The Lymphatic System
Chapter 20
The Lymphatic System
Lymphatic System Series of vessels,
tissues and organs
performing 2 major
functions:
1. Vessels return to the
blood any fluids and
proteins that leaked
into tissues
2. Tissues and organs
house cells for
immune functioning
Figure 20.1
Lymphatic Vessels
Lymphatic vessels – the series of
vessels draining lymph, returning it to
the blood in a unidirectional flow
Lymph – the protein-rich, interstitial
fluid that has entered the lymphatic
vessels
Lymphatic capillaries – smallest
vessels near capillary beds that
intake lymph
• Minivalves – flap-like
openings between endothelial
cells making lymphatic
capillaries highly permeable
• Collagen filaments – anchor
endothelial cells to surrounding
connective tissue, encourages
minivalve opening
Figure 20.1
Lymphatic Vessels
Lacteals – specialized
lymphatic capillaries of the
small intestine that absorb
lymph that is high in fat
content, called chyle
Other Lymphatic Vessels:
Lymphatic capillaries merge
to form lymphatic
collecting vessels, which
merge to form lymphatic
trunks, which merge to
form lymphatic ducts,
which drain into venous
circulation
• The larger vessels have
tunics and valves similar to
veins
Figure 20.1
Lymphatic Vessels
Lymph Transport
• A unidirectional, low-pressure flow
returning protein-rich fluid and
absorbed fats to the blood (about
3L/day)
• No pump – lymph transport relies
on body movements and the
respiratory and muscular ‘pumps’
seen in veins
Figure 20.2a
Lymphatic Tissue
Lymphoid cells:
Lymphocytes – the T cells and B
cells that fight disease-causing
pathogens
Plasma cells – specialized B cells
that produce antibodies
Macrophages – phagocytize any
foreign cells or debris
Dendritic cells – specialized
phagocytes with cellular
projections
Reticular cells – produce the web
of reticular fibers (stroma) that
support the lymphoid cells
Figure 20.3
Lymphatic Tissue
Lymphatic Tissue:
• Lymphatic tissue - largely
reticular connective tissue, it
houses large populations of the
lymphoid cells on a stroma of
reticular fibers
• Diffuse lymphatic tissue –
scattered reticular tissue found in
and around all body organs
• Lymphatic follicles (nodules) –
dense masses of reticular tissue,
often with a lighter germinal center
of dendritic and B cells
Figure 20.3
Lymph Nodes
Lymph nodes – the most abundant lymphoid organs, scattered amongst the
lymphatic vessels, and clustered in the inguinal, axillary, and cervical regions
• Filter lymph of foreign cells and debris via phagocytes
• Activate the immune system by stimulating lymphocytes
Figure 20.4
Lymph Nodes
Lymph node structure:
• bean shaped, 2.5 cm in length
• Capsule – dense fibrous connective
tissue surrounding for shape/support
• Trabeculae – connective tissue walls
dividing the node into compartments
• Cortex – outer layer of lymphoid
follicles
• Medulla – central region with
medullary cord filled with B and T cells
• Sinuses – spaces allowing lymph
movement through the node
• Afferent vessels – bring lymph into
the node
• Efferent vessels – drain lymph from
the node
Figure 20.4a
Lymphatic Organs
• Lymph nodes – most abundant
lymphoid organs, and the only
ones to filter lymph
• Other lymphoid organs do
NOT filter lymph, include the
tonsils, thymus, spleen,
appendix, and Peyer’s patches
Figure 20.5
The Spleen
Spleen
• Largest lymphoid organ, in
left side of abdomen
• Capsule – dense fibrous
surrounding, prone to rupture
• White pulp – regions
performing immune
surveillance via lymphocytes
and macrophages
• Red pulp – area of enlarged
veins and macrophages
performing erythrocyte
breakdown
• Also stores platelets and
salvages iron
Figure 20.6
The Thymus
Thymus – a glandular structures in the lower neck that does not
contain reticular tissue, but still functions as a lymphoid organ
• Produces hormones used in the activation of T lymphocytes,
peak activity in childhood, then deteriorates after puberty
• Thymic corpuscles – specialized areas for T cell destruction
Figure 20.7
Tonsils
Tonsils – a group of simple lymphoid organs encircling the throat that gather
and remove potential pathogens that entered the mouth or nose
Tonsillar crypts – invaginations that trap bacteria and other debris, allowing
lymphoid cells in the nearby follicles to destroy them
Figure 20.8
M.A.L.T.
Peyer’s patches – clusters of
lymphoid follicles in the wall of
the small intestine
Appendix – a cluster of
follicles hanging from the start
of the large intestine
M.A.L.T (mucosa-associated
lymphatic tissue) – the
appendix, Peyer’s patches,
and tonsils all line the mucosa
of the digestive tract, trapping
and destroying bacteria and
generating immunological
memory
Figure 20.9