174
UNIT 3
Organ Systems
Figure 9-17A.
A representation of a medium vein.
In general, the
tunica media
of veins is thinner than the
tunica adventitia
, and smooth muscle cells in this layer are
not as well organized as in arteries. The
tunica adventitia
of
medium veins
is more prominent and much thicker than
the tunica media. Medium veins have various sizes and
structural features depending on their location. The
tunica
intima
consists of an
endothelium
and a thin
subendothe-
lial layer
. The tunica media contains a few layers of circu-
larly arranged smooth muscle cells, which usually do not
form a distinct sheet. In some areas, large-sized medium
veins have a structure similar to that of large veins. Their
tunica intima is thinner, and the tunica adventitia contains
fewer bundles of
longitudinal smooth muscle
than large
veins. However, this characteristic is not prominent in
small segments of the medium veins. Valves are prominent
in medium veins, especially in the extremities of the body.
D. Cui /T. Yang
Tunica
adventitia
Tunica
media
Tunica
intima
Connective tissue
Longitudinal smooth
muscle bundle
Circular smooth
muscle layer
Endothelium
A
B
Smooth
Smooth
muscle bundles
muscle bundles
Smooth
muscle bundles
Longitudinal smooth
muscle bundles
Circular smooth
muscle layer
Tunica
intima
Tunica
media
Tunica
adventitia
Figure 9-17B.
Medium vein, large segment of medium
vein.
H&E,
3
68; inset
3
182
This sample of a large segment of
medium vein
has a
similar structure to a large vein but has fewer longitudi-
nal smooth muscle bundles in the tunica adventitia (Fig.
9-18B). The
tunica intima
is thinner than in the large veins,
and the
tunica media
contains several layers of circular
smooth muscle. The
tunica adventitia
is the thickest and
the most prominent layer in most medium veins. There are
some
longitudinal smooth muscle bundles
(
inset
) and con-
nective tissue in the tunica adventitia. In the extremities,
such as the legs and arms, there are many valves at vary-
ing intervals in the innermost layer of veins. They prevent
blood backfl
ow against gravity. Veins carry blood toward
the heart under lower pressure than arteries and have a
great capacity for housing blood (about 65% of blood is
in the venous system); for this reason, they are sometimes
called
capacitance vessels
.
C
Internal
elastic
lamina
Lumen of the
medium artery
Lumen of the
medium vein
Arteriole
Venule
Nerve
fibers
Figure 9-17C.
Medium vein, small segment of medium
vein.
H&E,
3
68
Veins
accompanying companion
arteries
are often found
in the tissues and organs. Structurally, medium arteries
have relatively smaller lumens, thicker walls, and more
prominent
IEL
than medium veins. The IEL is usually
absent in medium veins. In this example, a small segment
of a medium vein with a companion medium artery, the
vein has a larger
lumen
and a thinner wall than a compan-
ion artery. This medium vein does not show the promi-
nent tunica adventitia or the longitudinal smooth muscle
bundles usually seen in medium veins.
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