The Lymphatic System
Chapter 21

Introduction
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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
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The lymphatic vessels transport fluids that have escaped from the cardiovascular system

Lymphatic Vessels
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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
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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
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Once interstitial fluid enters the lymphatics ducts it is called lymph

Distribution of Lymphatic Vessels
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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
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Lymph capillaries weave between the tissue cells and blood capillaries in the loose connective tissue of the body

Distribution of Lymphatic Vessels
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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
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Although similar to blood capillaries, lymphatic capillaries are remarkably permeable
The great permeability is due to structural modifications
– Minivalves
– Anchoring filaments

Minivalves
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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
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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
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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
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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
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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
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From the lymphatic capillaries, lymph flows through successively larger channels
– Collecting vessels
– Trunks
– Ducts

The Lymphatic System
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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
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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
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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
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Each terminal duct empties the lymph into the venous circulation at the junction of the internal jugular vein on its side of the body

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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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Lymph nodes are small (2.5 cm), bean shaped structures surrounded by a fibrous capsule of connective tissue

Lymph Nodes
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Trabecula are connective tissue strands that extend inward to divide the node into compartments

Lymph Nodes
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Its internal of framework of reticular fibers physically supports the ever-changing population of lymphocytes

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Two histologically distinct regions in a lymph node are the cortex and the medulla
These areas contain densely packed follicles with dividing B cells
Lymph Nodes
Medulla
Cortex

Lymph Nodes
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The outer cortex contains densely packed follicles, many with germinal centers heavy with dividing B cells

Lymph Nodes
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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
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Medullary cords are thin inward extensions of the cortex containing lymphocytes and plasma cells

Lymph Nodes
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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
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Lymph enters the convex side of a lymph node through a number of afferent lymphatic vessels

Circulation in Lymph Nodes
Subcapsular sinus
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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
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Lymph meanders through these sinuses and finally exits the node at its hilus, via efferent lymphatic vessels

Circulation in Lymph Nodes
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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
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Inflammation of a node is caused by a large number of bacteria trapped in a node
– Inflammation results in swelling and pain
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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
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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
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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
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The soft, blood rich spleen is about the size of fist and is the largest lymphoid organ

The Spleen
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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
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The spleen is served by the large splenic artery and vein which enter at the hilus

The Spleen
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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
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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
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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
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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
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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
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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
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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
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The bilobed thymus has important functions primarily during the early years of life

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In infants, it is found in the inferior neck and extends into the mediastinum of the superior thorax where it partially overlies the heart
Thymus

The Thymus
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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
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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

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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
Tonsils

The Tonsils
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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
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The tonsils gather and remove many of the pathogens entering the pharynx in inhaled air or in food

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The lymphoid tissue of the tonsils contains follicles with obvious germinal centers surrounded by diffusely scattered lymphocytes
Tonsils
Germinal centers

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The tonsil masses are not fully encapsulated, and the epithelium invaginates deep into the interior forming blind ended structures called crypts
Tonsils

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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

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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
Tonsils

Aggregates of Lymphoid Follicles
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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
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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
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Lymphoid follicles are also heavily concentrated on the walls of the appendix

Peyer’s Patches
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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
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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
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This is the end of the material on the lymphatic system