The lymphatic system is a collection oforgans, ducts, and
tissues that has the dual role ofdraining tissue fluid (lymph)
back into the bloodstream and of fighting infection. Lymph is
drained bya system ofchannels (the lymphaticvessels).
White cells that are produced by the bone marrow, thymus,
and spleen are present in lymph nodes, or they circulate
through the lymphatic system and the bloodstream, providing
defences against infection.
The lymphatic network
The lymphatic system consists
of a network of lymph nodes
connected bylymphatic
vessels. The nodes generally
occur in clusters, mainly
around the neck, armpits, and
groin. The lymphatic system
also includes organs such as
the spleen and thymus.
T h o r a c ic d u c t
S p l e e n
C is t e r n a
c h y li
L y m p h n o d e
L y m p h a t ic
v e s s e l s
L iv e r
R i g h t l y m p h a t i c d u c t
T h o r a c i c d u c t
R i g h t s u b c l a v i a n v e in
L e f t s u b c l a v i a n v e in
L y m p h
S u p e r i o r v e n a c a v a
Lymphatic drainage
Just below the neck, the thoracic duct and right
lymphatic duct drain into the two subclavian
veins. These veins unite to form the superior
vena cava, which passes into the heart; in this
way, the lymph fluids rejoin the circulation.
L y m p h a t ic v e s s e l s —
C o l l a g e n o u s c a p s u l e
F o l l i c l e
A r t e r y
V e i n
L y m p h
W h ite b l o o d c e l l s
Lymph is constantly moving around
the body, but the lymphatic system
has no central pump equivalent to
the heart. Lymph is circulated by the
movement of the body’s muscles;
a system of one-way valves in the
lymphaticvessels ensures that
it moves in the right direction.
Exertion also pushes fluid from
body tissues into the bloodstream.
R e m a i n i n g fl u i d p a s s e s f r o m t i s s u e s t o
H e a r t
A r t e r ie s
V e in s
L y m p h a t ic
v e s s e l s
C a p ill a r ie s
Fluid exchange
During a 24-hour period, approximately 24 litres
of serumlike fluid pass from the bloodstream to
the body’s tissues. This fluid bathes the cells
and providesthem with oxygen and nutrients.
During the same period oftime, approximately
20 litres of fluid pass back from the tissues to
the bloodstream, carrying carbon dioxide and
other waste products. The remaining 4 litres pass
from the tissues to the lymphatic system and
return eventually to the circulation from there.
Structure of a lymph node
Any fluid absorbed into the lymphatic system
passes across at least one lymph node before
it returns to the circulation. The fluid filters
through a mesh oftightlypacked white blood
cells - some of which are grouped into follicles
consisting of similar cells - which attack and
destroy harmful organisms. Every lymph node
is supplied by its own tiny artery and vein.
Enlarged lymph nodes
This photograph shows a child with an enlarged
lymph node in the neck. One cause of such
enlargement is infection. Enlarged nodes may
also be a result of Hodgkin’s disease, a rare
cancer ofthe lymph nodes.
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