162
UNIT 3
Organ Systems
Figure 9-5.
A
representation of the general structure of blood vessel layers (tunicae) and a comparison of the medium artery and
the medium vein.
The walls of the
blood vessels
can be divided into three
layers
(
tunicae
). The innermost layer in contact with the blood is called the
tunica intima
. This layer contains endothelium and subendothelial connective tissue and may contain an
internal elastic lamina
(
IEL
)
in some vessels, particularly arteries. The
endothelium
is a layer of simple squamous cells, which forms the smooth surface of the
lumen.
Subendothelial connective tissue
is a thin layer of connective tissue beneath the endothelium. The IEL, if present, is a sheet
of elastic material that divides the tunica intima from the tunica media. The middle layer is called the
tunica media
. It primarily
contains circularly arranged smooth muscle cells (except in elastic arteries). Contraction and relaxation of these smooth muscle cells
will change the vessel diameter and affect blood pressure. The outermost layer, the
tunica adventitia
, is a layer of connective tissue
dominated by collagenous and elastic F
bers. In large and some medium veins, the tunica adventitia may also contain
longitudinal
smooth muscle bundles
. Tunica adventitia surrounds and covers the vessels for protection. Occasionally, small blood vessels called
vasa vasorum
are found in the tunica adventitia of large vessels. The vasa vasorum provide oxygen and nutrients for the large vessel
walls when the distance from the lumen is great and it is difF
cult to get nutrients from diffusion. Some differences between a medium
artery and a medium vein are listed here: (1) the artery has a smaller and more rounded lumen, whereas the vein has a larger and
oval or irregular-shaped lumen; (2) the artery has a thicker wall than does the vein; (3) in the artery, the tunica media is much thicker
than the tunica adventitia, but in the vein, the tunica adventitia is much thicker than the tunica media; (4) circularly oriented smooth
muscle cells form uniform sheets in the tunica media of the artery; however, smooth muscle cells are fewer and do not form a distinct
sheet in the vein; (5) There are a few longitudinal smooth muscle bundles in the tunica adventitia of medium veins in some locations,
whereas these smooth muscle bundles are more abundant in large veins. This pattern does not occur in arteries. Longitudinal smooth
muscle bundles in the tunica adventitia of the vein contract to help push blood back to the heart; and (6) valves are present in many
veins, especially in the medium veins of the extremities. Their function is to prevent gravitational backfl
ow of the blood and to help
blood return to the heart. Arteries do not have valves (except the aortic and pulmonary valves).
Fibroblast
Fibroblast
Tunica
adventitia
Tunica
adventitia
Tunica
media
Tunica
media
Tunica
intima
Tunica
intima
External elastic
lamina (
)
EEL
Internal elastic
lamina (
)
IEL
Connective
tissue
Connective
tissue
smooth muscle
with connective
tissue
Circular
smooth muscle
Circular
Longitudinal
smooth
muscle
Endothelial cells
Endothelial cells
Subendothelial
layer
Subendothelial
layer
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