Chapter 12: Organs of the Lymphatic System
Lymphatic System
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2 parts that work semi-independently from each other
o Lymphatic vessels  send fluids that leaked out of the blood back to the blood
o Lymphatic organs and tissues  home to phagocytes and lymphocytes that fight invading
microorganisms
Part 1: Lymphatic Vessels (LV) AKA Lymphatics
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Drain excess fluid from extracellular space (once in lymphatic vessels called lymph) and return it
to blood
Make a 1-way system where fluid flows towards the heart
How Does Excess Fluid Get Into Extracellular Spaces?...
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When blood goes into capillary beds the pressure in the arteriole end is high (going from a blood
vessel w/ a large diameter to a smaller one) high pressure pushes fluid out
Most fluid is reabsorbed at the venous end b/c pressure in venule is lower than pressure outside
venule
Fluid that is left behind (3 L/day) and not reabsorbed becomes part of the interstitial fluid (fluid
that bathes cells, part of extracellular fluid)
The fluid and plasma proteins that leak out of the blood vessels must be returned to the blood
otherwise blood volume will be too low for normal function and excess fluid will collect in the
tissues and cause swelling  edema (too much swelling inhibits tissue cells from exchanging
nutrients and waste w/ the interstitial fluid and ultimately the blood)
How do LV work?
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Microscopic lymph capillaries weave between blood capillaries and tissue cells to absorb fluid
LV are more permeable than blood capillaries
o Made by endothelial that overlap each other creating flaps
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o Flaps are anchored down by fine collagen fibers that
attach to surrounding tissues allowing the flaps (aka
minivalves) to open in only 1 direction
 When pressure outside LV is greater, minivalves
open and fluid moves in
 “ inside LV is greater, minivalves close and fluid is
trapped
o Proteins and larger substances like viruses, bacteria,
cancer cells, and remains of dead cells can enter LV esp in
inflamed areas b/c high pressure due to fluid
accumulation blood capillaries do not allow these
substances in
Potential problem: diseases can spread through the lymphatic
system
Which is why it’s important that lymph goes through lymph nodes where immune system cells
can go to work (GET INTO DETAIL LATER)
Lymph goes from capillaries to larger lymphatic vessels:
LC lymphatic collecting vessels  returned to venous system by 2 ducts that insert into subclavian
on their side:
1. Right lymphatic duct (drains R arm, R side of head and thorax)
2. Thoracic duct (gets from rest of body)
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LV have thin walls
Larger LV have valves to prevent backflow like veins b/c low pressure, pumpless system
Lymph is moved thru LV by:
o contracting skeletal muscles (milking action)
o Pressure changes in the thorax during breathing (squeeze LV)
Lymph Nodes
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Remove foreign substances, ex. bacteria & tumor cells, from lymph
Sizes vary ~2.5 cm or less
Kidney-shaped
Are thousands that filter lymph in LV buried in connective tissue
Large clusters in Axillary, inguinal, & cervical regions
Produce lymphocytes (type of WBC that fight foreign substances by producing protein particles that
capture invaders, such as viruses) that play a role in the immune response
o Lymphocytes- produced in red bone marrow but move to lymph nodes and organs where
they multiply
Have macrophages – engulf and destroy captured material before fluid is returned to the blood
When sick, lymph nodes well b/c of the trapping nature of lymph nodes
o If large # of viruses or bacteria get trapped nodes swell and are painful
o If cancer nodes swell but are not painful
Structure
Have a fibrous capsule with strands called trabeculae that extend inward to make the node
compartmentalized
Cortex- outer part of node, has collections of lymphocytes called follicles w/ dark-staining centers
called germinal centers and other cells called T-cells
o Germinal centers – enlarge when specific lymphocytes (B cells) are dividing into daughter
cells AKA plasma cells that release antibodies
o T cells circulate continuously between blood, lymph nodes, and lymphatic stream
“looking” for “invaders”
Medulla- center part of the lymph node, home to phagocytic macrophages
1. Lymph enters the convex (big-C) side of lymph node thru afferent lymphatic vessels
2. Lymph flows thru sinuses
3. Exits from the hilum, little-C, region via efferent lymphatic vessels
Are fewer efferent vessels than afferent so flow of lymph is slow  allows time for lymphocytes
and macrophages to look for “invaders”
Lymph has to pass thru several nodes to be cleaned
Other Lymphoid Organs (besides lymph nodes)
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All contain reticular connective tissue (make a net-like structure for support) and lymphocytes
NONE filter lymph to cleanse it BUT they do help protect the body
(reticular connective tissue in box)
(zoomed in, arrows: reticular fibers, dark spots:
reticular cells & lymphocytes)
1. Spleen
a. Soft organ on left side of abdominal cavity inferior to the diaphragm, curls around aterior
aspect of the stomach
b. Filters and cleanses blood of bacteria, viruses, and cellular debris
c. *** main function is to destroy old RBC and return certain pieces to liver  iron (in hB),
rest of hB protein to make bile
d. Also
i. Site of lymphocyte proliferation (multiplication) and immune suravlence
ii. Stores blood platelets for clotting
iii. Blood reservoir (liver too) so if there a hemorage organ contracts to release blood
to keep bl volume normal
2. Thymus gland
a. Lymphoid mass low in the throat, overlies the heart
b. Works at peak levels when young – produces hormones that help you program certain
lymphocytes so they can carry out their protective functions
3. Tonsils
a. Small masses of lymphoid tissue, ring around the pharynx
b. Trap and remove bacteria and pathogens in the throat
c. If trap too much bacteria & patheogens they swell and get sore – tonsillitis
4. Peyers Patches
a. Look like tonsils
b. Found in wall of small intestine
c. Contain macrophages that capture and destroy bacteria to prevent them from penetrating
the intestinal wall
MALT- mucosa-associated lymphoid tissue; includes peyers patches and tonsils; protect the upper
repiratory tract and entire digestive tract from foreign substances that constantly enter those
cavities.