Transcript Slide 1

Lymphatic System
Mike Clark, M.D.
Lymphatic System
• Composed of lymphoid cells, tissues,
organs, and vessels
• The function of the lymphatic system is
to (1) fight infection (2) carry lipids
absorbed from the GI tract to the
bloodstream and (3) return fluids and
plasma proteins that escaped from the
bloodstream back to the bloodstream
Lipids absorbed from GI tract into Central
Lacteal of Lymphatic
Microvilli
(brush border)
Absorptive cells
Lacteal
Goblet cell
Blood
capillaries
Mucosa
associated
lymphoid tissue
Intestinal crypt
Muscularis
mucosae
Duodenal gland
(b)
Vilus
Enteroendocrine
cells
Venule
Lymphatic vessel
Submucosa
Figure 23.22b
Cells and Fibers of the Lymphatic
System
1. Lymphocytes
2. Macrophages
3. Dendritic Cells (Antigen Presenting Cells)
4. Reticular Cells
5. Reticular Fibers
Lymph is a connective tissue
Lymphoid Tissues
1. Lymph follicles
2. Diffuse Lymphoid Tissue
MALT (Mucosal Associated Lymphoid Tissue)
GALT (Gut associated Lymphoid Tissue) –example
Peyer’s patches
BALT (Bronchial associated Lymphoid Tissue)
Lymphoid Organs
• In order to be lymphoid organ – need a partial or
complete connective tissue capsule surrounding the
organ
• Primary lymphoid organs – a lymphoid organ where
the lymphocytes receive immunocompetence ( Bone
marrow & Thymus)
• Secondary lymphoid organs – receive lymphocytes for
residence that have received immunocompetence in the
primary lymphoid organs (lymph nodes, spleen,
vermiform appendix, tonsils)
Lymphatic Vessels
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Lymph capillaries
Lymphatic collecting vessels
Lymphatic trunks
Lymphatic Ducts
Lymphatic Flow Vessels
• Lymphatic flow is a one way flow – from
lymphatic capillaries through lymphatic
collecting vessels to lymphatic trunks to
lymphatic ducts – then into the blood
stream
• Therefore lymphatic vessels drain the tissue
fluids (interstitial fluids) and return
substances to the bloodstream
• Unlike blood flow which is continuous and
circular- lymph begins in blind capillaries
Lymphatic Capillaries
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Similar to blood capillaries, with modifications:
Very permeable
Loosely joined endothelial mini-valves
Withstand interstitial pressure and remain open
The mini-valves function as one-way gates that:
Allow interstitial fluid to enter lymph capillaries
Do not allow lymph to escape from the
capillaries
Figure 20.1
Lymphatic Collecting Vessels
•Have the same three tunics as veins
•Have thinner walls, with more internal
valves
•Anastomose more frequently
•Collecting vessels in the skin travel with
superficial veins
•Deep vessels travel with arteries
•Nutrients are supplied from branching
vasa vasorum
Figure 20.2a
Lymphatic Trunks
Lymphatic trunks are formed by the
union of the largest collecting ducts
Major trunks include: Paired lumbar,
bronchomediastinal, subclavian, jugular
trunks and a single intestinal trunk
Lymphatic Ducts
Lymph is delivered into one of two large
ducts from the lymphatic trunks allowing
lymph to enter the bloodstream
•Right lymphatic duct – drains the right
upper arm and the right side of the head
and thorax
•Thoracic duct – arises from the cisterna
chyli and drains the rest of the body
Figure 20.2b
Lymph Transport
The lymphatic system lacks a pumping organ
•Vessels are low-pressure conduits
•Uses the same methods as veins to propel
lymph:
•Pulsations of nearby arteries
•Contractions of smooth muscle in the walls of
the lymphatics
Organization of Lymphatic System
•Cells
•Tissues – both diffuse and compact (nodules)
•Organs – must have a capsule
Lymph nodes, spleen, thymus, tonsils, appendix
Cells of the Lymphatic System
•Two main varieties of Lymphocytes:
T cells
B cells
Other cells of the Lymphatic System
Macrophages – phagocytize foreign
substances and help activate T cells
Dendritic cells – spiny-looking cells with
functions similar to macrophages
Reticular cells – fibroblast–like cells that
produce a stroma, or network, that supports
other cell types in lymphoid organs
Lymphoid Tissue
Diffuse lymphatic tissue – scattered reticular
tissue elements in every body organ
Larger collections appear in the lamina propria
of mucous membranes and lymphoid organs
Lymphatic follicles (nodules) – solid, spherical
bodies consisting of tightly packed reticular
elements and cells
Germinal center composed of dendritic and B
cells
Found in isolation and as part of larger
lymphoid organs
Figure 20.3
Lymph Nodes
Principal lymphoid organs of the body
Embedded in connective tissue and clustered along
lymphatic vessels
Aggregations of these nodes occur near the body
surface in inguinal, axillary, and cervical regions of
the body
Two basic functions:
Filtration – macrophages destroy microorganisms
and debris
Immune system activation – monitor for antigens
and mount an attack against them
Figure 20.2a
Figure 20.4
Figure 20.4a
Figure 20.4b
Thymus
•A bilobed organ that secretes hormones (thymosin and
thymopoietin) that cause T lymphocytes to become
immunocompetent
•The lobes are separate from one another – but are hooked
together by dense irregular collagenous connective tissue
•Size of the thymus varies with age: In infants, it is found in
the inferior neck and extends into the mediastinum where it
partially overlies the heart
•It increases in size and is most active during childhood
It stops growing during adolescence and then gradually
atrophies (involutes)
Figure 20.5
• The dense connective tissue capsule of
the Thymus – sends septa into the two
lobes of the Thymus – partitioning them
into lobules.
• Each lobule is composed of an outer
cortex and an inner medulla
Figure 20.7
Cells of Thymus
• The thymus does not have a reticular fiber
stroma like the other lymphoid organs –
instead it has “epithelial reticular cells”
Cortical Epithelial Reticular Cells are the
• Type I, II, and III – these three types of
epithelial reticular cells form tight junctions
around the blood vessels in the Thymus to
form – the Thymic- Blood Barrier
Medullary Epithelial Reticular
Cells
• Type IV, V and VI. The Type IV form the
Hassall’s corpuscles of the Thymus
• The Hassall’s corpuscles used to be thought of
as a place where T-cells were destroyed –
recent evidence shows that this is the site of
production of regulatory T-cells important in
preventing autoimmune actions
The thymus differs from other lymphoid organs in
important ways
1. It functions strictly in T lymphocyte maturation
It does not directly fight antigens (Blood – Thymus
Barrier)
2. The stroma of the thymus consists of starshaped epithelial cells (not reticular fibers)
These thymocytes secrete the hormones that
stimulate lymphocytes to become
immunocompetent
Spleen
•Largest lymphoid organ, located on the left side of the
abdominal cavity beneath the diaphragm
•Fibroelastic connective capsule with some smooth
muscle
•It is served by the splenic artery and vein, which enter
and exit at the hilus
•No cortex or medulla – white pulp, red pulp and
marginal zone
Functions:
•Site of lymphocyte proliferation
•Immune surveillance and response
•Cleanses the blood – old red blood cells
•Traps some platelets
Additional Spleen Functions
•Stores breakdown products of RBCs for later
reuse
•Spleen macrophages salvage and store iron
for later use by bone marrow
•Site of fetal erythrocyte production (normally
ceases after birth)
•Stores blood platelets
Structure of the Spleen
Surrounded by a fibrous capsule, it has
trabeculae that extend inward and contains
lymphocytes, macrophages, and huge numbers
of erythrocytes
Two distinct areas:
White pulp – containing mostly lymphocytes
suspended on reticular fibers and involved in
immune functions
Red pulp – remaining splenic tissue concerned
with disposing of worn-out RBCs and
bloodborne pathogens
Figure 20.6
Tonsils
Simplest lymphoid organs; form a ring of
lymphatic tissue around the pharynx
Location:
•Palatine tonsils – either side of the posterior
end of the oral cavity
•Lingual tonsils – lie at the base of the tongue
•Pharyngeal tonsil – posterior wall of the
nasopharynx
•Tubal tonsils – surround the openings of the
auditory tubes into the pharynx
Ring of Waldeyer
Figure 22.3b
Figure 20.8
MALT
MALT – mucosa-associated lymphatic tissue:
•GALT- Peyer’s patches, tonsils, and the
appendix (digestive tract)
•BALT- Lymphoid nodules in the walls of the
bronchi (respiratory tract)
MALT protects the digestive and respiratory
systems from foreign matter
Aggregates of Lymphoid Follicles
•Peyer’s patches – isolated clusters of lymphoid
tissue, similar to tonsils
•Found in the wall of the distal portion of the
small intestine
•Similar structures are found in the appendix
•Peyer’s patches and the appendix:
•Destroy bacteria, preventing them from
breaching the intestinal wall
•Generate “memory” lymphocytes for long-term
immunity
Figure 20.9