CHAPTER 9
Circulatory System
157
THE CAPILLARY SYSTEM
contains continuous capillaries,
which have a continuous basal lamina and
complete
endothe-
lial cells
. These structures allow only a very limited amount of
materials to pass through the capillary walls (Fig. 9-13A,B).
Fenestrated capillaries
have a continuous basal lamina
and
fenestrated
endothelial cells
(perforated by small pores).
A greater range of substances can, therefore, pass through the
capillary walls (Fig. 9-14A,B).
Discontinuous (sinusoidal)
capillaries have a
discontinuous
(or missing) basal lamina and
incomplete
endothelial cells
(perforated by large pores), which allow proteins and other
materials, even cells, to pass through the capillary walls freely
(Fig. 9-15A,B).
THE VENOUS SYSTEM
is composed of
venules
and
small
,
medium
, and
large veins
. Veins collect blood (under lower pres-
sure than arteries) from capillaries and transport it to the heart.
Veins have larger, fl at lumens and thinner walls than their com-
panion arteries (Figs. 9-5 and 9-6).
Venules
and
small veins
have very thin walls and few valves
.
Venules drain exchanged blood from the capillaries to the small
veins. Venules are the primary sites of many
infl
ammatory
reactions (Fig. 9-16A–C).
Medium veins
have various diameters, and their structure
varies based on their size and location. Segments of medium
veins in some locations have a thick
tunica adventitia
with a
few
longitudinal smooth muscle bundles
(Fig. 9-17A–C). Valves
are prominent in medium veins, with an abundance in the
extremities to
prevent blood from fl
owing backwards.
Large veins
have a thick
tunica adventitia
with numerous
longitudinal smooth muscle bundles
, which help to force blood
to fl
ow toward the heart (Fig. 9-18A–C). There are some large
valves in the large veins.
The Lymphatic Vascular
System
Lymphatic Vessels
The
lymphatic vessels
have a structure similar to small veins
(Fig. 9-19A,B). They have a large lumen, and valves are plenti-
ful in all sizes of lymphatic vessels.
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