The Lymphatic System Chapter 20
Dec 18, 2015
The Lymphatic System
Chapter 20
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
Introduction The lymphatic vessels transport fluids
that have escaped from the cardio-vascular system
The main components of the immune system (lymphocytes, lymphoid tissue, and lymphoid organs) fight infections and confer immunity to disease
The Lymphatic System An elaborate system of lymphatic vessels
runs throughout the body These vessels collect a fluid called lymph
from the loose connective tissue around blood capillaries and carry this fluid to the great veins at the root of the neck
The Lymphatic System Because lymph flows
only toward the heart, the lymphatic vessels form a one-way system rather than a full circuit
The Lymphatic System There are several
orders of vessels– Lymph capillaries
– Lymphatic collecting vessels
– Lymph nodes
– Lymph trunks
– Lymph ducts
The Lymphatic System Recall that all blood
capillaries are surrounded by a loose connective tissue that contains tissue fluid or interstitial fluid
The fluid arises from blood filtered through the capillary walls
The Lymphatic System Tissue fluid consists of
small molecules of blood plasma, water, various ions, nutrient molecules, and respiratory gases
The Lymphatic System Tissue fluid is
continuously leaving and re-entering the blood capillaries
For complex reasons slightly more fluid arises from the arteriole end than re-enters the venule end
This amounts to about 3 liters a day
The Lymphatic System The lymphatic
vessels function to collect this excess fluid and return it to the bloodstream
Any blockage of the lymphatic vessels causes the affected body region to swell with excess tissue fluid resulting in edema
The Lymphatic System The lymphatic vessels also perform another
related function Blood proteins leak slowly from blood
capillaries into the surrounding tissue fluid Lymph vessels return leaked proteins to the
bloodstream This is important because proteins in blood
generate osmotic forces that are essential for keeping water in the bloodstream
Lymph Capillaries
Lymph capillaries are permeable vessels that receive the tissue fluid
They are located near blood vessels in the loose connective tissue
Like blood capillaries their walls consists of a single layer of endothelial cells
Lymph Capillaries Lymph capillaries
are very permeable The permeability
results from the structure and arrangement of the endothelial cells
They have few intercellular junctions
Lymph Capillaries In lymph capillaries
the adjacent edges of cells overlap to form easily opened mini- valves
These valves open to allow tissue fluid to enter the lymphatic capillary
Collagen filaments anchor the cells to the connective tissue
Lymph Capillaries The minivalves work because the collagen
filaments anchor portions of the cell wall to the surrounding connective tissue
Any fluid pressure in the volume of the tissue fluid separates the minivalves
This opens gaps in the wall of the capillary allowing fluid to enter
Once in the lymphatic capillary, it cannot leak out, because backflow pressure forces the minivalve flaps together
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
Lymph Capillaries Once interstitial fluid enters the
lymphatic ducts it is called lymph Although the high permeability of lymph
capillaries allows the uptake of large quantities of tissue fluid and protein molecules it also allows bacteria, viruses, or cancer cells in the loose connective tissue to enter these capillaries with ease
Lymph Capillaries These pathogenic agents can then travel
throughout the body via the lymphatic vessels
However, most pathogenic agents are destroyed in the lymph nodes by various antibodies before reaching the general circulation
Cancer cells can be the most problematic and can actually use lymph nodes as a site to metastasize further
Lymph Capillaries Lymph
capillaries are widespread, occurring almost everywhere blood capillaries occur
Lymph Capillaries Lymph capillaries are absent from bone
and teeth, from bone marrow, and from the entire CNS
In the CNS excess fluid drains into the cerebrospinal fluid and then returns it to the blood at the superior sagittal sinus
Lymph Collecting Vessels From the lymph
capillaries, lymph enters lymphatic collecting vessels which accompany blood vessels
Lymph Collecting Vessels In general, the
superficial lymphatic collecting vessels in the skin travel with superficial veins
Deep lymphatic collecting vessels of the trunk and digestive viscera travel with the deep arteries
Lymph Collecting Vessels Lymphatic collecting vessels are narrow
and delicate and are usually not seen in a laboratory dissection
The vessels have the same tunics as blood vessels but their walls are always thinner
The thinness reflects the fact that lymph flows under very low pressure and have no pump to move the lymph along
Lymph Collecting Vessels To direct the flow of
lymph, lymphatic collecting vessels contain more valves than do veins
At the base of each valve, the vessel bulges, forming a pocket in which lymph collects and forces the valve shut
Lymph Collecting Vessels Because of these
bulges, each collecting vessel resembles a string of beads
This distinctive appearance, which characterizes the larger lymph ducts as well allow MD’s to recognize lymph vessels in X-rays
Lymph Collecting Vessels Unaided by pressure from a pump,
lymph is propelled through lymph vessels by a series of weaker mechanisms
The action of contracting skeletal muscle and the pulsation of nearby arteries push on lymph vessel, squeezing lymph through them
Lymph Collecting Vessels The muscular tunica media of the lymph
vessels also contacts to help propel the lymph
Additionally, the normal movements of the limbs and trunk keep the lymph flowing
Despite these mechanisms, the transport of lymph is slow
People who are inactive or who stand for long times often develop edema
Lymph Nodes Lymph nodes, which
cleanse the lymph of pathogens are bean shaped organs situated along lymphatic collecting vessels
The term lymph gland is not correct because they are not glandular in their function
There are about 500 lymph gland in the body
Lymph Nodes Large clusters of
superficial lymph nodes in the cervical, axillary, and inguinal regions
The superficial cervical nodes along the jugular and carotid arteries receive lymph from the head and neck
Lymph Nodes Axillary nodes in the
armpit filter lymph from the upper limbs
Lymph Nodes The inguinal nodes in
the superior thigh filter lymph from the lower limb
Lymph Nodes Nodes in the
mediastinum such as the deep tracheobronchial nodes receive lymph from the thoracic viscera
Lymph Nodes Deep nodes along the
abdominal aorta, called aortic nodes, filter lymph from the posterior abdominal wall
Lymph Nodes Finally, deep nodes
along the iliac arteries, called iliac nodes, filter lymph from the pelvic organs and the lower limbs
Lymph Nodes The microscopic
anatomy of a lymph node suggests its role as a body filter
The node is surrounded by a fibrous capsule of dense connective tissue
Lymph Nodes Fibrous strands of
connective tissue called trabecule extend inward to divide the node into compartments
Lymph Nodes Lymph enters the
convex aspect of the node through several afferent lymphatic vessels and exits from the indented region on the other side, the hilus, through efferent lymphatic vessels
Lymph Nodes Between the
afferent and efferent vessels, lymph percolates through lymph sinuses
These large lymph capillaries are spanned internally by a crisscrossing network of reticular fibers
Lymph Nodes The reticular fibers are
covered by star-shaped endothelial cells
Many macrophages live on this fiber network, phagocytizing pathogens and foreign particles in the lymph that flows through the sinuses
Lymph Nodes Most lymph passes through several nodes It is usually free of pathogens by the time
it leaves its last node and enters the lymph trunks on its way to the great veins in the neck
Lymph Nodes Along with its
lymph sinuses, a lymph node also contains tadpole shaped masses of lymphoid tissue
This tissue is divided into outer (cortex) and inner (medulla) regions and are part of the immune system
Lymph Trunks After leaving the
lymph nodes, the largest lymphatic collecting vessels converge to form lymph trunks
These trunks drain large areas of the body
Lumbar Trunks The paired lumbar
trunks lie along either side of the aorta in the inferior abdomen
They receive all lymph draining from the lower limbs, pelvic organs and some of the anterior abdominal wall
Intestinal Trunk The unpaired
intestinal trunk lies near midline on the posterior abdominal wall
It receives fatty lymph (chyme) from the stomach, intestines, and other digestive organs
Brachiomediastinal Trunks These paired
trunks ascend near the sides of the trachea
They collect lymph from the thoracic viscera and thoracic wall
Subclavian Trunks These are paired
trunks located near the sides of the trachea
These trunks receive lymph from the upper limbs
They also drain the inferior neck and the superior thoracic wall
Jugular Trunks These trunks are
located in the neck at the base of each internal jugular vein
These trunks drain lymph from the head and neck
Lymph Ducts The lymph trunks drain into the largest
vessels the lymph ducts The number of ducts in an individual
may vary from two to only only duct
Thoracic Duct The thoracic duct
is present in all individuals
Its most inferior part, located at the union of the lumbar and intestinal trucks is the cisterna chyli which lies on the bodies of vertebrae L1 + L2
Thoracic Duct The thoracic duct
ascends along the vertebral bodies
In the superior thorax, it turns left and empties into the venous circulation at the junction of the internal jugular and left subclavian veins
Right lymphatic Duct Some people have
a short right lymphatic duct formed by the union of the right jugular, subclavian, and broncho-mediastinal trunks
Right lymphatic Duct When present, this
duct empties into the neck veins at or near the junction of the right internal jugular and subclavian veins
The right lymphatic duct drains the upper quarter of the body
Right lymphatic Duct In individuals
without a right lymphatic duct, the three trunks empty separately into the neck veins
Lymphatic Vessels In summary, the lymphatic vessels….
– Return excess tissue fluid to the bloodstream– Return leaked proteins to the blood– Carry absorbed fat from the intestine to the
blood through lacteals
Immune System The immune system is central to the
body’s fight against disease Unlike the body’s other defense systems,
it recognizes and attacks specific foreign molecules
It destroys pathogens more and more effectively with each new exposure
Immune System The immune system centers around the
key defense cells from lymphocytes But it also includes lymphoid tissue, and
the lymphoid organs these include…– Lymph nodes, spleen, thymus, tonsils,
aggregated lymphoid nodules in the small intestine, and appendix
Lymphocytes Infectious microorganisms that penetrate
the epithelial barriers of the body enter the underlying loose connective tissues, where they are attacked by the inflammatory response, by macrophages and finally, by lymphocytes of the immune system
Lymphocytes are white blood cells and that each lymphocyte recognizes and attacks its own type of foreign molecule, called an antigen
Lymphocytes B lymphocytes multiply to become
plasma cells that secrete antibodies Cytotoxic (CD8+) T lymphocytes destroy
antigen bearing cells by penetrating their membranes and inducing programmed cell death
Lymphocytes B and T cells continuously travel in the
blood and lymph streams to reach infected connective tissues throughout the body, where they fight infection
They repeatedly enter and exit these connective tissues, including the often infected lymphoid tissue, by squeezing through the walls of capillaries and venules
Lymphocytes This repeated movement of activated
lymphocytes between the circulatory vessels and the connective tissues, called recirculation, ensures that lymphocytes reach all infection sites quickly
Lymphocyte Activation Immature lymphocytes go through
several stages before they are able to attack antigens
Most lymphocytes pass through these stages during infancy and childhood, by many do so in adulthood as well
Lymphocyte Activation
This illustration provides an overview of lymphocyte activation
Lymphocyte Activation Lymphocytes
originate in the bone marrow from lymphoid stem cells, some of which travel in the bloodstream to the thymus in the thorax and become T lymphocytes (T is for Thymus)
Lymphocyte Activation Other
lymphocytes stay in the bone marrow and become B lymphcytes
Lymphocyte Activation These new T and B lymphocytes divide
rapidly and generate many lymphocyte families (clones), each of which is able to recognize one unique type of antigen (this is called gaining immunocompetence)
Lymphocyte Activation Young T or B
lymphocytes travel through the bloodstream to an infected connective tissue, where it binds to its specific antigen, an encounter called antigen challenge
Lymphocyte Activation As a consequence of the antigen challenge
the lymphocyte becomes fully activated, gaining the ability to attack its antigen, proliferates rapidly, and produces mature lymphocytes that recirculate throughout the body seeking pathogens to attack
Lymphocyte Activation During the antigen challenge, an activating
lymphocyte interacts with several other cells types
The lymphocyte receives its antigen from an antigen presenting cell, such as a macrophage that has recently phagocytized the antigen, or a star shaped dendritic cell, a professional antigen gathered that patrols the body seeking antigens and carries them to places where lymphocytes gather
Lymphocyte Activation A distinct type of lymphocyte, called a
helper (CD4+) T lymphocyte, secretes chemical signals that greatly stimulate the proliferation of activating B and cytotoxic T lymphocytes
Helper T cells are important because their signals amplify and fine-tune the immune response
Lymphocyte Activation The importance of helper T lymphocytes
is illustrated by acquired immune deficiency syndrome (AIDS), a viral disease in which a drastic decline in the body’s helper T cells greatly weakens the immune system
Lymphocyte Activation As more activating T or B cells
proliferates within infected connective tissue, it produces two types of mature lymphocytes, effector and memory
Short lived effector lymphocytes attack the pathogen immediately and then die
Memory lymphocytes, by contrast, wait until the body encounters their antigen again - maybe decades later
Lymphocyte Activation When a memory lymphocyte finally
encounters its antigen, its proliferating response and its attack are most vigorous and rapid
Memory lymphocytes are the basis for acquired immunity
They guard against subsequent infections and prevent us from getting many diseases more than once
There are T and B varieties of memory lymphocytes
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
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 is the most important
tissue of the immune system The tissue is an often infected connective
tissue in which vast quantities of lymphocytes gather to fight invading microorganisms
This tissue had two general locations– Mucous membranes– Lymphoid organs
Lymphoid Tissue Mucous membranes
– Found within the digestive, respiratory, urinary and reproductive tracks where it is called mucosa-assocaited lymphoid tissue (MALT)
Lymphoid Organs– Lymph nodes, spleen, thymus, tonsils,
aggregated lymphoid nodules, and appendix
Lymphoid Tissue This tissue is the main battleground in
the fight against infection Lymphoid tissue is where most
lymphocytes become activated and most effector and memory lymphocytes are generated
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 The structural features
of lymphoid tissue serves its infection fighting role
It is a reticular connective tissue whose basic framework is a network of reticular fibers secreted by reticular cells (fibroblasts)
Lymphoid Tissue Within the spaces
of this network reside the many T and B lymphocytes that arrive continuously from venules coursing through the tissue
Lymphocytes
Macrophage
Reticularfiber
Lymphoid Tissue Macrophages on
the fiber network kill invading microorganisms by phagocytosis and along with dendritic cells, they activate nearby lymphocytes by presenting them with antigens
Lymphocytes
Macrophage
Reticularfiber
Lymphoid Tissue Evident within
lymphoid tissues are scattered, spherical clusters of densely packed lymphocytes, called lymphoid nodules or follicles
These nodules often exhibit lighter staining germinal centers
Lymphoid Tissue Nodules derive from
the activation of a single B cell, whose rapid proliferation generates the thousands of lymphocytes in the nodule
Newly produced B cells migrate away from the nodule to become plasma cells
Lymphoid Organs Lymphoid organs are
the lymph nodes, spleen, thymus, aggregated lymphoid nodules in the small intestine, and appendix
Lymphoid Organs Lymph nodes are
more than filters The regions of the
node between the lymph sinuses are tadpole shaped masses of lymphoid tissue
Lymphoid Organs As lymph moves
through the sinuses, some of the contained antigens leak out through the sinus wall into the lymphatic tissue
Most antigen challenges in the human body occur in the lymph nodes
Lymphoid Organs In the lymph nodes
antigens are destroyed and B and T lymphocytes are activated
The activation adds to the body’s supply of memory lymphocytes that offer long term immunity
Lymphoid Organs Lymph nodes have
two histologically distinct regions, an external cortex and a medulla
All the lymphoid nodules and most B cells occupy the lymphoid tissue of the most superficial part of the cortex
Lymphoid Organs Deeper in the cortex
the lymphocytes are primarily T cells, especially helper T cells that increase the activity of B cells in the nearby nodules
Lymphoid Organs Thin, inward
extensions from the cortical lymphoid tissue help define the medulla
These cord like medullary extensions contain both T and B lymphocytes, plus plasma cells
Spleen The soft, blood rich
spleen is the largest lymphoid organ
Its size varies greatly among individual, but on average it is the size of a fist
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
Spleen The large splenic
vessels enter and exit the spleen on the anterior surface along a line called the hilus
Spleen The spleen has two main blood cleansing
functions– The removal of blood-borne antigens (its
immune function)– The removal and destruction of aged or
defective blood cells Additionally, the spleen is a site of
hematopoiesis in the fetus and stores blood platelets throughout life
Spleen The spleen is
surrounded by a fibrous capsule from which trabeculae extend inward
The larger branches of the splenic artery run in the trabeculae and send smaller arterial branches into the substance of the spleen
Spleen The arterial branches
are called central arteries because they are enclosed by thick sleeves of lymphoid tissue that collectively constitute the white pulp of the spleen
Spleen Blood borne antigens
enter this lymphoid tissue and are destroyed as they activate the immune response
Surrounding the white pulp is red pulp which has two parts– Venous sinuses
– Splenic cords
Spleen Venous sinuses are
sinusoid capillaries that arise from the distal branches of the central arteries outside of the white pulp
Spleen Splenic cords consist
of reticular connective tissue that is exceptionally rich in macrophages
Whole blood leaks from the sinuses into this connective tissue where macrophages then phagocytize any defective blood cells
Spleen Red pulp is responsible for the spleen’s
ability to dispose of worn-out blood cells White pulp provides the immune
function of the spleen
Thymus The two lobed
thymus lies in the anterior thorax and inferior neck, just posterior to the sternum
Thymus The thymus is the site at which immature
lymphocytes develop into T lymphocytes The thymus secrets thymic hormones
such as thymopoietin which causes T lymphocytes to gain immunocompetence
Thymus Prominent in newborns, the thymus
continues to increase in size during childhood when it is most active
During late adolescence, it begins to atrophy gradually, as its functional tissue is slowly replaced with fibrous and fatty tissue
Thymus At age 20 it still has about 80% of its
functional tissue but at age 40 it typically retains only 5% of its functional tissue
By age only 2% of functional tissue remains and the thymus is a fatty mass that is difficult to distinguish from surrounding connective tissue
However, even as it atrophies, the thymus continues to produce immunocompetent cells throughout adulthood (reduced rate)
Thymus
Again, the thymus lies in the superior thorax
Thymus
The thymus contains numerous lobules arranged like fronds in the head of a cauliflower
Each lobule contains an outer cortex and an inner medulla
Thymus The cortex is packed with rapidly
dividing T lymphocytes gaining immunocompetence
The medulla contains fewer lymphocytes In addition, the medulla contain the
thymic (Hassall’s) corpuscles which seem to be collections of degenerating epithelial reticular cells
The number and size of these corpuscles increases with age
Thymus The thymus differs from other lymphoid
organs in two basic ways (First) – It functions strictly in lymphocyte
maturation and thus is the only lymphoid organ that does not directly fight antigens
– The blood-thymus barrier, keeps blood-borne antigens from leaking out of thymic capillaries and prematurely activating the immature thymic lymphocytes
Thymus The thymus differs from other lymphoid
organs in two basic ways (Second)– The tissue framework of the thymus is not a
true lymphoid connective tissue– The thymus arises like a gland from the
epithelium lining the embryonic pharynx– Its basic tissue framework consists of star
shaped epithelial cells rather that reticular fibers
Thymus The thymus differs from other lymphoid
organs in two basic ways (Second)– These epithelial reticular cells secrete the
thymic hormones that stimulate T cells to become immunocompetent
– The thymus has no lymphoid nodules because it lack B cells
The Tonsils The tonsils are perhaps the simplest
lymphoid tissue They are mere swellings of the mucosal
lining of the pharynx
The Tonsils
There are four groups of tonsils, palatine, lingual, pharyngeal, and tubal
The Tonsils The palatine tonsils lie directly posterior
to the mouth and palate on the lateral sides of the pharyngeal wall
These are the largest tonsils and the ones most often infected and removed during childhood (tonsillectomy)
The lingual tonsil lie on the posterior surface of the tongue
The Tonsils The pharyngeal tonsil (adenoids) lie on
the pharyngeal roof The tubal tonsils are just behind the
openings of the pharyngotympanic tubes into the pharynx
The Tonsils The four tonsils are arranged in a ring
around the entrance to the pharynx to gather and remove many pathogens that enter the pharynx in inspired air and swallowed food
The tonsils process the antigens, then set up immune responses
The Tonsils The tonsils
consist of an epithelium underlain by a connective tissue lamina propria
The Tonsils In the tonsils the
underlying propria consists of abundant mucosa associated lymphoid tissue (MALT) packed with lymphocytes and scattered lymphoid nodules
Germinalcenters
The Tonsils The overlying
epithelium invaginates deep into the interior forming blind ended structures called crypts that trap bacteria and particulate matter
The Tonsils The trapped
bacteria work their way through the epithelium into the underlying lymphoid tissue causing the activation of lymphocytes
The Tonsils The trapping of bacteria in the crypts
leads to many infections during childhood
But it also generates a great variety of memory lymphocytes for long-term immunity
The 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 Many bacteria permanently inhabit the
hollow interior of the intestines and are constantly infecting the intestinal walls
To fight these invaders, MALT is especially abundant in the intestine
In two parts of the intestine, MALT is so large, permanent, and densely packed with lymphocytes that is said to form lymphoid organs: Aggregated lymphoid nodules and the appendix
Aggregates of Lymphoid Follicles Aggregated
lymphoid nodules (Peyer’s patches) are clusters in the walls of the distal part of the ileum of the small intestine
About 40 of these patches are present averaging about 1 cm and 1 cm wide
Peyer’s Patches
Aggregates of Lymphoid Follicles Aggregates of lymphoid follicles are ideally
situated to destroy bacteria thereby preventing these pathogens from breaching the intestinal wall
The Appendix The appendix is a
tubular offshoot of the first part of the (cecum) of the large intestine
The Appendix Lymphoid tissue is
also heavily concentrated on the walls of the appendix
Histological sections reveal that dense lymphoid tissue uniformly occupies over half the thickness of the wall of the appendix
The Appendix Beside destroying the microorganisms
that invade them, the aggregated lymphoid nodules and the appendix sample many different antigens from within the digestive tube and generate a wide variety of memory lymphocytes to protect the body