TM

Lymphatic system

  • Lymphatic system: core roles

    • Returns interstitial (extracellular) fluid to the circulatory system
    • Absorbs dietary fats from the digestive tract (via lymphatic vessels in the gut)
    • Provides immune surveillance and defense against pathogens throughout the body
    • Works alongside the cardiovascular system; vessels run side-by-side and drain back into the venous system

  • Major lymphoid organs

    • Thymus
    • Location: mediastinum in the thoracic cavity; sits along the midline
    • Function: maturation of T lymphocytes (T cells)
    • Size/function across life: large in infancy/childhood, considerably smaller in adulthood; immunity can diminish as the thymus shrinks
    • Spleen
    • Location: upper left quadrant of the abdomen
    • Structure: two main tissue regions – white pulp (macrophages and lymphocytes) and red pulp (red blood cells)
    • Function: immune surveillance; filters blood; produces immune responses to blood-borne pathogens; distinguishes white vs red pulp under microscopy
    • Lymph nodes
    • Distribution: dispersed throughout the body (cervical, axillary, inguinal, popliteal, supratrochlear, pelvic, abdominal, thoracic regions, etc.)
    • Structure: encapsulated discrete organs; lymphocytes reside within; afferent lymphatic vessels bring lymph in; efferent lymphatic vessels transport lymph out
    • Clinical notes: enlarged cervical nodes can be felt; sentinel lymph nodes are the first nodes to receive drainage from a tumor and may be biopsied
    • Function: immune surveillance; filter lymph for pathogens; house macrophages and lymphocytes

  • Lymphoid tissues beyond discrete organs

    • MALT (mucosa-associated lymphoid tissue)
    • Unencapsulated, diffuse lymphoid tissue found in respiratory, digestive, reproductive, and urinary tracts
    • Appendix and tonsils are examples of MALt tissue
    • Encapsulated vs unencapsulated lymphoid tissue
    • Encapsulated organs (e.g., lymph nodes, spleen, thymus) are discrete
    • MALt tissue is unencapsulated and diffuse

  • Pathogens and lymphatic defense

    • Pathogens: disease-causing agents (bacteria, viruses, fungi, etc.)
    • Bacteria: discrete organisms; viruses: not organisms in the traditional sense; particles of DNA or RNA that hijack host cells
    • Lymphatic system tasks:
    • Recognize foreign material and initiate defense
    • Help eliminate interstitial fluid from tissues

  • Immunity: two broad classes

    • Innate (nonspecific) immunity
    • General defenses that do not target specific pathogens
    • Components include: mechanical barriers (skin, hair, tears, saliva, urine washout), physiological barriers (stomach acid), chemical barriers, inflammation, phagocytes, natural killer (NK) cells, and fever
    • Hallmarks of inflammation: heat, pain, redness, swelling
    • Major players in nonspecific defense: neutrophils, monocytes/macrophages (phagocytosis), NK cells, fever
    • Adaptive (specific) immunity
    • Targets specific pathogens or antigens; highly specific and remembers past exposures
    • Key players: B lymphocytes (B cells) and T lymphocytes (T cells)
    • B cells mature in red bone marrow; T cells mature in the thymus
    • Plasma cells (differentiated B cells) produce antibodies (immunoglobulins, gamma globulins)
    • Cytotoxic cells destroy infected cells; cytokines regulate immune responses
    • Memory cells (both B and T) persist after an infection to provide faster, stronger responses on re-exposure

  • Cells of the immune system and their roles

    • Lymphocytes: primary white blood cells of the immune system; include B cells, T cells, and NK cells
    • B lymphocytes (B cells)
    • Maturation: red bone marrow
    • Role: differentiate into plasma cells that produce antibodies (immunoglobulins / gamma globulins)
    • Part of adaptive immunity
    • T lymphocytes (T cells)
    • Maturation: thymus
    • Role: various subsets (e.g., helper T cells, cytotoxic T cells); cytotoxic T cells destroy infected or abnormal cells
    • Part of adaptive immunity; memory T cells contribute to faster responses
    • Plasma cells
    • Differentiated B cells; key antibody producers
    • Antibodies (immunoglobulins / gamma globulins)
    • Functions: neutralize pathogens; mark them for attack by other parts of the immune system
    • Cytokines
    • Helper signaling molecules that enhance the immune response to antigens
    • Cytotoxic (destructive) concept
    • Cytotoxic activity refers to destroying target cells (e.g., virus-infected cells or cancer cells)

  • Lymphocyte development and key connections

    • B cells: mature in red bone marrow; fate includes becoming plasma cells producing antibodies
    • T cells: mature in the thymus; help orchestrate immune responses; cytotoxic T cells destroy target cells
    • Memory cells: long-lived; enable rapid antibody and cell-mediated responses upon re-exposure
    • Gamma globulins: large plasma proteins (antibodies) that contribute to the immune response; antibodies are a class of gamma globulins
    • Plasma proteins and osmotic pressures in blood vessels:
    • Fibrinogen, albumin, and globulins are large plasma proteins that mostly remain in the blood vessels
    • They contribute to colloid osmotic (oncotic) pressure that helps draw fluid back into capillaries
    • When albumin production is reduced (e.g., liver dysfunction), colloid osmotic pressure falls and edema can occur

  • Interstitial fluid dynamics and filtration forces

    • Interstitial fluid is extracellular and formed from capillary filtration
    • Forces governing movement across capillaries (Starling-like context in the lecture):
    • Hydrostatic pressure in the capillary drives fluid out into the interstitium
    • Colloid (osmotic) pressure from plasma proteins pulls fluid back into the capillary
    • Net filtration pressure (NFP) is the balance of these forces
    • Reported values from the lecture (illustrative):
    • At the arterial (outward) end of a capillary: ext{NFP}_{arterial} = 13 ext{ mmHg}
    • At the venous (inward) end of a capillary: ext{NFP}_{venous} = -7 ext{ mmHg}
    • Fluid fate:
    • Approximately frac{9}{10} of the fluid that leaves the capillary at the arterial end reenters at the venous end
    • Approximately frac{1}{10} of the fluid enters the lymphatic capillaries and is returned to the bloodstream via the lymphatic system
    • Lymphatic capillaries structure
    • One cell-thick layer of simple squamous epithelium; highly permeable to fluids and solutes
    • Valves prevent backflow; act as a one-way system to move fluid toward larger vessels
    • Lymphatic pathway overview
    • Capillary → lymphatic capillary → lymphatic vessel → lymph node (afferent vessels bring lymph in; efferent vessels carry lymph out) → lymphatic trunk → collecting duct → subclavian vein
    • Major ducts: thoracic duct (drains most of the body) and right lymphatic duct (drains the right upper quadrant)
    • What happens to the filtered fluid
    • Most enters venous blood, but a portion enters lymphatic vessels to be returned to the venous system via the ducts

  • Clinical relevance and practical implications

    • Edema and ascites
    • Edema can occur when hydrostatic pressure exceeds osmotic pressure (or when colloid osmotic pressure is reduced, e.g., liver dysfunction and low albumin)
    • Ascites: fluid accumulation in the abdominal cavity due to such imbalances
    • Cancer and lymphatic spread
    • Lymphatic system can transport cancer cells (metastasis) via lymph flow
    • Sentinel lymph node biopsy helps determine if cancer has spread; removal of axillary lymph nodes in breast cancer can result in lymphedema of the arm due to obstructed drainage
    • Exercise and lymph movement
    • Lymph has no central pump like the heart; movement is aided by muscle contractions (e.g., gastrocnemius), breathing, and overall physical activity
    • Aging and immunity
    • Thymus shrinks with puberty; reduced production of new T cells with age can lead to diminished adaptive immunity and higher susceptibility to infections and some cancers

  • Summary connections and takeaways

    • The lymphatic and cardiovascular systems are tightly integrated in circulating fluids and immune surveillance
    • Immunity comprises two major pillars: innate (nonspecific) and adaptive (specific); memory cells provide long-term protection
    • Lymphoid organs and tissues (thymus, spleen, lymph nodes, MALTs) play distinct roles in maturation, surveillance, and response
    • Proper fluid balance depends on a balance between hydrostatic and colloid osmotic pressures; disruption can cause edema or ascites
    • Physical activity and proper lymphatic movement are important for preventing fluid buildup and supporting immune function

  • Quick glossary of key terms (definitions in brief)

    • Pathogen: disease-causing agent (bacteria, viruses, fungi)
    • Antigen: substance that elicits an immune response
    • Antibody / Immunoglobulin: proteins produced by plasma cells that target antigens
    • Plasma proteins: fibrinogen, albumin, globulins (including gamma globulins)
    • Oncotic/Colloid osmotic pressure: osmotic pressure due to plasma proteins that holds fluid in capillaries
    • Memorable keywords: HEAT, PAIN, REDNESS, SWELLING (hallmarks of inflammation)
    • Afferent/efferent: vessels entering/leaving a lymph node
    • MALt: mucosa-associated lymphoid tissue (diffuse lymphoid tissue in mucosal sites)
    • Sentinel lymph node: first lymph node to which cancer cells are likely to spread from a primary tumor
    • Ascites: fluid buildup within the abdominal cavity

  • Notable figures and concepts to remember for exams

    • Location and roles of thymus, spleen, and lymph nodes
    • The dual nature of immunity (innate vs adaptive) and their major components
    • Blood vs lymph dynamics: capillary filtration, interstitial fluid formation, and lymph return pathway
    • Mechanisms behind edema and the importance of albumin in maintaining oncotic pressure
    • Lymph flow and clinical scenarios: cancer metastasis, lymphedema after lymph node removal, and role of exercise in lymph movement