The Lymphatic System

• The body contains ten times more bacterial cells than human cells
• Some bacteria are beneficial, and some cause disease
• The immune system is a cell population that protects the body from disease agents
• The lymphatic system is a true organ system of network of organs and vein-like vessels that recover fluid

Lymphatic System Functions

• Includes the network of organs and vein-like vessels that recover fluid
• Inspects it for disease agents
• Activates immune responses
• Returns fluid to the bloodstream

Expected Learning Outcomes

• List the functions of the lymphatic system
• Explain how lymph forms and returns to the bloodstream
• Name the major cells of the lymphatic system and state their functions
• Name and describe the types of lymphatic tissue
• Describe the structure and function of the red bone marrow, thymus, lymph nodes, tonsils, and spleen

The Lymphatic System

• Composed of lymphatic vessels and lymphoid tissues and organs
• Transports fluids back to the blood
• Plays roles in body defense and resistance to disease
• Absorbs digested fat at the intestinal villi

Lymphatic Characteristics

• Lymph is excess tissue fluid carried by lymphatic vessels
• One way system toward the heart
• No pump exists in the lymphatic system
• Lymph moves toward the heart by milking action of skeletal muscle
• Lymph moves toward the heart by Rhythmic contraction of smooth muscle in vessel walls
• Main functions are to maintain fluid balance and protect body from infection and disease

Lymphatic System Components

• The recovered fluid
• Transports the recovered fluid
• Lymphatic tissues are composed of lymphocytes and macrophages
• Lymphatic tissues are aggregates of lymphocytes and macrophages that populate many organs in the body
• Defense cells are concentrated in lymphatic organs
• Lymphatic organs are separated from surrounding organs by connective tissue capsules
• Lymph is clear, colorless fluid, similar to plasma, but with less protein
• Lymph originates as extracellular fluid drawn into lymphatic capillaries
• Chemical composition varies in different places such as the intestines and lymph nodes
• Lymphatic capillaries (terminal lymphatics) penetrate nearly every tissue of the body
• Absent from the central nervous system, cartilage, cornea, bone, and bone marrow
• Capillary wall is endothelial cells overlapping each other like roof shingles
• The lymphatic capillaries are closed at one end
• Cells tethered by protein filaments
• Gaps between cells are large enough for bacteria and cells to enter
• Endothelium creates valve-like flaps that open when interstitial fluid pressure is high, and close when it is low
• Larger vessels composed of three layers

Layers of Lymphatic Vessels

• Tunica interna: endothelium and valves
• Tunica media: elastic fibers, smooth muscle
• Tunica externa: thin outer layer
• Vessels converge into larger vessels
• Collecting vessels course through many lymph nodes

Lymphatic Trunks

• Six lymphatic trunks drain major portions of the body
• Jugular, subclavian, bronchomediastinal, intercostal, intestinal (unpaired), lumbar trunks

Collecting Ducts

• Right lymphatic duct receives lymph from right arm and right side of head/thorax, and empties into right subclavian vein
• Thoracic duct is larger, begins as cisterna chyli, receives lymph from below the diaphragm, left arm, left side of head, neck, and thorax and empties into left subclavian vein
• Subclavian veins collect from thoracic duct

Lymph Flow

• Lymph flows under forces similar to venous return (except heart)
• Lymph flows at low pressure and slower speed than venous blood
• Lymph is moved along by rhythmic contractions of lymphatic vessels
• Stretching of vessels stimulates contraction
• Flow aided by skeletal muscle pump
• Arterial pulsation rhythmically squeezes lymphatic vessels
• Thoracic pump aids flow from abdominal to thoracic cavity
• Valves prevent backward flow
• Rapidly flowing blood in subclavian veins, draws lymph into it
• Exercise increases lymphatic return

Lymphatic Cells

• Natural killer (NK) cells are large lymphocytes that attack/destroy bacteria, transplanted tissue, host cells infected with viruses/cancer
• T lymphocytes (T cells) mature in thymus
• B lymphocytes (B cells) activation causes proliferation and differentiation into plasma cells that produce antibodies
• Macrophages are avidly phagocytic cells of connective tissue, develop from monocytes, phagocytize tissue debris, dead neutrophils, bacteria, and foreign matter
• Macrophages process foreign matter, display antigenic fragments to certain T cells alerting immune system
• Antigen-presenting cells APCs
• Dendritic cells are branched, mobile APCs in epidermis, mucous membranes, and lymphatic organs
• Dendritic cells alert immune system to pathogens that breached body surface
• Reticular cells are branched stationary cells that contribute to stroma of lymphatic organ

Lymphatic Tissue

• Lymphatic (lymphoid) tissue—aggregations of lymphocytes in the connective tissues of mucous membranes and various organs
• Diffuse lymphatic tissue is the simplest form
• Lymphocytes are scattered (not clustered)
• Prevalent in body passages open to the exterior like respiratory, digestive, urinary, and reproductive tracts
• Mucosa-associated lymphatic tissue (MALT)
• Lymphatic nodules (follicles) are dense masses of lymphocytes and macrophages that congregate in response to pathogens
• Constant feature of the lymph nodes, tonsils, and appendix
• Peyer patches: dense clusters in the ileum, the distal portion of the small intestine

Lymphatic Organs

• Lymphatic organs are anatomically well-defined
• Have connective tissue capsule that separates lymphatic tissue from neighboring tissues
• Primary lymphatic organs: red bone marrow and thymus
• Site where T and B cells become immunocompetent and recognize/respond to antigens
• Secondary lymphatic organs: lymph nodes, tonsils, and spleen
• Immunocompetent cells populate these tissues

Red Bone Marrow

• Red bone marrow is involved in hemopoiesis (blood formation) and immunity
• It is soft, loosely organized, highly vascular material
• Separated from osseous tissue by endosteum of bone
• Blood cells mature, they push thru reticular/endothelial cells to enter the sinus and flow away in bloodstream

Thymus

• Part of the endocrine, lymphatic, and immune systems
• Houses developing lymphocytes
• Secretes hormones regulating activity
• Bilobed organ in superior mediastinum between sternum and aortic arch
• Undergoes degeneration (involution) with age
• Fibrous capsule gives off trabeculae (septa) that divide the gland into several lobes
• Lobes have cortex and medulla populated by T lymphocytes
• Reticular epithelial cells seal off cortex from medulla, forming blood–thymus barrier
• Signaling molecules produced are thymosin, thymopoietin, thymulin, interleukins, and interferon

Lymph Nodes

• Lymph nodes are the numerous lymphatic organs
• About 450 in typical young adult
• Serves two functions of cleansing lymph and Act as a site of T and B cell activation
• Elongated, bean-shaped structure with hilum
• Enclosed with fibrous capsule with trabeculae that divide interior into compartments
• Stroma of reticular fibers and reticular cells
• Parenchyma divided into cortex and medulla
• Germinal centers are where B cells multiply and differentiate into plasma cells
• Afferent lymphatic vessels lead into node along convex surface
• Lymph leaves the node through one to three efferent lymphatic vessels that leave the hilum

Lymph Node Types

• Cervical lymph nodes consist of deep/superficial groups in the neck; monitor lymph coming from head/neck
• Axillary lymph nodes are concentrated in armpit
• Axillary lymph nodes receive lymph from upper limb and female breast
• Thoracic lymph nodes are in thoracic cavity, embedded in mediastinum; receive lymph from mediastinum, lungs, and airway
• Abdominal lymph nodes occur in posterior abdominopelvic wall and monitor lymph from the urinary/reproductive systems
• Intestinal and mesenteric lymph nodes are found in mesenteries, adjacent to appendix/intestines; monitor lymph from digestive tract
• Inguinal lymph nodes are in groin and receive lymph from the entire lower limb
• Popliteal lymph nodes occur on back of knee and receive lymph from the leg proper
• With lymphadenitis, a lymph node is swollen and painful due to a foreign antigen challenging the node
• Lymphadenopathy is the collective term for all lymph node diseases
• With metastasis, cancerous cells break free from original tumor travel to other sites in body, and form new tumors
• Metastasizing cells enter lymphatic vessels
• Metastasizing cells tend to lodge in first lymph node encountered and will multiply/destroy the node
• Swollen, firm, and usually painless upon lodging
• Tends to spread to next node downstream
• Treatment of breast cancer involves lumpectomy, mastectomy, along with removal of nearby axillary nodes

Tonsils

• Patches of lymphatic tissue located at entrance to the pharynx
• Guards against ingested/inhaled pathogens
• Covered with epithelium
• Deep pits: tonsillar crypts are lined with lymphatic nodules
• Tonsillitis is inflammation of the tonsils
• Tonsillectomyis removing the tonsils

Three main sets of tonsils

• Palatine tonsils - pair at posterior margin of oral cavity/ most often infected
• Lingual tonsils - pair at root of tongue
• Pharyngeal tonsil (adenoids) - single tonsil on wall of nasopharynx
• All tonsils are covered by epithelium
• Pathogens get into tonsillar crypts and encounter lymphocytes

Spleen

• Parenchyma exhibits two types of tissue
• Red pulp: sinuses filled with erythrocytes
• White pulp: lymphocytes and macrophages surrounding small branches of splenic artery
• It is the body’s largest lymphatic organ

Spleen Functions

• Healthy red blood cells (RBCs) pass through
• Designated the "erythrocyte graveyard" for old, fragile RBCs
• Blood cell production in the fetus and anemic adults
• Monitors blood for foreign antigens and keeps an army of monocytes for release when needed
• Stabilizes blood volume through plasma transfers to lymphatic system
• Spleen is highly vascular and vulnerable to trauma/infection
• Ruptured spleen often requires splenectomy, resulting in person susceptible to future infections, premature death