CHAPTER 9
Circulatory System
177
The Lymphatic Vascular System
CLINICAL CORRELATION
Figure 9-19C.
Lymphangioma, Skin.
H&E,
3
44
Lymphangioma
is a congenital malformation of the
lymphatic system that involves the skin and subcutane-
ous tissues. It occurs most often in the head and the
neck. It is characterized by multiple clusters of translu-
cent vesicles that contain lymph fl uid varying from clear
to pink to dark red. These vesicles are separated from
the normal network of lymphatic vessels but have com-
munications with the superF cial lymph vesicles through
vertical and dilated lymph channels. Skin lesions range
from minute vesicles to cystic and cavernous spaces.
Clinically, it appears as a raised, soft, spongy, and
pinkish-white lesion. Surgical excision may be chosen if
vital structures are involved or for cosmetic correction,
but it has a high recurrence rate after surgery.
Lymphatic vessels
Lymphatic vessel
C
Figure 9-19A.
Small lymphatic vessels, lymph node.
H&E,
3
272; inset
3
767
The
lymphatic vascular system
is composed of
lymphatic
capillaries
,
lymphatic vessels
, and
lymphatic ducts
, which col-
lect and drain interstitial fl uid from the tissue into the large veins
(for subclavian veins, see Chapter 10, “Lymphoid
System,”
±ig. 10-6).
Lymph
(fl uid in the lymphatic system) contains lym-
phocytes, immunoglobulins, plasma, foreign antigens, and other
substances.
Lymphatic vessels
carry lymph through the
lymph
nodes
along the lymphatic vessels.
Lymph nodes
F lter lymph
and expose lymphocytes to antigens as part of the immune
response (see Chapter 10, “Lymphoid System”). After F ltration,
the lymph is transported via large lymphatic vessels to lymphatic
ducts (the thoracic right lymphatic ducts) and F nally enters the
subclavian veins and becomes part of the blood plasma. An
example of
small lymphatic vessels
is shown in the hilus of a
lymph node. Small lymphatic vessels have large lumens and very
thin walls, which are composed of a layer of endothelium and a
little connective tissue with a few smooth muscle cells.
Small
artery
Small
artery
Small
lymphatic
vessel
Small
lymphatic vessel
Valve
A
Smooth
Smooth
muscle
muscle
cells
cells
Lymphocytes
Lymphocytes
Valve
Valve
Lymphatic
Lymphatic
vessels
vessels
Lymphatic
vessels
Coagulated
Coagulated
plasma
plasma
Coagulated
Coagulated
plasma
plasma
Coagulated
plasma
Coagulated
plasma
Smooth
muscle cells
Lymphocytes
Valve
B
Figure 9-19B.
Large lymphatic vessels, lymph node.
H&E,
3
136; inset
3
422
Large lymphatic vessels
have thicker walls than small
lymphatic vessels. Large lymphatic vessels are composed of
connective tissues and multiple layers of
smooth muscle cells
.
Contraction of smooth muscle cells helps to move the lymph
forward. Large lymphatic vessels are structurally similar to
small veins, except they have larger lumens and prominent
valves
. Valves are present in all sizes of lymphatic vessels.
They prevent lymph from fl
owing backward. Lymphatic ves-
sels are often distinguished by lumens that contain clusters of
lymphocytes and
coagulated plasma
. Lymphatic vessels can be
found in most of the tissues of the body but not in the central
nervous system, the bone marrow, or the hard tissues.
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