Transcript the lymphatic system

THE LYMPHATIC SYSTEM
• Consists of two semi-independent parts:
– 1. A meandering network of lymphatic vessels:
– Transport back to the blood any fluids that have
escaped from the blood vascular system
– 2. Various lymphoid tissues and organs scattered
throughout the body:
– Contain phagocytes and lymphocytes which
play essential roles in:
» Resistance of the body to disease
» Defensive mechanisms of the body
LYMPHATIC VESSELS
• Hydrostatic and colloid osmotic pressures operating at
capillary beds force fluid out of the blood at the arterial ends of
the beds and cause most of the fluid to be reabsorbed at the
venous ends
– The fluid that remains behind in the tissues spaces (3L
daily) becomes part of the interstitial fluid
– This leaked fluid, plus any plasma proteins that escape from
the bloodstream, must be carried back to the blood to
ensure that the cardiovascular system has sufficient blood
volume to operate properly
– When interstitial fluid enters the lymph vessels it is called
lymph (means clear water)
LYMPHATIC VESSELS
• The lymphatic vessels form a one-way
system in which lymph flows only toward the
heart:
– The lymphatic transport system starts with the
microscopic blind-ended lymphatic capillaries,
found between the tissue cells and blood capillaries,
in the loose connective tissue
• Widespread but absent from bones and teeth, bone
marrow, and the entire central nervous system (where
excess fluid drains into the cerebrospinal fluid)
LYMPHATIC CAPILLARIES
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Owe their permeability to two
unique structural modifications:
– 1. Endothelial cells forming the
walls are not tightly joined
• They overlap loosely, forming
easily opened, flaplike
minivalves
– 2. Collagen filaments anchor the
endothelial cells to surrounding
structures so that any increase in
interstitial fluid volume opens the
minivalves, rather than causing
the lymphatic capillaries to
collapse
• One-way swinging door
• Pressure outside greater:
valves open
• Pressure inside greater:
valves close
Distribution and special features of
Lymphatic Capillaries
LYMPHATIC VESSELS
• Proteins in the interstitial space are unable to enter
blood capillaries, but they enter lymphatic capillaries
easily
• When tissues are inflamed, lymphatic capillaries
develop openings that permit uptake of even larger
particles such as cell debris, pathogens, and cancer
cells
– The pathogenic agents and cancer cells can then use the
lymph system to travel throughout the body
• Threat to the body is partly resolved by the fact that the lymph
takes detours through lymph nodes, where it is cleansed of
debris and “examined” by cells of the immune system
LYMPHATIC CAPILLARIES
• Highly specialized lymphatic capillaries
called lacteals are present in the
fingerlike villi of the intestinal mucosa
– Lymph draining from the digestive viscera is
milky white rather than clear because the
lacteals play a major role in absorbing
digested fats from the intestine
• This fatty lymph, called chyle (juice), is also
delivered to the blood via lymphatic stream
LYMPHATIC VESSELS
• The lymph capillaries flow
into the lymphatic collecting
vessels and carry the lymph
to the lymphatic trunks
– The lymphatic collecting
vessels have the same three
tunics as veins, but the
collecting vessels are thinnerwalled, have more internal
valves, and anastomose
(branch connections) more
• In skin travel along with
superficial veins
• Trunk and digestive viscera
travel with deep arteries
LYMPHATIC SYSTEM
LYMPHATIC VESSELS
• The lymphatic trunks
drain fairly large areas
of the body and
eventually empty the
lymph back into the
circulatory system via
the thoracic duct or the
right lymphatic duct
– Major trunks:
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Lumbar
Bronchomediastinal
Subclavian
Jugular
Intestinal (single)
Major veins in the Superior Thorax showing the entry points
of the Thoracic and Right Lymphatic Ducts
LYMPHATIC VESSELS
• Lymph is eventually
delivered to one of two large
ducts in the thoracic region
– The right lymphatic duct
drains lymph from the right
upper arm and the right side
of the head
– The much larger thoracic
duct receives lymph from
the rest of the body
– Each terminal duct empties
its lymph into the venous
circulation at the junction of
the internal jugular vein and
subclavian vein on its own
side of the body
LYMPHATIC SYSTEM
Major veins in the Superior Thorax showing the entry points
of the Thoracic and Right Lymphatic Ducts
HOMEOSTATIC IMBALANCE
• Lymphangitis:
– When lymphatic vessels are severely
inflamed, the related vessels of the vasa
vasorum (minute blood vessels that are
distributed to the walls of the larger veins,
arteries, or lymph vessels) become congested
with blood
• The pathway of the associated superficial
lymphatics becomes visible through the skin as red
lines that are tender to the touch
LYMPH TRANSPORT
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Lymphatic system lacks an organ that acts as a pump
Transport is slow and sporadic
About 3 L of lymph enters the bloodstream every 24 hours
Lymphatic vessels are low-pressure vessels that use the same
mechanisms as veins to return the lymph to the circulatory system:
– Milking action of active skeletal muscles
– Pressure changes in the thorax during breathing
– Valves to prevent backflow
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Lymphatics are usually bundled together in connective tissue sheaths along
with blood vessels, and pulsations of nearby arteries also promote lymph
flow
Smooth muscles in the walls of the trunks and ducts contracts
rhythmically, helping to pump the lymph along
When physical activity or passive movements increase, lymph flows
much more rapidly (balancing the greater rate of fluid loss from the
blood)
– Hence, it is a good idea to immobilize a badly infected body part to hinder
flow of inflammatory material from that region
HOMEOSTATIC IMBALANCE
• Lymphedema: short-term but severe
localized edema
– Cause:
• Anything that prevents the normal return of lymph
to the blood
– Blockage of the lymphatics by tumors
– Removal of lymphatics during cancer surgery
• However, lymphatic drainage is eventually
reestablished by regrowth from the vessels
remaining in the area
LYMPHOID CELLS
• Infectious microorganisms that manage to penetrate the body’s
epithelial barriers quickly proliferate in the underlying loose
connective tissues
• Lymphocytes:
– Main warriors of the immune system
– Produced in the red marrow (along with other formed elements)
– Mature into one of the two main varieties of immunocompetent
cells: Cells that protect the body against antigens
» Antigens are anything the body perceives as foreign, such as bacteria and
their toxins, viruses, mismatched RBCs, or cancer cells
• T cells (T lymphocytes):
– Activated T cells manage the immune response and some of them directly attack
and destroy foreign cells
• B cells (B lymphocytes):
– B cells protect the body by producing plasma cells, daughter cells that secrete
antibodies into the blood (or other body fluids)
» Antibodies immobilize antigens until they can be destroyed by phagocytes
or other means
LYMPHOID CELLS
• Macrophages play an
important role in body
protection by phagocytizing
foreign substances and in
activating T lymphocytes
• Dendritic cells, found in
lymphoid tissue, also play a
role in T lymphocyte activation
• Reticular cells produce the
reticular fiber stroma, which is
the network that supports the
other cell types in the lymphoid
organ
Scanning electron micrograph of reticular tissue in
a human lymph node
LYMPHOID TISSUE
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Houses and provide a proliferation
site for lymphocytes
Furnishes an ideal surveillance site for
lymphocytes and macrophages
Largely composed of a type of loose
connective tissue called reticular
connective tissue:
– Dominates all the lymphoid
organs except the thymus
– Macrophages live on the fibers of
the reticular network
– Lymphocytes live in the spaces of
the network
• Temporary resident
• Cycle between the circulatory
vessels, lymphoid tissues,
and loose connective tissues
of the body seeking infected
or damaged sites
LYMPH NODES
• The principle lymphoid
organs in the body are the
lymph nodes, which act as
filters to remove and destroy
microorganisms and other
debris from the lymph before
it is transported back to the
bloodstream:
– Hundreds of these small
organs
– Usually embedded in
connective tissue
– Large clusters occur near
body surfaces in the inguinal,
axillary, and cervical regions,
places where the lymphatic
collecting vessels converge to
form trunks
LYMPH NODES
• Two basic functions: both concerned with
body protection:
– 1. They act as lymph “filters”:
• Macrophages in the nodes remove and destroy
microorganisms and other debris that enter the lymph from
the loose connective tissues, effectively preventing them
from being delivered to the blood and spreading to other
parts of the body
– 2. They help activate the immune system:
• Lymphocytes, also strategically located in the lymph nodes,
monitor the lymphatic stream for the presence of antigens
and mount an attack against them
Structure of Lymph Node
• Vary in shape and size
– Most are bean shaped and
less than 2.5 cm (1 inch) in
length
• Each lymph node is
surrounded by a dense
fibrous capsule from which
connective tissue strands
called trabeculae extend
inward to divide the node
into a number of
compartments
– The node’s internal framework
or stroma of reticular fibers
physically supports the everchanging population of
lymphocytes
Structure of Lymph Node
• Histologically distinct
regions
• Cortex:
– Superficial portion contains
densely packed follicles, many
with germinal centers heavy
with dividing B cells
– Dendritic cells nearly
encapsulate the follicles and
abut the deeper portions of the
cortex, which primarily houses
T cells in transit
• The T cells circulate
continuously between the
blood, lymph nodes, and
lymph, performing their
surveillance role
Structure of Lymph Node
• Histologically distinct
regions
• Medulla cords:
– Thin inward extensions from
the cortical tissue, contain
both types of lymphocytes
plus plasma cells
– Lymph sinuses: large lymph
capillaries spanned by crisscrossing reticular fibers
• Numerous macrophages
reside on these reticular
fibers and phagocytize
foreign matter in the lymph
as it flows by in the sinuses
STRUCTURE OF A LYMPH NODE
Circulation in the Lymph Nodes
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Lymph enters the convex side of a
lymph node through a number of
afferent lymphatic vessels
It then moves through a large,
baglike sinus, the subcapsular
sinus, into a number of smaller
sinuses that cut through the cortex and
enter the medulla
The lymph meanders through these
sinuses and finally exits the node at
its hilus, the indented region on the
concave side, via efferent lymphatic
vessels
– Since there are fewer efferent
vessels than afferent vessels,
the lymph stagnates in the
nodes allowing time for
cleansing
• Lymph passes through
several nodes before it is
completely cleansed
STRUCTURE OF A LYMPH NODE
HOMEOSTATIC IMBALANCE
• Swollen Glands: erroneous name
– Node is overwhelmed by the agents it is trying to
destroy
• Becomes inflamed, swollen, and tender to the touch
– Infected lymph node
• Called buboes (most obvious symptom of bubonic plague in
the late Middle Ages)
• Secondary Cancer Sites:
– Metastasizing (spreading) cancers
• Cells enter and become trapped
– The fact that cancer-infiltrated lymph nodes are swollen but not
painful helps distinguish cancerous lymph nodes from those
infected by microorganisms
LYMPHOID ORGANS
SPLEEN
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Although all lymphoid organs
help protect the body, only the
lymph nodes filter lymph
Spleen:
– Largest lymphoid organ
– Size of a fist
– Located in the left side of the
abdominal cavity directly
below the diaphragm
– The spleen’s macrophages
function to remove old and
defective RBCs and
platelets as well as foreign
matter and debris from the
blood (blood-cleansing)
• It also provides a site for
lymphocyte proliferation
and immune surveillance
LYMPHOID ORGANS
LYMPHOID ORGANS
SPLEEN
• Site of erythrocyte
production in the fetus
• Stores platelets
• Stores some of the
breakdown products of red
blood cells for later reuse
• The spleen is surrounded by a
fibrous capsule and contains
both lymphocytes found in
white pulp (immune functions),
and macrophages found in red
pulp (disposal of worn-out red
blood cells and bloodborne
pathogens)
SPLEEN
HOMEOSTATIC IMBALANCE
• Because the spleen’s capsule is
relatively thin, a direct blow or severe
infection may cause it to rupture,
spilling blood into the peritoneal cavity
– Splenectomy: removal of the spleen
• Surgical removal of the spleen seems to create
few problems because the liver and bone
marrow take over most of its functions
– In children under 12, the spleen will regenerate if a small
part of it is left in the body
LYMPHOID ORGANS
THYMUS
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Bilobed
Important functions primarily
during the early years of life
– During adolescence its growth
stops, and it starts to atrophy
gradually
Secretes hormones (thymosin and
thymopoietin) that cause T
lymphocytes to become
immunocompetent:
– Enables them to function against
specific pathogens in the immune
response
– Functions strictly in T
lymphocyte maturation and
thus is the only lymphoid organ
that does not directly fight
antigens
The thymus is made up of thymic
lobules containing an outer cortex and
an inner medulla
LYMPHOID ORGANS
THYMUS
• Hassall’s (thymic)
corpuscles: appear to
be areas of
degenerating cells,
but their significance
is unknown
• Because the thymus
lacks B cells, it has
no follicles
THYMUS
LYMPHOID ORGANS
TONSILS
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Simplest lymphoid organs
Form a ring of lymphoid tissue around
the opening to the pharynx (throat)
Appear as swellings of the mucosa
that gather and remove many of the
pathogens entering the pharynx in
food or inhaled air
Named according to location:
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Palatine Tonsils:
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Lingual Tonsils:
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Paired lumpy collections of lymphoid
follicles
Lie at the base of the tongue
Pharyngeal Tonsil: referred to as
adenoids if enlarged
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Paired tonsils located on either side at
the posterior end of the oral cavity
Largest tonsils and the most often
infected
In the posterior wall of the nasopharynx
Tubal Tonsils:
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Surround the openings of the auditory
tubes into the pharynx
PALATINE TONSILS
LYMPHOID ORGANS
TONSILS
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Crypts: Blind-ended invaginations
deep into the interior of the tissue
of the tonsil
– Trap bacteria and particulate
matter:
• The bacteria work their
way through the mucosal
epithelium into the
lymphoid tissue, where
most are destroyed
• It seems a bit dangerous to
“invite” infection this way,
but this strategy produces
a wide variety of immune
cells that have a “memory”
for the trapped pathogens
– Body takes a
calculated risk early on
(during childhood) for
the benefits of
heightened immunity
and better health later
PALATINE TONSILS
LYMPHOID ORGANS
• Peyer’s patches:
– Large isolated clusters of
lymphoid follicles
– Structurally similar to
tonsils
– Located in the wall of the
distal portion of the small
intestine
– Heavily concentrated in the
wall of the appendix
– Destroy bacteria
– Generate many “memory”
lymphocytes for long-term
immunity
LYMPHOID ORGANS
PEYER’S PATCHES
MALT
• Mucosa-Associated Lymphatic Tissue
(MALT): collection of small lymphoid
tissues that protects the digestive and
respiratory tracts from the never-ending
onslaughts of foreign matter entering
these cavities
– Digestive tract:
• Peyer’s patches, appendix, tonsils
– Respiratory tract:
• Lymphoid follicles in the walls of the bronchi
DEVELOPMENTAL ASPECTS
OF
THE LYMPHATIC SYSTEM
• By the fifth week of embryonic development, the
beginnings of the lymphatic vessels and the
main clusters of lymph nodes are apparent and
develop from the budding of lymph sacs from the
developing veins
• The thymus is an endodermal derivative, while
the rest of the lymphoid organs derive from the
mesoderm
• Except for the spleen and the tonsils, the
lymphoid organs are poorly developed at birth