CHAPTER 9
Circulatory System
167
Figure 9-10A.
A representation of a small artery and an
arteriole.
Small arteries
(
left
) have the general structure of muscular
arteries but with a smaller diameter and no EEL. The
tunica
media
usually contains about three to six layers of smooth
muscle cells. An
IEL
is usually present but not prominent.
Small arteries help control and modulate blood pressure.
Arterioles
(
right
) are the smallest arteries, leading blood fl
ow
into capillary beds. They play an important role in regulat-
ing blood pressure and controlling the blood fl
ow entering
capillaries. Arterioles have a very small diameter and narrow
lumen. There are only one to two layers of
smooth muscle
cells
in the arteriolar walls, and the tunica adventitia is barely
visible. Both an IEL and a subendothelial layer are often
absent.
Small arteries and arterioles are sometimes referred to as
resistance vessels
because of their function in reducing and
stabilizing blood pressure before blood fl
ows into the capil-
lary network.
A
D. Cui /T. Yang
Internal elastic
Small artery
Arteriole
lamina
Tunica
media
D. Cui /T. Yang
Endothelial
cell
Smooth
muscle cell
Lumen of
Lumen of
small vein
small vein
Arteriole
Arteriole
Nuclei of smooth
Nuclei of smooth
muscle cells
muscle cells
Small arteries
Small arteries
Internal
Internal
elastic
elastic
lamina
lamina
Nuclei of smooth
muscle cells
Internal
elastic
lamina
Arteriole
Lumen of
small vein
Small arteries
B
Figure 9-10B.
Small arteries, small intestine (ileum).
H&E,
3
136; inset
3
408
An example of a
small artery
in the tissue of the small
intestine
is shown. Small arteries can be found in various types of tis-
sues and organs where oxygen, nutrients, and other materials
must be exchanged. Small arteries have a thin tunica intima
and tunica adventitia. The
tunica media
, the most obvious
layer in small arteries, contains three to six layers of circu-
larly arranged
smooth muscle cells
. An
IEL
is often present.
Smooth muscle is innervated by the autonomic nervous sys-
tem and regulates blood pressure by causing vasoconstriction
(sympathetic nervous system) and vasodilation (parasympa-
thetic nervous system).
Lumen of
venule
Arteriole
Endothelial cell
Smooth muscle cell
C
Figure 9-10C.
Arterioles, tongue.
H&E,
3
680; inset
3
1,020
Arterioles
have a very small, round
lumen
(<0.5 mm in diam-
eter) and one or two layers of
smooth muscle cells
in the arte-
riolar wall. Arterioles are the last segment of the arterial sys-
tem. They connect small arteries to the capillary network. The
endothelium
of these small vessels is able to sense changes in
blood pressure, blood fl ow, and oxygen tension and to respond
to these changes by releasing signals, such as
endothelin
(
vaso-
constrictor
) and
nitric oxide
(
vasodilator
). These signals regu-
late the tone of adjacent smooth muscle cells. Changes in muscle
tone control blood fl ow into the capillaries. Arterioles can be
structurally distinct depending on their location. For example,
arterioles in the lung are not structured identically to those in
the kidney. A venule accompanying an arteriole is shown here.
Venules have very thin walls consisting mainly of endothelium.
Smooth muscle cells are not prominent in venules.
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