Download Skeletal System

The Lymphatic System
Chapter 21
Introduction


The lymphatic system supports the
function of the cardiovascular and
immune systems of the body
The lymphatic system consists of two
semi-independent parts
– A network of lymphatic vessels
– Lymphoid organs scattered throughout the
body

The lymphatic vessels transport fluids
that have escaped from the cardiovascular system
Lymphatic Vessels


As blood circulates through the body,
exchanges of nutrients, wastes, and gases
occur between the blood and the
interstitial fluid
The fluid that remains behind in the
tissue spaces, as much as 3 liters a day,
become part of interstitial fluid
Lymphatic Vessels



These leaked fluids, as well as any plasma
proteins that escape from the blood-stream,
must be carried back to the blood if the
cardiovascular system is to sufficient blood
volume to operate properly
The lymphatics are elaborate system of
drainage vessels that collects the excess
protein-containing interstitial fluid and
returns it to the bloodstream
Once interstitial fluid enters the lymphatics
ducts it is called lymph
Distribution of Lymphatic Vessels


The lymphatic vessels form a one-way system in which
lymph flows only toward the heart
The system begins with the lymph capillaries
Distribution of Lymphatic Vessels

Lymph capillaries weave between the tissue cells and
blood capillaries in the loose connective tissue of the
body
Distribution of Lymphatic Vessels


Lymph capillaries are widespread,
occurring almost everywhere blood
capillaries occur
Lymph capillaries are absent from bone
and teeth, bone marrow, and the entire
central nervous system
Distribution of Lymphatic Vessels


Although similar to blood capillaries,
lymphatic capillaries are remarkably
permeable
The great permeability is due to
structural modifications
– Minivalves
– Anchoring filaments
Minivalves

The endothelial cells
forming the walls of the
lymph capillaries are
not tightly joined;
instead their edges
loosely overlap forming
easily opened, flaplike
minivalves
Anchoring Filaments

Bundles of fine
filaments anchor the
endothelial cells to
surrounding structures
so that any increase in
interstitial fluid volume
separates the cell flaps,
exposing gaps in the
wall and allowing fluid
to enter rather than the
capillary collapsing
Lymphatic Vessels


These structural
modifications create a
system where the valves
gap open when fluid
pressure is greater in
the interstitial space,
allowing fluid to enter
the lymphatic capillary
Pressure inside the
lymphatic capillary
forces the minivalve
flaps together preventing a leak back out
Lymphatic Vessels



Proteins present in the interstitial fluid
are prevented from entering the blood
capillaries but enter lymphatic capillaries
In addition, when tissues are inflamed,
lymphatic capillaries develop openings
that permit uptake of even larger
particles such as cell, pathogens, bacteria,
viruses, and cancer cells
Thus cancer cells can use lymphatic
capillaries to travel throughout the body
Lymphatic Vessels




Highly specialized lymphatic capillaries called
lacteals are present in the fingerlike villa of
the intestinal mucosa
The lymph draining from the digestive viscera
is milky white rather than clear because the
lacteals also receive digested fat from the
intestine
This creamy lymph, called chyme, is also
delivered to the blood via the lymphatic
system
This concept discussed further in Chap 24
The Lymphatic System

From the lymphatic
capillaries, lymph
flows through
successively larger
channels
– Collecting vessels
– Trunks
– Ducts
The Lymphatic System


Collecting vessels have
the same three tunics
as veins, but they are
thinner-walled, have
more internal valves,
and anastomose more
In general the collecting vessels in the skin
travel along with
superficial veins of the
CV system while deep
vessels of the trunk
travel with arteries
The Lymphatic System


The lymphatic trunks
are formed by the
union of the largest
collecting vessels, and
drain fairly large
areas of the body
The trunks are named
for the areas from
which they collect
lymph
– Lumbar
– Bronchomediastinal
– Subclavian
The Lymphatic System



Lymph is delivered to
one of two large ducts
in the thoracic region
The right lymphatic
duct drains lymph
from the upper arm
and the right side of
the head and thorax
The larger thoracic
duct receives lymph
from the rest of the
body
The Lymphatic System

Each terminal duct empties the lymph into the venous
circulation at the junction of the internal jugular vein
on its side of the body
Lymph Transport




Unlike the cardiovascular circulation, the
lymphatic system lacks an organ that acts as a
pump
Under normal conditions, lymphatic vessels
are very low pressure conduits
Compression of skeletal muscle, pressure
changes associated with respiration and valves
to prevent back flow, aid the movement of
lymph
Smooth muscle in the lymphatic duct
contracts rhythmically to move lymph along
Lymph Transport



About 3 liters of lymph enters the bloodstream every 24 hours, a volume that
almost equal to the amount of fluid lost to
the tissue spaces from the bloodstream in
the same time period
Movement of the adjacent tissues are
extremely important in propelling lymph
through the lymphatics
Physical activity or passive movement
increase lymph flow
Lymphoid Cells

In order to understand some of the basic
aspects of the lymphatic system’s role in
body protection and immunity it is
necessary to understand the components
– Lymphoid cells
– Lymphoid tissues
Lymphoid Cells


Infectious microorganisms, such as
bacteria and viruses, that manage to
penetrate the body’s epithelial barrier
begin to quickly proliferate in the
underlying loose tissue
These invaders are fought off by the
inflammatory response by phagocytes
(macrophages) and lymphocytes
Lymphoid Cells


Lymphocytes, the main warriors of the
immune system, arise in red bone
marrow
They then mature into one of the two
main varieties of immunocompetent cells
– T cells (T lymphocytes)
– B cells (B lymphocytes)

These cells act to protect the body against
antigens (bacteria and their toxins,
viruses, mismatched RBC’s, or cancer
cells
Lymphoid Cells



Activated T cells manage the immune
response and some of them directly
attack and destroy foreign cells
B cells protect the body by producing
plasma cells, daughter cells that secrete
antibodies into the blood
Antibodies immobilize antigens until they
can be destroyed by phagocytes
Lymphoid Cells



Lymphoid marcophages play a crucial
role in body protection and in the
immune response by phagocytizing
foreign substances and helping to activate
T cells
Dendritic cells found in lymphoid tissue
also activate T cells
Reticular cells are fibroblast cells that
produce the reticular fiber stroma or
network that supports the other cells
types in the lymphoid organs
Lymphoid Tissue

Lymphoid tissue is an important
component of the immune system
because it
– Houses and provides a proliferation site for
lymphocytes
– Furnishes an ideal surveillance vantage point
for both lymphocytes and macrophages
Lymphoid Tissue


Lymphoid tissue, a
type of loose
connective tissue
called reticular
connective tissue,
dominates all
lymphoid organs
except the thymus
The dark staining
areas represent the
connective tissue
fibers
Lymphoid Tissue


Macrophages live
on the fibers of the
network
Within the spaces
of this network are
huge numbers of
lymphocytes
Macrophage
Lymphocytes
Reticular
fiber
Lymphoid Tissue


Lymphocytes squeeze through the walls
of capillaries and venules to reside
temporarily in the lymphoid tissue and
then leave to patrol the body
The cycling of lymphocytes between the
circulatory vessels, lymphoid tissues, and
loose connective tissues of the body
ensures that lymphocytes reach infected
or damaged sites quickly
Lymphoid Organs


Lymphoid organs
as exemplified by
lymph nodes, the
spleen, and the
thymus are discrete
collections of
lymphoid tissue
The exact pattern
of the lymphoid
tissue differs in the
various lymphoid
organs
Lymphoid Organs


Lymphoid organs are discrete,
encapsulated collections of diffuse
lymphoid tissue and nodules
The exact pattern of lymphoid tissue
differs in the various lymphoid organs
Lymph Nodes

As lymph is transported back to the bloodstream, it is
filtered through lymph nodes that cluster along the
lymphatic vessels of the body
Lymph Nodes



There are hundreds of
lymph nodes that are
usually imbedded in
connective tissue an
not seen
Large clusters of
lymph nodes occur
near the body surface
in the inguinal,
axillary, and cervical
regions of the body
Located where vessels
form large trunks
Lymph Nodes

Lymph nodes have two basic functions,
both concerned with body protection
– They act to filter lymph
• Phagocytic macrophages in the nodes remove
and destroy microorganisms and other debris
that enter the lymph from the loose connective
tissue, effectively preventing further spread
– They play a role in activating the immune
system
• Lymphocytes in the lymph nodes monitor the
lymphatic stream for the presence of antigens
and attack them
Lymph Nodes

Lymph nodes are small (2.5 cm), bean shaped
structures surrounded by a fibrous capsule of
connective tissue
Lymph Nodes

Trabecula are connective tissue strands that extend
inward to divide the node into compartments
Lymph Nodes

Its internal of framework of reticular fibers physically
supports the ever-changing population of lymphocytes
Lymph Nodes


Two
histologically
distinct regions
in a lymph
node are the
cortex and the
medulla
These areas
contain densely
packed follicles
with dividing B
cells
Cortex
Medulla
Lymph Nodes

The outer cortex contains densely packed follicles,
many with germinal centers heavy with dividing B
cells
Lymph Nodes


Dendritic cells nearly encapsulate the
follicles and abut the rest of the cortex,
which primarily houses T cells in transit
The T cells circulate continuously
between the blood, lymph nodes, and
lymphatic stream, performing their
surveillance role
Lymph Nodes
Medullary
cords

Medullary cords are thin inward extensions of the
cortex containing lymphocytes and plasma cells
Lymph Nodes



Throughout the node are lymph sinuses
which are large lymph capillaries spanned by
reticular fibers
Numerous marcophages reside on these
reticular fibers and phagocytize foreign
matter in the lymph as it flows by the sinuses
Lymph borne antigens in the lymph leak into
the surrounding reticular tissue, where they
activate some of the strategically positioned
lymphocytes to mount an immune response
Circulation in Lymph Nodes

Lymph enters the convex side of a lymph node
through a number of afferent lymphatic vessels
Circulation in Lymph Nodes
Subcapsular sinus

Lymph moves through a large, baglike sinus, the subcapsular sinus, into a number of smaller sinuses that
cut through the cortex and enter the medulla
Circulation in Lymph Nodes

Lymph meanders through these sinuses and finally
exits the node at its hilus, via efferent lymphatic
vessels
Circulation in Lymph Nodes


Because there are fewer efferent vessels
draining the node than there afferent
vessels feeding it, the flow of lymph
through the node stagnates somewhat,
allowing time for the lymphocytes and
macrophages to carry out their protective
functions
In general, lymph passes through several
nodes before its cleansing process is
completed
Lymph Nodes: Clinical

Inflammation of a node is caused by a
large number of bacteria trapped in a
node
– Inflammation results in swelling and pain

Lymph nodes can become secondary
cancer sites, particularly in metastasizing
cancers that enter lymphatic vessels and
become trapped
– Cancer infiltrated nodes are swollen but not
painful
Other Lymphoid Organs


Lymph nodes are just one type of many
types of lymphatic tissue
Other lymphoid organs include
•
•
•
•
Spleen
Thymus gland
Tonsils
Peyer’s patches
Other Lymphoid Organs


The common feature of all lymphoid
organs is that they are all composed of
reticular connective tissue
Additionally, all lymphoid tissues help
protect the body
Spleen

The soft, blood rich spleen is about the size of
fist and is the largest lymphoid organ
The Spleen


Located in the left side of the abdominal
cavity just beneath the diaphragm
It extends to curl around the anterior
aspect of the stomach
Spleen

The spleen is served by the large splenic artery
and vein which enter at the hilus
The Spleen



The spleen provides a site for lyphocyte
proliferation and immune surveillance
and response
However, even more important is the
blood cleaning functions
It extracts aged and defective blood cells
and platelets from the blood, its macrophages remove debris, foreign matter,
bacteria, viruses, and toxins from blood
flowing through its sinuses
The Spleen

The spleen also performs three additional
and related functions
– It stores some of the breakdown products of
red blood cells for later use and releases
others to the blood for processing by the
liver
– Spleen marcophages salvage and store iron
for later use by the bone marrow in making
hemoglobin
The Spleen

The spleen also performs three additional
and related functions
– It is a site for erythrocyte production in the
fetus (ends after birth)
– It stores blood platelets
Spleen



The spleen is surrounded by a fibrous capsule
and has trabeculae which extend inward to
divide the organ
It contains both lymphocytes and macrophages
Consistent with its blood processing functions,
it also contains huge numbers or erythocytes
Spleen



Areas composed mostly of erythrocytes
suspended in reticular fibers are called white
pulp.
The white pulp clusters or forms “cuffs”
around the central arteries
Red pulp is essentially all remaining splenic
tissue
Spleen



The red pulp consist of venous sinuses
These regions of reticular connective tissue are
exceptionally rich in macrophages
Red pulp is more concerned with disposing of
worn out red blood cells and blood born
pathogens
Spleen



White pulp is involved with the immune
function of the spleen
It dispatches macophages to circulate in the
blood
It is mobilzed to combat infections
Thymus

The bilobed thymus has important functions
primarily during the early years of life
Thymus

In infants, it is
found in the
inferior neck and
extends into the
mediastinum of the
superior thorax
where it partially
overlies the heart
The Thymus


By secreting hormones the thymus enables
T lymphocytes to function against specific
pathogens in an immune response
The thymus varies with age
–
–
–
–
–
Prominent in newborns
Size increases in childhood
Growth stops during adolescence
It atrophies in adulthood
By old age it has been largely replaced by
fibrous and fatty connective tissue
The Thymus

The thymus differs from other lymphoid
organs in two important ways
– It functions strictly in T lymphocyte
maturation and thus is the only lymphoid
organ that does not directly fight antigens
– The stroma of the thymus consists of starshaped epithelial cells rather than reticular
fibers. These thymocytes secrete the
hormones that stimulate the lymphocytes to
become immunocompetent
Tonsils



The tonsils are
perhaps the
simplest lympoid
organs
They form a ring
of lymphatic
tissue around the
entrance to the
pharynx
They appear as
swellings of the
mucosa
The Tonsils

The tonsils are named according to
location
– Palatine tonsils are located on either side at
the end of the oral cavity
– The lingual tonsils lies at the base of the
tongue
– The pharyngeal tonsils (adenoids if enlarged)
are found on the posterior wall of the
nasopharynx
– The tubal tonsils surround the openings to the
auditory tubes into the pharyx
The Tonsils

The tonsils gather and remove many of
the pathogens entering the pharynx in
inhaled air or in food
Tonsils

The lymphoid
tissue of the tonsils
contains follicles
with obvious
germinal centers
surrounded by
diffusely scattered
lymphocytes
Germinal
centers
Tonsils

The tonsil masses
are not fully
encapsulated, and
the epithelium
invaginates deep
into the interior
forming blind
ended structures
called crypts
Tonsils

The crypts trap
bacteria and
particulate matter,
and the bacteria
work their way
through the
muscosal epithelium
into the lymphoid
tissue where most
are destroyed
Tonsils


By inviting an
infection the tissue
produces a wide
variety of immune
cells with a
“memory” for the
trapped pathogens
The early risk
during childhood
results in better
health in adulthood
Aggregates of Lymphoid Follicles

In addition to the lymphoid organs
previously described there are two
additional forms of lymphoid tissues that
appear as isolated follicles of tissue
– Peyer’s patches
– Mucosa-associated lymphoid tissue (MALT)
Peyer’s Patch


Peyer’s patches are
large isolated clusters
of lymph follicles
Structurally similar
to the tonsils, they
are found in the wall
of the distal portion
of the small intestine
Peyer’s Patch

Lymphoid follicles
are also heavily
concentrated on the
walls of the appendix
Peyer’s Patches


Peyers patches and the appendix are
ideally situated to destroy bacteria thereby
preventing these pathogens from breaching
the intestinal wall
In addition these tissues develop “memory”
lymphocytes for long-term immunity
MALT

Collectively MALT acts to protect the
digestive and respiratory tracts from
foreign matter and bacteria
– Peyer’s patches, tonsils and appendix are all
located in the digestive tract
– Lymphoid nodules in the walls of the bronchi
protect the respiratory tract
Lymphatic System

This is the end of the material on the
lymphatic system