Overview of the Lymphatic System
- A meandering, one-way network of lymphatic vessels that returns interstitial fluid to the cardiovascular system.
- Fluid terminology
- Interstitial fluid → enters lymphatic vessels → becomes lymph ("clear water").
- Structural–functional bridge to immunity
- Lymphoid organs/tissues (spleen, thymus, tonsils, MALT, lymph nodes) provide the structural basis of the immune system.
- Lymph nodes act as the keystone, linking vessel function with immune functions.
Components of the System
Lymphatic Vessels
- Functions
- Return excess tissue fluid to blood.
- Return leaked plasma proteins → maintain plasma colloid osmotic pressure.
- Transport absorbed fats (via lacteals) from small intestine to blood.
- Carry pathogens & antigens to lymph nodes.
- One-way flow: always toward the heart.
Lymphatic Capillaries
- Microscopic, blind-ended tubes interwoven with blood capillary beds.
- Absent in bone, bone marrow, teeth; present in limited areas of CNS meninges (drain brain interstitial & CSF).
- High permeability due to two specializations
- Overlapping endothelial cells form flap-like minivalves (one-way swing doors).
- Collagen anchoring filaments tether endothelial cells; ↑ interstitial volume → pulls flaps open.
- Admit large particles (proteins, cell debris, pathogens, cancer cells).
- Specialized intestinal capillaries = lacteals; lymph inside is chyle (milky, fat-laden).
Collecting Vessels → Trunks → Ducts
- Same 3 tunics as veins; thinner walls, more valves, frequent anastomoses.
- Superficial vessels travel with superficial veins; deep lymphatics track deep arteries.
- Major trunks: paired lumbar, bronchomediastinal, subclavian, jugular + single intestinal trunk.
- Terminal ducts
- Right lymphatic duct → drains right upper limb, right head, right thorax; empties at junction of right internal jugular & right subclavian veins.
- Thoracic duct (larger) → drains remainder; often begins as cisterna\,chyli (enlarged sac) at T{12}-L{2} level; empties into left venous junction.
Lymph Transport Mechanics
- No pump; low-pressure system.
- Propulsion mechanisms
- Skeletal-muscle milking.
- Respiratory thoracic pressure changes.
- Pulsations of adjacent arteries.
- Rhythmic contraction of smooth muscle in vessel walls (except smallest).
- Flow is sporadic & slow; physical movement ↑ rate.
Quantitative Highlights
- Fluid left behind at capillary beds ≈ 3\;\text{L day}^{-1} → must be reclaimed.
Homeostatic Imbalances (Vessels)
- Lymphangitis: inflamed lymphatics; vasa vasorum congest → red tender streaks.
- Lymphedema: blocked/removed lymphatics → severe localized edema.
Lymphoid Cells & Tissues
Cellular Players
- Lymphocytes
- T cells: manage immune response; directly attack infected cells.
- B cells: differentiate into plasma cells → secrete antibodies that tag antigens.
- Macrophages: phagocytize & activate T cells.
- Dendritic cells: capture antigens, migrate to nodes, present to T cells.
- Reticular cells: produce reticular fiber stroma scaffold.
Tissue Types
- Diffuse lymphoid tissue: loose arrangement; nearly every body organ (lamina propria of mucosae).
- Lymphoid follicles (nodules): solid, spherical B-cell bodies; possess germinal centers.
- Isolated as Peyer’s patches & appendix follicles or as part of larger organs (nodes, tonsils).
Lymphoid Organs
- Primary: sites of lymphocyte maturation
- Red bone marrow → origin & B-cell maturation.
- Thymus → T-cell maturation.
- Secondary: first antigen encounter & activation
- Lymph nodes, spleen, MALT (tonsils, Peyer’s patches, appendix).
- Only lymph nodes filter lymph directly (afferent & efferent vessels).
Lymph Nodes
Distribution
- Hundreds; largest clusters in cervical, axillary, inguinal regions where vessels converge.
Functions
- Filter lymph: macrophages remove microorganisms/debris before lymph returns to blood.
- Immune activation: site where lymphocytes encounter antigens (often delivered by dendritic cells).
Histological Architecture
- Size: < 2.5\;\text{cm} (≈ 1\;\text{inch}), bean-shaped.
- Capsule → inward trabeculae create compartments.
- Two regions
- Cortex: follicles with germinal centers (dividing B cells); deeper cortex rich in T cells; abundant dendritic cells.
- Medulla: medullary cords (T & B cells) + medullary sinuses.
- Sinuses: subcapsular → cortical → medullary; criss-crossed by reticular fibers laden with macrophages.
Lymph Flow Pattern
\text{Afferent vessels} \rightarrow \text{subcapsular sinus} \rightarrow \text{cortical sinuses} \rightarrow \text{medullary sinuses} \rightarrow \text{hilum} \rightarrow \text{efferent vessels}
- Fewer efferent than afferent routes → stagnation → increased filtration & immune interaction time.
Clinical Notes
- Buboes: infected, pus-filled nodes (e.g., bubonic plague).
- Cancer metastasis often seeds nodes; nodes become hard, non-tender, enlarged.
Spleen
Position & Blood Supply
- Left upper abdomen, curling around stomach; supplied by splenic artery & vein (branches of celiac trunk & hepatic portal system respectively).
Functions
- Immune surveillance & lymphocyte proliferation.
- Blood cleanser
- Removes aged/defective RBCs & platelets.
- Macrophages phagocytize debris & pathogens.
- Additional roles
- Salvages & stores iron; sends other Hb breakdown products to liver.
- Stores \sim 1/3 of body’s platelets + monocytes.
- Fetal erythrocyte production (may persist in adults under stress).
Microanatomy
- White pulp: lymphocyte-rich islands around central arteries → immune functions.
- Red pulp: splenic cords + sinusoids; site of RBC & pathogen destruction.
Pathology
- Thin capsule; rupture → intraperitoneal hemorrhage; historically splenectomy, now conservative management.
- Splenomegaly: enlargement (e.g., mononucleosis, malaria, leukemia, septicemia).
MALT (Mucosa-Associated Lymphoid Tissue)
- Scattered, strategically placed lymphoid tissue in mucous membranes → guards entry points.
- Major aggregations
- Tonsils (pharyngeal ring)
- Palatine (largest, most often infected), lingual, pharyngeal (adenoids when enlarged), tubal.
- Tonsillar crypts trap bacteria → immune memory formation.
- Peyer’s patches: large nodules in distal small intestine wall.
- Appendix: tubular offshoot of cecum; high follicle density.
- Additional diffuse MALT in respiratory, genitourinary, and rest of GI tracts.
Thymus
Location & Development
- Inferior neck → superior thorax, deep to sternum; bilobed.
- Prominent in newborns; enlarges during first year; starts involuting at puberty; residual tissue persists, still produces some T cells.
Structure
- Lobulated (like cauliflower).
- Cortex: densely packed, rapidly dividing T lymphocytes + macrophages.
- Medulla: fewer lymphocytes + thymic corpuscles (Hassall’s corpuscles) → regulatory T-cell development.
- Epithelial-cell stroma, not reticular fibers.
- Blood–thymus barrier isolates developing T cells from antigens.
- Lacks B cells & follicles; no direct antigen fight—serves strictly for T-cell maturation.
Comparative Summary (Key Differentiators)
- Capsule present: nodes, spleen, thymus; MALT generally lacks full capsule (tonsils partially encapsulated).
- Lymphoid follicles rich in B cells: nodes (cortex), spleen (white pulp), MALT; absent in thymus.
- Stroma: reticular connective tissue (nodes, spleen, MALT) vs epithelial tissue (thymus).
- Unique features: efferent+afferent vessels (nodes), red/white pulp (spleen), crypts (tonsils), cisterna chyli (duct), thymic corpuscles (thymus).
Numerical & Equation Snapshot
- Daily capillary fluid loss → 3\;\text{L} returns via lymph.
- Lymph node length < 2.5\;\text{cm} (≈ 1\;\text{in}).
- Platelet reservoir in spleen ≈ \frac{1}{3} total platelets.
Developmental Notes
- Lymphatic vessels & main node clusters arise by week 5 embryonically from lymph sacs budding off veins.
- Thymus (endodermal) first lymphoid organ; others (mesodermal mesenchyme) follow; lymphocytes populate organs shortly after birth.
Selected Clinical & Vocabulary Terms
- Elephantiasis: lymphedema due to filarial worms.
- Hodgkin’s lymphoma: malignant, Reed–Sternberg cells, high cure rate.
- Non-Hodgkin’s lymphoma: all other lymphoid cancers; variable prognosis.
- Sentinel node: 1st draining node examined for cancer spread.
- Tonsillitis: inflamed/infected tonsils.
- Lymphangiography: imaging with radiopaque dye.
Integrated Questions / Self-Check Prompts
- Define lymph; trace its origin path.
- Identify regions drained by right lymphatic vs thoracic ducts.
- Summarize forces propelling lymph.
- Analyze edema development when lymphatics are blocked—refer to capillary hydrostatic & colloid osmotic pressures.
- Distinguish primary vs secondary lymphoid organs.
- Sketch & label a lymph node (afferent, efferent, hilum, cortex, medulla, follicles).
- Explain why fewer efferent vessels benefit immune surveillance.
- Relate splenic artery/vein to celiac trunk & hepatic portal vein hierarchy.
- Predict consequences of thymic removal in a neonate vs an adult.
These bullet-point notes encompass every major and minor concept in the transcript, elaborate on complex ideas, include quantitative data in LaTeX, integrate clinical correlations, and connect developmental and functional perspectives—forming a comprehensive stand-alone study guide for Chapter 20 on the lymphatic system.