The Lymphatic System and Lymphoid Tissue – Comprehensive Study Notes

Objectives

• Identify and list the core purposes of the lymphatic (lymphoid) system
  • \text{Fluid balance}: reclaim excess interstitial fluid (ISF) and return it to blood circulation
  • \text{Protection / Immunity}: house, mature, and mobilise leukocytes (lymphocytes, macrophages, etc.)
  • \text{Fat absorption}: pick up dietary lipids in intestines via lacteals and shunt them to blood
• Describe and compare the micro-anatomy of lymphatic capillaries vs. blood capillaries
• Trace the one-way flow of lymph from tissues → lymphatic capillaries → vessels → ducts → venous angles
• Locate and explain the functions of the five major lymphoid tissues/organs (lymph nodes, spleen, thymus, MALT, red bone marrow)
• Recognise common lymphatic disorders and outline nursing priorities / expected outcomes

Foundational Review (Ch. 15 & Earlier)

• Capillary exchange leaves ~\tfrac{1}{10} of plasma volume in tissues → forms ISF
• Mass balance principle: whatever exits capillaries must re-enter circulation (blood + lymph)
• Relevant background chapters
  • Barriers (Ch 4): skin, mucosae – 1st line of defence
  • Flow & Gradients (Ch 5): fluids move down pressure gradients \Delta P
  • Mass balance (Ch 13): input = output to maintain homeostasis

Major Components of the Lymphatic System

• Fluid: Lymph
• Conduits: lymphatic capillaries, vessels, trunks, ducts, plexuses
• Filters: lymph nodes (≈600 clustered units)
• Cells: T-cells, B-cells, NK cells, macrophages, dendritic cells, reticular cells
• Organs/Tissues (encapsulated or diffuse)
  • Spleen, thymus
  • MALT (tonsils, Peyer patches, appendix, etc.)
  • Red bone marrow (lymphopoiesis)

Lymph (the Fluid)

• Clear, colourless; composition closely mirrors ISF
• Derived from blood plasma but contains less protein (large plasma proteins cannot exit blood capillaries easily)
• May contain:
  • Hormones, wastes, cell debris, pathogens
  • Chylomicrons (fat-laden lipoproteins) after intestinal absorption → "chyle" (milky-white appearance)

Functional Triad of the Lymphatic System

• Fluid Balance
  • Returns ≈3\,\text{L/day} ISF to venous blood → prevents tissue oedema, maintains BP & BV
• Protection / Immunological Surveillance
  • Lymph organs provide sites for antigen presentation, clonal expansion, and cytotoxic responses
• Absorption of Fats
  • Lacteals in villi of small intestine pick up fats → thoracic duct → venous blood → bypasses hepatic portal vein (fats do not go directly to liver)

Lymphatic Circulation (One-Way Path)

1. Blind-ended lymphatic capillaries in tissues
2. Larger collecting vessels (with valves)
3. Lymph nodes interposed at intervals for filtration
4. Lymphatic trunks
5. Two terminal ducts
   • \text{Right lymphatic duct} drains right upper quadrant (R head, neck, thorax, upper limb) → empties into R subclavian–internal jugular junction
   • \text{Thoracic duct} (larger) drains rest of body; origin = cisterna chyli (L₂ level); empties into L venous angle

Lymphatic Capillaries

• Endothelium: flattened simple squamous cells
• More permeable than blood capillaries due to:
  • Overlapping endothelial flaps → act as one-way mini-valves
  • Anchoring filaments tie flaps to surrounding matrix; ↑ tissue pressure pulls flaps open
• Begin blindly (closed at one end); no continuous basement membrane
• Specialised type: Lacteals in intestinal villi → absorb emulsified fats

Fluid Entry Mechanism

• ↑ ISF volume → ↑ tissue hydrostatic pressure P_{t}
• Creates larger \Delta P between ISF & low-pressure lymph capillary lumen
• Endothelial flaps open → fluid, proteins, pathogens, metastatic cells enter
• Once inside, fluid called lymph; entry lowers ISF pressure restoring homeostasis

Propulsion / Movement of Lymph Toward the Heart

• No pump; relies on multiple low-pressure aids
  • One-way valves in vessels prevent backflow
  • Intrinsic smooth-muscle contraction (segmental pumping)
  • Skeletal-muscle pump: contraction compresses vessels (importance of exercise)
  • Respiratory pump: inspiration ↓ thoracic pressure & ↑ abdominal pressure → lymph drawn upward
  • Pulsations of neighbouring arteries

Lymphatic Vessels & Ducts – Gross Anatomy

• Vessel walls thinner than veins; possess more valves
• Converge into 9 major trunks → 2 ducts
• Regions drained (see colour map)
  • Right lymphatic duct region = R head/neck, R thorax, R upper limb
  • Thoracic duct region = L head/neck, L thorax, L upper limb, abdomen, pelvis, both lower limbs
• Clinical note: obstruction of thoracic duct may cause bilateral lower-limb & abdominal oedema

Lymph Nodes

• Bean-shaped ≈ 1{-}25\,\text{mm}; encapsulated; located along vessels
• Afferent vessels bring lymph into node; single efferent vessel exits at hilum (creates slowed flow → more filtration time)
• Organised layers
  • Outer cortex: lymphoid follicles; germinal centres = B-cell proliferation
  • Paracortex: T-cell dependent zone
  • Inner medulla: medullary cords (B-cells, plasma cells) + sinuses rich in macrophages
• Major regional clusters
  • Cervical, Axillary, Tracheobronchial, Mesenteric, Inguinal
• Functions
  • Mechanical filtration (trap debris, pathogens, cancer cells)
  • Immune activation → antigen presentation, clonal expansion

Lymphoid Organs & Tissues

Spleen

• Largest lymphoid organ; LUQ, posterior to stomach; delicate capsule
• Multilobed circulation:
  • White pulp: lymphoid tissue surrounding arteries – immune surveillance
  • Red pulp: venous sinuses + splenic cords – blood filtration
• Key roles
  • Filters & cleanses blood (not lymph) – removes microbes & old RBCs
  • Site of fetal erythropoiesis (before bone marrow matures)
  • Reservoir storing \approx 350\,\text{mL} blood
  • Recycles iron & globin
• Post-splenectomy → ↑ risk of sepsis by encapsulated bacteria (Strep. pneumoniae, H. influenzae)

Thymus

• Bi-lobed gland in mediastinum; most active in childhood
• Site of T-lymphocyte maturation & selection (positive/negative)
• Secretes hormones (thymosin, thymopoietin) that guide T-cell development
• Involution after puberty → replaced by adipose; immune competence largely maintained via memory T-cells

MALT (Mucosa-Associated Lymphoid Tissue)

• Diffuse system guarding mucosal surfaces where pathogens first contact body
• Populations
  • GALT (gut-associated): Peyer patches, appendix, tonsils, isolated intestinal follicles
  • BALT (bronchus-associated), NALT (nasal), VALT (vulvo-vaginal), etc.
• Mechanisms: IgA secretion, phagocytosis, immune tolerance to food antigens

Peyer Patches

• 50–80 aggregates mainly in distal ileum (some proximal jejunum)
• Monitor intestinal bacteria & generate memory lymphocytes

Appendix

• Vermiform tube off cecum; dense lymphoid follicles
• Safe-house for commensal microbiota; participates in GALT immunity

Tonsils

• Ring (Waldeyer’s) guarding pharynx; partially encapsulated; crypts trap material
  • Palatine ("the tonsils") – lateral oropharynx
  • Pharyngeal (adenoids) – roof of nasopharynx
  • Lingual – base of tongue
• Provide first-line immune sampling of inhaled/ingested pathogens

Disorders of the Lymphatic System

Lymphadenopathy

• General term for diseased/enlarged lymph nodes (tender or non-tender; local or generalised)
• Common causes
  • Viral: Infectious mononucleosis (EBV), HIV, CMV
  • Bacterial: TB, cat-scratch, bacterial pharyngitis
  • Malignancy: lymphoma, metastases
• Nursing focus: node size, consistency, fixation, overlying skin, systemic signs

Lymphadenitis

• Inflammation of node(s); nodes painful, warm; may suppurate
• Typically secondary to local infection

Lymphangitis

• Acute inflammation of lymphatic vessels (often streptococcal)
• Clinical sign: red "streak" extending proximally from wound toward trunk
• Risk: septicemia if untreated

Lymphedema

• Chronic interstitial fluid accumulation due to lymphatic obstruction/insufficiency
• Aetiologies
  • Primary (congenital) – hypoplastic vessels
  • Secondary – infection (filariasis), surgery (lymph node dissection), radiation, malignancy
• Staging
  1. Latent (asymptomatic) – abnormal flow, no swelling
  2. Spontaneously reversible – pitting oedema ↓ with elevation
  3. Spontaneously irreversible – non-pitting; fibrosis
  4. Lymphostatic elephantiasis – massive enlargement, skin thickening, papillomatosis
• Management
  • Complete decongestive therapy (manual drainage, compression, exercise, skin care)
  • Pharmacologic (benzopyrones), surgery (lymphovenous bypass)

Splenomegaly

• Spleen enlargement (>13\,\text{cm} length, >500\,\text{g})
• Causes (mnemonic "CHILD SPLEN")
  • Congestion (portal HTN), Haemolysis, Infection (mono, malaria), Leukaemia/lymphoma, Deposit/storage (Gaucher)
• Consequences: hypersplenism → cytopenias

Lymphoma

• Malignancy of lymphocytes in lymphoid tissue

Hodgkin Lymphoma (HL)

• B-cell origin; hallmark Reed–Sternberg cells (giant binucleate)
• Bimodal age peaks ~20 y & ~65 y
• Often arises in single node/group → orderly contiguous spread
• S/Sx: painless node, "B" symptoms (fever, night sweats, wt loss)
• Highly curable with chemo-radiation

Non-Hodgkin Lymphoma (NHL)

• Heterogeneous; B-cell > T-/NK-cell
• More common; often disseminated at dx; may involve extranodal sites (GI, skin, CNS)
• Risk ↑ with immunodeficiency (HIV, post-transplant)
• Absence of Reed–Sternberg cells

Nursing Concerns & Interventions (Selected)

• Monitor limb girth and skin integrity in lymphedema; educate on compression garments
• Post-splenectomy → vaccinate vs. encapsulated organisms; watch for fever as emergency
• For lymphangitis: prompt antibiotic therapy; marking the streak for progression
• In lymphoma: manage fatigue, risk of infection, educate re: chemo side-effects

Rapid-Fire Review / Q&A

• 3 main lymphatic functions → fluid balance, protection, fat absorption
• Lymphatic vs blood capillary differences
  • Lymph caps are more permeable & blind-ended
• Name the two terminal ducts → right lymphatic & thoracic ducts
• Node function → filter lymph; site of lymphocyte activation
• Spleen filters blood; thymus matures T-cells
• Acronyms: \text{MALT} = Mucosa-Associated Lymphoid Tissue; \text{GALT} = Gut-Associated Lymphoid Tissue
• Tonsils location → around pharynx (oropharynx & nasopharynx)
• Blocked lymph flow → lymphedema; enlarged spleen → splenomegaly
• Malignant lymphocyte cancers → Hodgkin & Non-Hodgkin lymphomas
• NOT true of lymph vessels? "They transport lymph to the tissues" (they drain FROM tissues)
• Lymph from arm/breast drains primarily into axillary nodes
• Hodgkin lymphoma = malignant lymphoma of nodes with Reed–Sternberg cells

Clinical / Ethical / Practical Connections

• Understanding lymphatic drainage critical for oncologic surgery (sentinel node biopsy)
• Public health: filarial lymphedema (elephantiasis) targeted by WHO for eradication
• Ethical issue: access to compression therapy devices varies globally → health disparity
• Vaccination strategy post-splenectomy highlights importance of patient education & preventive medicine