CHAPTER 9
Circulatory System
173
The Venous System
Figure 9-16B.
Venule and arteriole, colon.
H&E,
3
680
This
venule
(
muscular venule
) and companion
arteriole
were taken
from the colon of the large intestine. Venules have larger lumens
than arterioles. In histological sections, they often have blood
cells remaining inside the lumen. Venules are the smallest veins;
they drain blood from capillaries into the small veins. They are
also involved in exchange of metabolites with tissues and allow
blood cells to migrate from the vessels into the tissues (
diapedesis
).
The
postcapillary venules
are the primary sites involved in the
infl ammatory response; they allow leukocytes (white blood cells)
to migrate into the affected tissues in cases of infl ammation or
infection. Note that the companion artery has a small, round
lumen and a single layer of
smooth muscle cells
. There are fewer
or no
blood cells
in the lumen of an arteriole specimen, because of
the higher pressure and stronger contraction of the arteriole wall.
Venule
Blood cells
Arteriole
Endothelial cell
Smooth
muscle
cells
B
Figure 9-16C.
Small vein, colon.
H&E,
3
272; inset
3
527
Small veins
have one to three layers of isolated
smooth muscle cells
in the walls. The tunica adventitia of small veins is a little thicker
than that of venules. Small veins have a thinner wall and less dis-
tinguishable
smooth muscle
layers than do small arteries (also
see Fig. 9-10B). Small veins gradually increase in size to become
medium veins.
Microvessels
include
arterioles
,
capillaries
, and
venules
and play a key role in the response to infl ammation. During
infection or tissue injury, immune cells (such as mast cells and
basophils) release histamine and heparin. In response, the endothe-
lium of microvessels allows fl uid leakage (
edema
) and relaxation of
smooth muscle cells, leading to vessel dilation (redness and heat).
Endothelial
cell
Lumen of a
small vein
Lumen of a
small vein
Arteriole
Smooth
muscle
nuclei
Smooth muscle nucleus
C
Exudation
is the escape of blood fl uid and protein from the
lumen of the vessel into the interstitial tissue when the cell-to-
cell junctions of the endothelium loosen.
Transudation
is move-
ment, due to hydrostatic imbalance, of the blood plasma fl
uid
from the lumen to the interstitial tissue, with large protein and
cells ± ltered out by the normal endothelial wall.
Figure 9-16A.
A representation of a venule and small vein.
The
venous system
is composed of
venules
,
small veins
,
medium
veins
, and
large veins
. These vessels carry blood from capillar-
ies in the tissues back to the heart. Small veins and venules are
very similar in structure and are sometimes dif± cult to distinguish
from one another. In general,
small veins
have larger lumens
and more visible smooth muscle cells than venules.
Venules
, the
smallest veins, receive blood from the capillary network. Venules
have small lumens and very thin walls, with only a single layer
of endothelium and a small amount of underlying connective
tissue (similar to large capillaries). They may have a few scat-
tered smooth muscle cells in the vessel walls. Venules gradually
increase in size to form small veins. They can be categorized as
postcapillary venules
(10–30
μ
m in diameter),
collecting venules
(30–50
μ
m) and
muscular venules
(50–100
μ
m).
Postcapillary
venules
connect directly to capillaries and drain blood into the
collecting venules.
Muscular venules
often accompany arterioles.
D. Cui /T. Yang
D. Cui /T. Yang
Smooth muscle cell
Venule
Small vein
Endothelial
cell
Endothelium
Connective tissue
A
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