170
UNIT 3
Organ Systems
The Capillary System
Figure 9-13A.
Continuous capillaries, skeletal muscle.
H&E,
3
272; inset
3
870
Capillaries
, the smallest vessels (5–10
μ
m in diameter) in the
blood circulatory system, connect arterioles to venules. Capil-
laries contain one layer of endothelial cells with a basal lamina
sometimes wrapped by pericytes. According to the structure
of the endothelial cells and the continuity of the basal lamina,
capillaries can be classiF ed into three types: (1)
continuous
capillaries
, (2)
fenestrated
capillaries
, and (3)
discontinuous
(
sinusoidal
)
capillaries
.
Continuous capillaries
have
complete
endothelial cells
(no pores), interrupted only by
intercellular
clefts
between the cells, and a
continuous basal lamina
with
no breaks in the endothelial barrier between the vascular
compartment and the surrounding tissues. Such structures
in the continuous capillaries allow very limited amounts of
materials to pass through their walls. Continuous capillaries
can be found in muscles, connective tissues, nervous tissues,
lungs, exocrine glands, and the thymus.
Arteriole
Muscle fibers
Capillaries
Endothelial cells
A
Figure 9-13B.
Continuous capillary, loose connective tissue.
EM
3
5,100
The caliber of this
continuous capillary
can be inferred from the
erythrocyte
that occupies the lumen. Nuclei of two endothelial cells
are prominent, and portions of
pericytes
can be seen external to the endothelial cells. A continuous layer of endothelial cell cyto-
plasm forms a barrier between the capillary lumen and the surrounding tissue compartment.
Junctions
between adjacent endothe-
lial cells can be of both the
occludens
and the
adherens
types.
Marginal folds
, such as the one beneath the erythrocyte, are typical
features at the junctions between capillary endothelial cells. Each endothelial cell has numerous
caveolae
beneath its plasmalemma,
although these structures are difF cult to discern at this magniF
cation. The caveolae are a manifestation of active transcytosis of
materials between the plasma and the tissue space.
T. Yang
Basal lamina
Endothelial cell
Caveolae
Caveolae
Erythrocyte
Erythrocyte
Marginal fold
Marginal fold
Pericyte
Pericyte
Intercellular
Intercellular
junction
junction
Intercellular
Intercellular
junction
junction
Basal lamina of
Basal lamina of
pericyte
pericyte
Erythrocyte
Marginal fold
Pericyte
Intercellular
junction
Caveolae
Intercellular
junction
Basal lamina of
pericyte
B
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