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 cardio-vascular 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 prevent-ing 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 collect- ing 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 blood-

stream 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

Lymphocytes

Macrophage

Reticularfiber

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 are thin inward extensions of the cortex containing lymphocytes and plasma cells

Medullarycords

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

Lymph moves through a large, baglike sinus, the sub- capsular sinus, into a number of smaller sinuses that cut through the cortex and enter the medulla

Subcapsular sinus

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 macro- phages 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 star- shaped 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

Germinalcenters

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