Lymphatic System

Lymph & Lymphatic Vessels Lymphatic system functions Transport and house lymphocytes and other immune cells Return excess fluid in body tissues to blood to maintain blood volume Lymphatic system components Lymph vessels, lymphatic issues and organs Lymph is the fluid transported within lymph vessels

Lymph & Lymphatic Capillaries Characteristics of lymph Some fluid leaves blood capillaries and is not reabsorbed by them This interstitial fluid moves into lymphatic capillaries where it is then called lymph Components of lymph Water, dissolved solutes, and small amount of protein Sometimes cell debris, pathogens, or cancer cells

Lymphatic capillaries Small, closed-ended vessels that absorb interstitial fluid Interspersed around most blood capillaries Absent in avascular tissues, red marrow, spleen, and CNS Slightly larger than blood capillaries; no basement membrane Walls are made of overlapping endothelial cells Have flaps between cells through which fluid enters but can’t exit Anchoring filaments hold endothelial cells to nearby structures

Movement of lymph into lymphatic capillaries Hydrostatic pressure of interstitial fluid pushes it into capillary Anchoring filaments linking endothelial cells to surrounding structures prevent vessel collapse Pressure of lymph inside vessel forces intercellular openings (“flaps”) of capillary wall to close with lymph inside Lymph moves through vessels of larger and larger size Lymphatic capillaries, lymphatic vessels, lymphatic trunks, and lymphatic ducts Ultimately, fluid is returned to blood circulation

Lymphatic Vessels, Trunks, and Ducts Lymphatic vessels are fed by lymphatic capillaries Located adjacent to arteries and veins Have all three vessel tunics (intima, media, externa) Have valves to prevent pooling and backflow of lymph Lymphatic system lacks a pump; moves lymph using: Skeletal muscles and respiratory pumps (as seen in veins) Pulsatile movement of blood in nearby arteries Rhythmic contraction of smooth muscle in larger lymph vessel walls Some vessels connect to lymph nodes for lymph filtration

Lymphatic trunks are fed by lymphatic vessels Jugular trunks drain lymph from head and neck Subclavian trunks drain upper limbs, breasts, and superficial thoracic wall Bronchomediastinal trunks drain deep thoracic structures Intestinal trunks drain most abdominal structures Lumbar trunks drain lower limbs, abdominopelvic wall, and pelvic organs

Lymphatic ducts are fed by lymphatic trunks Largest lymphatic vessels Bring lymph to venous blood circulation There are two: right lymphatic duct and thoracic duct Right lymphatic duct is near right clavicle Drains upper right quadrant of body Right side of head and neck, right upper limb, and right side of thorax Delivers lymph to junction of right subclavian and right internal jugular veins Thoracic duct is largest lymphatic vessel Runs from diaphragm to junction of left subclavian and left jugular veins Saclike cisterna chyli at its base Receives lipid-rich chyle from GI tract Drains lymph from left side of head and neck, left upper limb, left side of thorax, abdomen, and both lower limbs

Lymphedema Accumulation of interstitial fluid due to interference with lymphatic drainage Causes swelling and pain in affected area May interfere with wound healing and contribute to infection Most caused by blockage of lymph vessels E.g., trauma or infection of lymph vessels or tumors E.g., radiation therapy scarring or surgical removal of lymph nodes Controlled with compression garments, exercise, massage Extreme cases known as elephantiasis Caused by infection with parasitic worms

Overview of Lymphatic Tissue and Organs Primary lymphatic structures Involved in formation and maturation of lymphocytes Red bone marrow and thymus Secondary lymphatic structures Do not form lymphocytes, but house them and other immune cells Sites of immune response initiation Include lymph nodes, spleen, tonsils, and lymphatic nodules Include MALT (mucosa-associated lymphatic tissue)

Primary Structures: Red Bone Marrow Located between trabeculae of spongy bone In flat bones of skull, ribs, and sternum, vertebrae, ossa coxae, heads of humerus and femur Site of hemopoiesis: production of blood’s formed elements Formed elements include T-lymphocytes and B- lymphocytes T-lymphocytes migrate to thymus to complete maturation

Primary Structures: Thymus The thymus is involved in T-lymphocyte maturation Located in mediastinum Larger in children than adults Grows until puberty, then regresses; gradually replaced by adipose tissue Consists of two thymic lobes, each surrounded by connective tissue capsule Trabeculae of capsules subdivide lobes into lobules Each lobule has outer cortex and inner medulla regions Both regions are lymphoepithelial (contain lymphatic cells and epithelial tissue) Cortex contains immature T-lymphocytes Medulla contains mature T-lymphocytes

Secondary Lymphatic Structures House lymphocytes and other immune cells within a reticular connective tissue matrix Organized into lymphatic organs and aggregates of lymphatic nodules Organs have a complete capsule of dense irregular connective tissue Spleen and lymph nodes Other lymphatic structures have an incomplete capsule or lack one Tonsils, MALT, diffuse lymphatic nodules

Secondary Structures: Lymph Nodes Lymph nodes filter lymph, remove unwanted substances Small, oval, encapsulated structures Located along deep and superficial pathways of lymph vessels Occur in clusters receiving lymph from body regions Cervical lymph nodes receive lymph from head, neck Axillary lymph nodes receive lymph from breast, axilla, and upper limb Inguinal lymph nodes in groin receive lymph from lower limb and pelvis

Lymph node components Afferent lymphatic vessels bring lymph to node An efferent vessel drains a lymph node Located at hilum (involuted portion of node) Dense connective tissue capsule encloses node Trabeculae of capsule subdivide node into compartments Outer cortex, inner medulla

The cortex contains lymphatic nodules Reticular fibers support a germinal center Houses proliferating B-lymphocytes and some macrophages Surrounding mantle zone contains T- lymphocytes, macrophages, and dendritic cells Cortical sinuses are tiny open channels lined by macrophages

Lymph flow through nodes Lymph enters through several afferent vessels Creates pressure to push it through node to the single exit vessel Lymph is monitored for presence of foreign material Macrophages remove foreign debris from lymph Lymphocytes may initiate immune response Lymphocytes proliferate especially in germinal centers Can cause enlarged nodes that can be palpated in neck and axilla Lymph exits node through efferent vessel May then pass to neighboring lymph node

Lymphoma Malignant neoplasm from lymphatic structures Nontender, enlarged lymph node Possible night sweats, fever, and weight loss Hodgkin lymphoma Affects young adults and people over 60 If caught early, cured by excision, radiation, and chemotherapy Non-Hodgkin lymphoma More common Some kinds aggressive, others slow growing

Secondary Structures: Spleen The spleen is the largest lymphatic organ Located in left upper abdominal quadrant Lateral to left kidney and posterolateral to stomach Posterolateral aspect is convex and rounded Anteromedial border is concave Hilum: indentation where blood vessels and nerves enter Splenic artery supplies blood; splenic vein drains it Surrounded by connective tissue capsule Trabeculae from capsule extend into spleen dividing red and white pulp

Tissues of the spleen White pulp Red pulp Clusters of T- and B-lymphocytes and macrophages around central artery Red pulp Contains erythrocytes, platelets, macrophages, and B-lymphocytes Splenic cords: cells and reticular connective tissue in red pulp Splenic sinusoids: very permeable capillaries Storage site for erythrocytes and platelets

Monitoring blood as it flows through the spleen The spleen filters and monitors blood (not lymph) White pulp monitors it for foreign materials and bacteria Macrophages in sinusoids of red pulp remove particles Phagocytize bacteria, debris, defective erythrocytes and platelets Path of flow: splenic artery, central artery, sinusoids, venules, splenic vein Summary of functions: eat foreign particles, clear defective erythrocytes and platelets, store erythrocytes and platelets In first 5 months of fetal life, spleen makes blood cells This function can be reactivated under certain conditions E.g., some hematologic disorders

Secondary Structures: Tonsils Immune surveillance of inhaled and ingested substances Tonsillar crypts Invaginations that trap material Contain lymphatic nodules Some with germinal centers

Secondary Structures: Nodules Clusters of lymphatic cells with some extracellular matrix Not completely surrounded by connective tissue capsule Scattered nodules termed diffuse lymphatic tissue Found in every body organ Help defend against infection In some areas, group together to form larger structures E.g., MALT

Secondary Structures: MALT Mucosa-associated lymphatic tissue Located in gastrointestinal, respiratory, genital, and urinary tracts Found in lamina propria of the mucosa Help defend against foreign substances Prominent in small intestines, especially ileum Peyer patches: large collections of lymphatic nodules within the wall of the GI tract