Figure 12.2 provides an overview of the primary and
secondary lymphatic organs. We will discuss the major
organs and tissues in more detail soon, particularly the
spleen and lymph nodes.
InSight
P rim a ry a n d s e c o n d a r y o r g a n s a n d t is s u e s
o f t h e ly m p h a tic s y s te m
Figure 12.2
THE PLANNER^
Primary organs and tissues
Red Bone Marrow
In red bone marrow, stem cells
divide and develop into mature
B and T lymphocytes (immune
cells).
Thymus
Thymus is the organ where stem
cells develop into mature T
lymphocytes. It atrophies (shrinks)
after puberty.
S ec o n d ary o rg an s and tissues
Tonsils
Palatine, pharyngeal, and
lingual tonsils are large
aggregations of lymphatic
nodules (not capsulated). They
protect against inhaled or
ingested foreign substances.
Lymphatic Vessels
Although similar to veins,
lymphatic vessels have thinner
walls and more valves. They
carry lymph away from tissues.
Palatine
tonsil
Lingual
tonsil
Tongue
Thoracic and Right
Lymphatic Ducts
The thoracic duct and
right lymphatic duct
receive lymph from
lymphatic vessels and
empty into the junction
between the jugular and
subclavian veins.
Right lymphatic
duct
Jugular vein
i
Subclavian
vein
Thoracic
duct
Lymphatic Capillaries
Lymphatic capillaries drain
the interstitial spaces of
excess fluid. This fluid,
known as
lymph
once it
enters the capillary, is
passed on to the lymphatic
vessels.
Venule
Lymph
Blood
capillary
Arteriole
Lymphatic
capillary
Spleen
The largest lymphatic organ is
the spleen. It has macrophages
and immune cells that destroy
pathogens in the blood and
remove worn-out blood cells. It
also stores RBCs and platelets
during fetal life, produces red
blood cells.
Lymph Nodes
Lymph nodes are bean-shaped,
encapsulated tissues that filter
lymph. Fibers trap foreign
substances, which get broken down
by macrophages and by immune
responses involving lymphocytes.
There are about 600 lymph nodes
located mainly in the abdomen and
thorax and near the head, axiliae,
and groin.
346 CHAPTER 12
The Lymphatic System and Immunity
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