CHAPTER 9
Circulatory System
175
Figure 9-18A.
A representation of a large vein.
The
vena cava
and
pulmonary veins
are the largest
veins in the
venous system
. They connect directly to
the heart. The
tunica adventitia
is the most prominent
and distinct layer in large veins; it contains numerous
longitudinally arranged
smooth muscle bundles
and
some connective tissues (mostly
collagen ±
bers
). The
vasa vasorum
are also present in the tunica adventitia;
they provide blood and nutrient supply to the wall of
the large vein. The
tunica media
contains a few lay-
ers of smooth muscle cells that are loosely arranged
and do not form a distinct sheath. Contraction of the
circularly arranged smooth muscle cells in the tunica
media and the longitudinally arranged smooth muscle
bundles in the tunica adventitia help to push blood
toward the heart.
D. Cui /T. Yang
Tunica
adventitia
Tunica
media
Tunica
intima
Connective tissue
Longitudinal smooth
muscle bundle
Collagen fiber
Vasa vasorum
Endothelium
Circular smooth
muscle layer
A
Subendothelial
Subendothelial
layer
layer
Connective tissue
Connective tissue
Vasa
Vasa
vasorum
vasorum
Longitudinal
Longitudinal
smooth
smooth
muscle bundles
muscle bundles
Tunica
Tunica
intima
intima
Tunica
Tunica
media
media
Tunica
Tunica
adventitia
adventitia
Connective tissue
Longitudinal
smooth
muscle bundles
Tunica
intima
Subendothelial
layer
Tunica
media
Tunica
adventitia
Vasa
vasorum
B
Figure 9-18B.
Large vein, vena cava.
H&E,
3
68;
insets
3
185
The
tunica intima
of the
vena cava
consists of an
endothelial layer, a thick subendothelial layer (connec-
tive tissue), and an IEL. In general, the IEL is not well
developed in veins. Although it is not commonly seen
in veins, it may still be present in some large veins.
The
tunica media
contains a few layers of loosely
organized smooth muscle cells; the tunica media is
much thinner compared to its tunica adventitia. There
are densely arranged
smooth muscle bundles
in the
tunica adventitia
, which give the large vein a distinct
appearance. There is a thin layer of connective tissue
(mainly collagen F
bers) between the circular smooth
muscle and the longitudinal smooth muscle bundles in
the tunica adventitia (±ig. 9-18C). A
vasa vasorum
is
shown on the right, and a cross section of longitudinal
smooth muscle bundles is shown on the left.
Internal
Internal
elastic
elastic
lamina
lamina
Circular smooth
Circular smooth
muscle cells
muscle cells
Longitudinal
Longitudinal
smooth
smooth
muscle bundles
muscle bundles
Tunica
Tunica
intima
intima
Tunica
Tunica
media
media
Tunica
Tunica
adventitia
adventitia
Longitudinal
smooth
muscle bundles
Connective
Connective
tissue
tissue
Connective
tissue
Circular smooth
muscle cells
Internal
elastic
lamina
Tunica
intima
Tunica
media
Tunica
adventitia
C
Figure 9-18C.
Large vein, vena cava.
H&E,
3
186
A high power view of a
large vein
with smooth muscle
cells in the circular
smooth muscle layer
in the
tunica
media
. The
longitudinal smooth muscle bundles
are
surrounded by collagen F
bers in the
tunica adventitia
.
There are some connective tissues (mainly collagen
bundles) between circular smooth muscle and longi-
tudinal smooth layers. The
IEL
is clearly visible here
between the tunica intima and the tunica media. How-
ever, the IEL is not usually present in the veins.
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