Lymphatic System: Organs, Tissues, and Functions

Lymphatic System and Lymphoid Organs and Tissues

• The lymphatic system encompasses a variety of organs, tissues, and vessels.
• The lymphatic system has a diverse range of bodily functions.
• The lymphatic system is comprised of lymphatic vessels (lymphatics), lymph fluid, and lymph nodes.
• Lymphoid organs and tissues create the basis for the immune system.
• Phagocytic cells and lymphocytes are housed in lymphoid organs and tissues
• Structures include the spleen, thymus, tonsils, lymph nodes, and other lymphoid tissues.

Functions of the Lymphatic System

• Fluids and proteins that leak from the vascular system are returned to the blood via the lymphatic system.
• Lipids and lipid-soluble vitamins absorbed by the digestive canal are transported.
• The lymphatic system is responsible for carrying out the immune response.
• Non-specific resistance in the lymphatic system provides general protection against disease,
• Immunity in the lymphatic system offers specifc protection.

Lymphatic Vessels

• Lymphatic vessels begin as lymphatic capillaries, located in spaces between cells, and are larger than blood capillaries
• Lymphatic vessels are not found in avascular tissues portions of the spleen, and red bone marrow.
• Avascular tissues include cartilage, epidermis, and cornea of the eyes.

Lymphatic Capillaries

• Fluid flow is regulated by structure and function.
• Anchoring filaments from lymphatic endothelium attach to surrounding tissues
• Endothelial cells overlap, and high hydrostatic fluid pressure separates cells, allowing fluid to enter capillaries.
• Hydrostatic fluid pressure in the capillaries prevents fluid from moving out.

Lymph Flow

• More fluid exits blood capillaries through filtration than returns through reabsorption
• About 3L of lymph is generated per day.
• Proteins escaped from the blood or secreted by tissues are transferred back to the blood by the lymphatics.
• Lymph flow is aided by muscle pumps, the respiratory pump, and valves prevent backflow.
• Pulsations of nearby arteries and contractions of smooth muscle in walls of lymphatics also facilitate flow.
• In the skin, lymph vessels flow along the veins, while in viscera lymph flow follows arteries
• Lymph vessels mirror the organization and routing of the vascular tree.
• Lymphatic vessels have no separate pump, like the heart
• All lymph eventually returns to the vena cava and right side of the heart.
• Lymph returns to venous drainage via right and left lymphatic ducts at the junction of the internal jugular and subclavian veins.

Lymph Ducts

• Lymph ducts includes the right lymphatic duct and the thoracic (left) duct
• The right lymphatic duct is about 1/2 inch long.
• The right lymphatic duct drains lymph from the upper right side of the body.
• Location includes the arm, head, and thorax.
• The thoracic (left) duct is the main collecting duct of the lymphatic system.
• The thoracic duct is 38-45 cm long and drains 75% of the body.
• It begins as a dilation known as cisterna chyli, located anterior to lumbar disk #2.
• The direction of lymph flow is from smallest to largest: capillaries, vessels, trunks, ducts
• Lymph vessels anastomose, supplying and draining lymph nodes along their course.
• Skeletal muscle pumps and pressure changes in the thorax contribute to lymph flow through vessels

Lymphoid Cells and Tissues

• Lymphoid cells are found in lymphoid organs and connective tissue of other organs.
• Lymphoid cells consist of immune cells found in tissues along with supporting cells. B and T Lymphocytes, macrophages, dendritic cells and reticular cells are found in lymphoid tissue.
• Lymphoid tissue include diffuse lymphoid tissue and lymphoid follicles (nodules).

Lymphoid Cells - Lymphocytes

• Lymphocytes are formed in red marrow, they are warriors of the immune system protecting against antigens. Anything that provokes an immune response, such as bacteria and their toxins, viruses, mismatched RBCs, or cancer cells, are antigens
• Lymphocytes mature into two main types: B cells (B lymphocytes) and T cells (4 types).

Other Lymphoid Cells

• Macrophages phagocytize foreign substances, and dendritic cells capture antigens and transport them to lymph nodes.
• Reticular cells are similar to fibroblasts Produce reticular fibers (stroma) that support other cells in lymphoid organs.

Lymphoid Tissue - General

• Lymphoid tissue contains lymphocytes and reticular connective tissue.
• Lymphoid tissue is important to the immune system because it houses lymphocytes and provides a site for their activation and proliferation, providing a surveillance point for lymphocytes and macrophages.
• Diffuse lymphatic tissue is made of scattered reticular tissue parts, it is found in nearly every organ.
• Lymphoid follicles (nodules) are solid balls of tightly packed reticular fibers and lymphoid cells without a capsule.
• The germinal center contains dendritic and B cells.
• Follicles can form part of larger lymphoid organs, like lymph nodes.
• Isolated aggregations of lymphoid follicles occur in the intestinal walls as Peyer's patches, tonsils, and in the appendix.
• The right lymphatic duct drains lymph from the right upper limb and the right side of the head and thorax.

Lymphoid Organs and Tissues

• Primary lymphoid organs are where B and T cells mature.
• B cells mature in red bone marrow, T cells mature in the thymus.
• Secondary lymphoid organs is the site where mature B and T lymphocytes encounter their antigen and are activated.
• Lymph nodes, spleen, MALTs, tonsils, Peyer's patches, and the appendix are secondary lymphoid tissues.
• Lymphocytes are activated in diffuse lymphoid tissues.

Lymphoid Organs - Thymus Gland

• The thymus gland has two lobes between the sternum and the aorta.
• It atrophies with age, starting around age 20.
• The thymus gland is a primary lymphoid organ.
• The thymus differs from other lymphoid organs because it lacks follicles and B cells, does not directly fight antigens, functions strictly in T lymphocyte maturation and the stroma is made of epithelial cells, not reticular fibers

Lymphoid Organs - Lymph Nodes

• Lymph nodes are oval, bean-shaped structures along lymph vessels.
• They may be deep or superficial throughout the body, but are concentrated along the respiratory tree and GI tract, glands, axillae, and groin.
• The nodes filter lymph fluid, trapping foreign organisms, debris, and tumor cells.
• Lymph nodes are covered by a connective tissue capsule, with trabeculae extending from cortex to medulla.
• A supportive connective tissue network of reticular fibers, fibroblasts, capsule, and trabeculae exist.
• The cortex, which contains densely packed follicles, is the superficial part of the lymph node and contains germinal centers heavy with dividing B cells.
• The deeper part of the lymph node cortex contains T lymphocytes in transit, and dendritic cells associated with B and T lymphocytes are also present.
• Medullary cords of lymphocytes, macrophages, and plasma cells are also located in the medulla of the lymph node.
• Histologically, germinal centers, medulla of lymph node, capsule, lymphatic nodule, and cortex of lymph node are visible in lymph nodes.
• Afferent vessels carry lymph to the cortex, then the it enters a large subcapsular sinus before traveling into smaller sinuses.
• Reticular fibers filter and trap damaged cells, microorganisms, foreign substances, and tumor cells.
• Macrophages phagocytize some trapped materials; lymphocytes destroy others with immune defenses.
• Lymph stagnates in lymph nodes because there are fewer efferent vessels, allowing lymphocytes and macrophages time to carry out protective functions.

Homeostasis of the Lymph Nodes

• Buboes are inflamed and swollen lymph nodes that are painful.
• Metastasis occurs when cancer cells from a tumor are trapped in lymph nodes, this is generally not painful.

Lymphoid Organs - Spleen

• The spleen is the largest mass of lymphoid tissue in the body.
• It is encapsulated by a fibrous capsule with an artery, vein, and efferent lymph vessels.
• The spleen is located between the stomach and diaphragm.
• Spleen structure has white pulp where immune functions take place, consisting mostly of lymphocytes suspended on reticular fibers; red pulp is where worn-out red blood cells and blood pathogens are destroyed, containing macrophages.
• The spleen is a site of lymphocyte proliferation, and cleans the blood.
• Spleen macrophages salvage and store iron for later use by bone marrow.
• The spleen is a site of fetal erythrocyte production, though it normally ceases after birth. The spleen also stores blood platelets, with the red pulp acting as a blood storage area for release during emergencies.

Lymphoid Organs - MALT

• Lymphoid tissues in mucous membranes is throughout the body.
• MALT protects against pathogens trying to enter the body and is found in the mucosa of the respiratory tract, genitourinary organs, and digestive tract.
• The largest collections of MALT are found in tonsils and Peyer's patches.

Lymphoid Organs - Tonsils

• Tonsils are the simplest lymphoid organ.
• They form a ring of lymphoid tissue around the entrance of the throat.
• Follicles with germinal centers are surrounded by lymphocytes; tonsils are not fully encapsulated.
• Tonsillar crypts trap bacteria and particles, and bacteria move into tissue where they are destroyed.
• Palatine tonsils are the largest and most frequently infected.
• Peyer's Patches are also called aggregated lymphoid nodules.
• Large clusters of lymphoid follicles located in the wall of the distal portion of the small intestine, Peyers patches are structurally similar to tonsils

Lymphadenopathy

• This occurs when lymph nodes become enlarged and tender due to infection or cancer. It is often called "swollen glands".
• Lymphocyte leukemia can cause severe lymphadenopathy.
• Hodgkin's Lymphoma is a malignancy of lymphoid tissue with symptoms including painless, nontender enlargement of one or more lymph nodes, fatigue, intermittent fever and night sweats.
• Non-Hodgkin's Lymphoma includes all cancers of lymphoid tissues except Hodgkin's lymphoma.
• It involves uncontrolled metastasis and undifferentiated lymphocytes, with swellings of the lymph nodes, spleen, and Peyer's patches.
• Other organs may eventually be involved.

Burkitt Lymphoma

• A type of non-Hodgkin's lymphoma.
• The disease is found in central Africa and New Guinea.
• The African (endemic) Type is associated with Epstein-Barr virus and commonly involves the jaw.
• American (sporadic) type is not associated with EBV, exhibits extensive marrow replacement.
• Immunodeficiency-associated type is associated with 90% of HIV cases and can come from immunosuppressive drugs.
• The cancerous cell is a B cell and is nearly 50% curable with extensive chemotherapy.

Explore the lymphatic system's organs, tissues, and vessels. Learn about its functions, including fluid and protein return, lipid transport, and immune response. Discover non-specific resistance and immunity within the lymphatic system.