176
UNIT 3
Organ Systems
Types of
Arteries and
Veins
Tunica Intima
Tunica Media
Tunica Adventitia
Size
(Diameter)
Main Functions
Arterial System
Large/elastic
(conducting)
arteries
Endothelium;
subendothelial
layer (connective
tissue); IEL
Abundant fenestrated
elastic membranes;
interspersed smooth
muscle cells; EEL
Thinner than
its tunica media
(connective tissue);
vasa vasorum present
>10 mm
Conducting blood fl ow
from the heart to the
arterial system; being
able to recoil (elastic
membranes) to buffer
pressure against arte-
rial wall and
conserve
energy to force the blood
forward, even while the
ventricle is relaxing
Medium/
muscular
(distributing)
arteries
Endothelium;
subendothelial
layer (connective
tissue); prominent
complete IEL
Abundant, thick layer
of smooth muscle cells
(6–40 layers of muscle
cells); prominent EEL
in larger-sized medium
arteries
Thinner than
its tunica media
(connective tissue);
vasa vasorum not
prominent
10–0.5 mm
Distributing blood
fl ow to small
arteries
in various parts of
body; adjusting the
rate of blood fl
ow by
vasoconstriction and
vasodilation
Small arteries
Endothelium; thin
subendothelial
layer; thin IEL
3–6 layers of smooth
muscle cells; EEL
absent
Thinner than its
tunica media (loose
connective tissue);
no vasa vasorum
500–100
μ
m
Distributing blood
to arterioles and to
capillaries; participating
in vasoconstriction
and vasodilation to
adjust the blood fl ow
Arterioles
Endothelium; no
subendothelial
layer; IEL not
prominent
1–2 layers of smooth
muscle cells; no EEL
Very thin; a sheath of
loose connective tissue
100–30
μ
m
Leading blood to
the capillary beds;
regulating resistance;
controlling blood fl ow
to capillaries
Venous System
Venules
( postcapillary,
collecting,
and muscular
venules)
Endothelium; no
subendothelial
layer; no valves
Very thin; smooth
muscle cells not promi-
nent
Very thin; layer of
collagen F bers
10–100
μ
m
Draining exchanged
blood from
capillaries
to the small veins;
promoting leukocyte
migration from blood-
stream to infl amed
tissue; primary sites for
infl ammatory response
Small veins
Endothelium;
subendothelial
layer; few valves
Thin; 1–3 layers
isolated smooth muscle
cells
Thin; connective
tissue layer
0.1–1 mm
Collecting blood fl ow
from venules to the
medium veins
Medium veins
Endothelium;
subendothelial
layer; increased
number of valves
Thicker than small
vein; a few layers of
smooth muscle cells
that usually do not
form a distinct sheath
Thicker than its
tunica media; variable
number and size of
longitudinal bundles of
smooth muscle mixed
with connective tissue
1–10 mm
Carrying blood to large
veins leading toward
the heart; preventing
blood from backfl ow
Large veins
Endothelium; thick
subendothelial
layer; large valves;
IEL may be
present
Thinner than its tunica
adventitia layer; several
layers of circularly
arranged smooth
muscle cells but do not
form a distinct sheath;
some collagen F bers
Thickest layer in large
veins; composed of
many large bundles of
longitudinal smooth
muscle; cardiac
muscle may be present
near the atria
>10 mm
Transporting blood
from the venous system
back to the heart;
forcing blood toward
the heart by
constriction of circu-
lar and longitudinal
smooth muscle
TABLE 9-1
Blood Vessels
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