THE LYMPHATIC SYSTEM Consists of two semiindependent parts

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 (3 L 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 • 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: • • • 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 • • 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 • • • 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 • • • 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 • • • 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 • • 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 • • 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 thymus lacks B cells, it has no follicles

THYMUS

LYMPHOID ORGANS TONSILS • • 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: – Palatine Tonsils: • • Paired tonsils located on either side at the posterior end of the oral cavity Largest tonsils and the most often infected – Lingual Tonsils: • • Paired lumpy collections of lymphoid follicles Lie at the base of the tongue – Pharyngeal Tonsil: referred to as adenoids if enlarged • In the posterior wall of the nasopharynx – Tubal Tonsils: • Surround the openings of the auditory tubes into the pharynx

PALATINE TONSILS

LYMPHOID ORGANS TONSILS • 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