Basics of the Lymphatic System – Vocabulary Review

Overview

• Lymphatic system = network of vessels, organs, and tissues that returns excess interstitial fluid to blood, supports immunity, and aids fat absorption.

• Color‐coded green in diagrams; empties into subclavian veins.

• Intertwined with cardiovascular, immune, and digestive systems.

Major Functions

• Fluid balance

  • Blood capillaries leak plasma water continuously.

  • ~10\% of filtered water is NOT reabsorbed osmotically → becomes lymph.

  • Lymphatic capillaries collect this fluid and channel it back to blood.

• Immune support

  • Storage & maturation of lymphocytes (T & B cells).

  • Provides travel routes for WBCs to infection sites.

  • Numerous “check-points” (nodes, MALT, spleen) filter fluids and expose pathogens to immune cells.

• Digestive involvement

  • Specialized intestinal lymphatics (lacteals) absorb dietary fats.

Primary vs. Secondary Lymphatic Organs

• Primary (sites of lymphocyte production & maturation)

  • Red bone marrow

  • Generates ALL formed elements.

  • B cells: produced & mature here → exit via sinusoidal capillaries.

  • T cells: produced but NOT fully mature; migrate to thymus.

  • Thymus

  • Location: mediastinum, anterior–superior to heart.

  • T-cell maturation & selection.

• Secondary (filter extracellular fluids; house mature lymphocytes)

  • Encapsulated

  • Lymph nodes → filter lymph.

  • Spleen → filters blood.

  • Unencapsulated / partially encapsulated = MALT (mucosa-associated lymphoid tissue)

  • Diffuse or nodular clusters embedded in mucosal/connective layers.

Lymphatic Vessel Hierarchy & Flow

• Lymph capillaries → collecting (lymphatic) vessels → trunks → ducts → subclavian veins.

• Analogous to veins: large diameter, thin walls, low pressure, valves.

• Ducts

  • Right lymphatic duct

  • Drains R upper limb, R thorax, R head/neck.

  • Thoracic duct (larger, longer)

  • Drains remainder: both lower limbs, abdomen, pelvis, L upper limb, L thorax, L head/neck.

Fluid Dynamics at Capillary Level

• Blood capillaries retain plasma proteins → high capillary oncotic pressure \pi_c.

• Interstitial fluid almost protein-free ⇒ oncotic gradient negligible.

• Net driving force for lymph entry = hydrostatic pressure gradient:
  P{interstitial} > P{lymph\,capillary} (though both low).

• Lymph capillary features

  • Blind-ended “terminal” sacs.

  • Simple squamous endothelial cells overlap → flaplike one-way micro-valves.

  • Anchoring filaments prevent collapse under tissue pressure.

Collecting Vessels – Structural/Functional Parallels with Veins

• Tunics: endothelium + thin smooth muscle layer (perivascular cells) + adventitia.

• Possess valves; rely on

  • Skeletal-muscle pump.

  • Respiratory pump.

  • Sympathetic (α-adrenergic)–mediated constriction.

• Extremely compliant due to thin walls & low P.

Lymph Nodes

• Definition: encapsulated, bean-shaped organs inserted along collecting vessels.

• Basic anatomy

  • Dense irregular collagen capsule → extensions (trabeculae) partition lobules.

  • Cortex (outer): densely packed diffuse lymphatic tissue & lymphatic nodules.

  • Medulla (inner): fewer cells, medullary cords/sinuses.

  • Afferent (inbound) vs. efferent (outbound) vessels—labels depend on node referenced.

• Histology

  • Reticular CTP network → physical scaffolding + filtration mesh.

  • Lymphatic nodules

  • Circular aggregates of lymphocytes.

  • Central germinal centers = active B/T cell proliferation.

• Functional role

  • “Immune checkpoints”; expose lymph to resident WBCs.

  • Capture pathogens/debris via reticular mesh for recognition.

MALT (Mucosa-Associated Lymphoid Tissue)

• Non-capsulated nodules within mucosal CT layers; extra protection at body tract portals.

• Key sites

  • Tonsils (6 total)

  • Pharyngeal (adenoids), palatine, lingual.

  • Stratified squamous epithelium with crypts that trap pathogens.

  • Peyer’s patches (ileum of small intestine).

  • Vermiform appendix (large clusters within wall).

  • Scattered in respiratory, urinary, reproductive tracts.

• Rationale near ileocecal region: guard against translocation of large-intestinal flora into small intestine/body.

Spleen

• Location: upper L quadrant, posterior to stomach & pancreas.

• Vascular supply: splenic artery (from celiac trunk) in; splenic vein → hepatic portal vein → liver.

• Capsule & trabeculae partition lobules.

• Two pulps

  • White pulp: lymphocytes clustered around arterioles → immune surveillance of blood.

  • Red pulp: sinusoidal capillaries + macrophages → remove senescent RBCs; store platelets, healthy RBC reservoir.

• Functions

  • Filters bloodborne pathogens.

  • Hematologic recycling of erythrocytes (macrophage phagocytosis).

  • Blood reservoir; potential to release RBCs during high demand (species-dependent).

Thymus

• Primary lymphatic organ for T-cell education.

• Anatomy

  • Two lobes; each subdivided into lobules via connective tissue trabeculae.

  • Lobule structure: dark cortex (densely packed immature T cells) → lighter medulla (maturing cells exit via vessels).

• Epithelial cells secrete thymosins → orchestrate maturation.

• Positive/negative selection

  • T cells exposed to self-antigens; self-reactive clones undergo apoptosis (autoimmunity safeguard).

• Involution

  • Active in childhood; atrophies in adulthood, replaced by inert CT/fat once adequate T-cell repertoire (millions of clones) produced.

Connections & Cross-System Relevance

• Cardiovascular: Returns \approx 3\,\text{L/day} of fluid to venous circulation; vessels share pumps/valves.

• Immune: Houses & transports leukocytes; major filter for pathogens.

• Digestive: Lacteals absorb chylomicrons; MALT guards GI tract; spleen/liver sequential blood filtration.

• Endocrine: Thymic hormones (thymosins) regulate immunity.

• Clinical tie-ins

  • Blocked lymphatics → edema.

  • Splenectomy ↑ infection risk; liver takeover of RBC recycling.

  • Tonsillitis, appendicitis = inflamed MALT structures.

  • Autoimmune diseases stem from failed T/B cell selection checkpoints.

Ethical & Philosophical Notes

• Body maintains strict self/non-self discrimination; failure → autoimmunity, raising questions about immunological tolerance.

• Spleen/thymus studies inform transplantation ethics and age-related immunity.

Numerical/Statistical References & Equations

• Plasma filtration: \sim 10\% not reabsorbed; \approx 3\,\text{L} daily enters lymphatics.

• Hydrostatic vs. oncotic relationships: P{if} > P{lymph} while \pi_{if}\approx 0 (protein-free).

Key Terminology Recap

• Lymph, lymphocyte, lacteal, MALT, node vs. nodule, pulp, thymosin, apoptosis.