relative
volumes of blood in the veins and other blood vessels.
Table 15.3
summarizes the characteristics of blood ves-
sels. Clinical Application 15.2 examines disorders of blood
vessels.
PRACTICE
39
How does the structure of a vein diF
er from that of an artery?
40
What are the functions of veins and venules?
41
How does venous circulation help to maintain blood pressure
when hemorrhaging causes blood loss?
15.5
BLOOD PRESSURE
Blood pressure is the force the blood exerts against the
inner walls of the blood vessels. Although this force is pres-
Systemic
veins
60–70%
Capillaries
4–5%
Systemic
arteries
10–12%
Heart
8–11%
Lungs
10–12%
Large
veins
Small
veins
and
venules
100
90
80
70
60
50
40
30
20
10
0
Percent distribution
FIGURE 15.32
Most blood is in the veins and venules.
Toward
heart
(a)
(b)
FIGURE 15.31
Venous valves. (
a
) allow blood to move toward the
heart, but (
b
) prevent blood from moving backward away from the heart.
TABLE
15.3
|
Characteristics of Blood Vessels
Vessel
Type of Wall
Function
Artery
Thick, strong wall with three layers—an endothelial lining, a middle layer
of smooth muscle and elastic tissue, and an outer layer of connective tissue
Carries blood under relatively high pressure from the heart to
arterioles
Arteriole
Thinner wall than an artery but with three layers; smaller arterioles have
an endothelial lining, some smooth muscle tissue, and a small amount of
connective tissue
Connects an artery to a capillary, helps control the blood ±
ow into a
capillary by vasoconstricting or vasodilating
Capillary
Single layer of squamous epithelium
Provides a membrane through which nutrients, gases, and wastes are
exchanged between the blood and tissue ±
uid; connects an arteriole
to a venule
Venule
Thinner wall than an arteriole, less smooth muscle and elastic tissue
Connects a capillary to a vein
Vein
Thinner wall than an artery but with similar layers; the middle layer is more
poorly developed; some have ±
aplike valves
Carries blood under relatively low pressure from a venule to the
heart; valves prevent a back±
ow of blood; serves as blood reservoir
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