Lymphatic System: Composition and Function

Lymphatic System Basics

• It includes lymph, lymphatic vessels, lymph nodes, the spleen, thymus, tonsils, and Peyer's patches in the small intestine.
• Lymphatic vessels transport lymph, and lymph nodes filter it, key for maintaining fluid balance and immune function.
• Three liters of fluid remain post hydrostatic and osmotic pressure balance.
• Leaked fluid and plasma proteins in the interstitial space must return to circulation via the lymphatic system.
• Lymph is the name for fluid once it enters lymphatic vessels.

Composition and Function of Lymph

• Lymph is interstitial fluid that has entered lymphatic vessels.
• This clear to yellowish fluid is found throughout the body.
• It collects bacteria, unwanted materials, and fat, filtering them through the lymphatic system.
• Lymph contains lymphocytes and proteins, circulating to collect harmful substances.
• The lymphatic system is a network of vessels, nodes, and lymph, playing a crucial role in immunity.

Types and Flow of Lymphatic Vessels

• Lymph flows in a one-way system towards the heart, similar to venous blood flow.
• Lymphatic vessels include capillaries, collecting vessels, trunks, and ducts.
• Lymph flows from capillaries through larger, thicker-walled channels.
• Lymphatic capillaries are similar to blood capillaries but with modifications.
• Lymphatic capillaries are more permeable, even to proteins that cannot enter blood capillaries.
• Loosely joined endothelial cells create minivalves in lymphatic capillaries.
• Minivalves are one-way gates, allowing interstitial fluid to enter but preventing lymph from escaping.
• They withstand interstitial pressure, remaining open.
• During inflammation, lymph capillaries absorb cell debris, pathogens, and cancer cells, unlike blood capillaries.
• Lymph nodes cleanse and examine this debris.
• Lacteals, specialized lymph capillaries in the intestinal mucosa, absorb digested fat and deliver chyle to the blood.

Lymphatic Collecting Vessels

• Lymphatic collecting vessels have the same three tunics as veins, but thinner walls and more internal valves.
• They anastomose more frequently than veins.
• In the skin, collecting vessels travel with superficial veins, while deep vessels travel with arteries.
• Nutrients are supplied via branching vasa vasorum.

Lymphatic Trunks and Ducts

• Lymphatic trunks are formed where collecting vessels unite.
• Major trunks include lumbar (R/L), bronchomediastinal (R/L), subclavian (R/L), jugular (R/L), and a single intestinal trunk.
• Lymph is delivered into either the right lymphatic duct or the thoracic duct.
• The right lymphatic duct drains the right upper arm, head, and thorax, emptying into venous circulation.
• It drains at the junction of the right internal jugular and subclavian veins.
• The thoracic duct arises from the cisterna chyli and drains the rest of the body.
• It empties into venous circulation at the junction of the left internal jugular and subclavian veins.

Lymph Transport Mechanisms

• The lymphatic system lacks a pump.
• Vessels are low-pressure conduits.
• It uses similar methods as veins to propel lymph.
• Mechanisms include pressure changes in the thorax during breathing and smooth muscle contractions in vessel walls.
• Minivalves and pulsations of nearby arteries aid lymph flow.
• Physical activity increases lymph flow.

Lymph Nodes

• Lymph nodes are the principal lymphoid organs.
• They are embedded in connective tissue and clustered along lymphatic vessels.
• Aggregations occur in inguinal, axillary, and cervical regions.
• Lymph node functions include filtration, where lymphatic cells destroy pathogens and debris.
• Lymph nodes also activate the immune system, monitoring for antigens and mounting attacks.
• Trabeculae divide nodes into compartments.
• Medullary cords from the cortex contain B, T, and plasma cells, essential for immune responses.
• Lymph sinuses are crisscrossed by reticular fibers throughout the node.
• Macrophages reside on these fibers, phagocytizing foreign matter.
• Lymph enters via afferent vessels, flows into the subcapsular sinus then smaller sinuses.
• It meanders through these sinuses and exits via efferent vessels at the hilus.
• Lymph stagnates due to fewer efferent vessels, allowing lymphocytes and macrophages to act.

Lymph Node Structure

• Nodes are bean-shaped, with a fibrous capsule and two distinct regions called the cortex and medulla.
• The cortex contains follicles with germinal centers where B cells and macrophages divide.
• The deep cortex houses T cells and macrophages in transit.
• T cells circulate continuously among the blood, lymph nodes, and lymphatic stream.
• Dendritic cells nearly encapsulate the follicles.

Other Lymphoid Organs

• These include the spleen, thymus gland, tonsils, and Peyer's patches.
• These are composed of reticular connective tissue, protecting the body.
• Only lymph nodes filter lymph.
• Lymphoid tissue can be diffuse or in follicles.
• Diffuse lymphoid tissue is found in every organ, made of reticular connective tissue.
• It houses lymphoid cells like T cells, B cells, macrophages, and dendritic cells, patrolling the body.
• Lymphoid follicles are compact, spherical bodies of lymphoid cells and reticular fiber.
• B cells are predominant in follicles, found in Peyer's patches and the appendix, becoming larger lymphoid organs such as lymph nodes.

Lymphoid Cells

• T cells manage the immune response, attacking and destroying foreign cells.
• B cells produce plasma cells, which secrete antibodies that immobilize antigens.
• Macrophages phagocytize foreign substances and help activate T cells.
• Dendritic cells are spiny-looking cells that function similarly to macrophages.
• Reticular cells are fibroblast-like cells that produce a network, supporting other cell types in lymphoid organs.

Spleen

• The largest lymphoid organ lies on the left side of the abdominal cavity beneath the diaphragm.
• It is served by the splenic artery and vein, entering and exiting at the hilus.
• Spleen functions include lymphocyte proliferation, immune surveillance and response, and blood cleansing.
• Spleen macrophages salvage and store iron from old RBCs.
• It is a site of fetal RBC production and stores blood platelets.
• Surrounded by a fibrous capsule, it contains trabeculae with lymphocytes, macrophages, and erythrocytes.
• White pulp contains lymphocytes and is involved in immune functions.
• Red pulp is concerned with disposing of worn-out RBCs and bloodborne pathogens.

Tonsils

• Tonsils, simplest lymphoid organs, form a lymphatic tissue ring around the pharynx.
• Palatine tonsils are on either side of the oral cavity's posterior end.
• Lingual tonsils lie at the base of the tongue.
• Pharyngeal tonsils are on the nasopharynx's posterior wall.
• Tubal tonsils surround the auditory tubes' openings.
• Lymphoid tissue of tonsils contains follicles with germinal centers.
• The epithelium overlying the tonsil destroys bacteria and particulate matter.