CHAPTER 9
Circulatory System
165
Figure 9-8A.
A representation of a medium artery
(muscular or distributing artery).
The
medium arteries
are also called
muscular arteries
, refer-
ring to the fact that the arterial wall is dominated by smooth
muscle. There are about 6 to 40 layers of smooth muscle cells
that form a distinct sheath in the
tunica media
. In general,
the tunica media is much thicker than the
tunica adventitia
.
Vasoconstriction
can be caused by smooth muscle contrac-
tion that is controlled by the sympathetic nervous system.
Internal and external elastic laminae are obvious in medium
arteries. The
IEL
separates the tunica intima from the tunica
media; the
external elastic lamina
(
EEL
) separates the tunica
media from the tunica adventitia. The
tunica adventitia
is
composed of connective tissues (F broblasts and connective
tissue F bers).
Vasa vasorum
may also be present. Medium
arteries, also sometimes called
distributing arteries
, distribute
blood to the small arteries in various organs of the body.
D. Cui /T. Yang
Tunica
adventitia
Tunica
media
Tunica
intima
Connective tissue
Fibroblast
Smooth
muscle cell
External elastic
lamina
Internal elastic
lamina
Vasa vasorum
Nerve
A
B
Smooth
Smooth
muscle
muscle
cells
cells
Smooth
muscle
cells
Tunica
intima
Tunica
media
Tunica
adventitia
Vein
Lumen of the
medium artery
Internal elastic
Internal elastic
lamina
lamina
Internal elastic
lamina
External
External
elastic lamina
elastic lamina
External
elastic lamina
Figure 9-8B.
Medium artery (small muscular artery).
H&E,
3
68; inset
3
207
The size of
medium arteries
(
muscular arteries
) varies
depending upon the location. This is an example of a small
segment, which has 10 to 12 layers of smooth muscle cells
in the
tunica media
. Smooth muscle cells are arranged in
a
circular orientation
around the lumen, are connected
to each other by
gap junctions
, and are surrounded by
a network of
extracellular matrix
(see ±ig. 6-13). This
arrangement allows the smooth muscle cells to function
as one unit. When smooth muscle contracts, the lumen
size decreases and blood pressure increases, thereby keep-
ing blood moving forward from medium arteries to small
arteries. Smooth muscle cells in the tunica media are the
targets of various
neural
and
endocrine substances
. Both
the IEL and EEL are prominent, appearing red-pink with
H&E stain.
C
Internal
Internal
elastic
elastic
lamina
lamina
Internal
elastic
lamina
Endothelium
Endothelium
Subendothelium
Subendothelium
Tunica
intima
Tunica
media
Tunica
adventitia
Endothelium
Subendothelium
Figure 9-8C.
Medium artery (large muscular artery).
H&E,
3
68; inset
3
218
This is an example of a large segment of a
medium artery
,
which has about 20 to 40 layers of smooth muscle cells. The
tunica intima
is composed of
endothelium
,
subendothelial
connective tissue
, and an
IEL
. The thin subendothelial
connective tissue layer may become thicker with age and in
atherosclerosis
. The IEL is a wavy pink sheet forming the
boundary between the tunica intima and tunica media. In
general, the
tunica media
is the thickest layer, composed of
smooth muscle cells. The
tunica adventitia
is composed of
a layer of dense irregular connective tissue.
Vasa vasorum
and
nerve ±
bers
can be found in this layer but are not as
prominent as in elastic arteries.
Damage to
endothelial cells
can lead to various types
of cardiovascular diseases, such as
atherosclerosis
and
arteriosclerosis
(see Synopsis 9-2).
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