Lymphatic System – Core Lecture Points

Learning Outcomes

• By the end of this unit you should be able to:
  • Describe the structural features & general functions of the lymphatic system.
  • Identify and describe every component of the major lymphatic pathways.
  • Explain how tissue fluid & lymph are formed, and list all functions of lymph.
  • Discuss the mechanisms that maintain lymph flow and predict the consequences of lymphatic obstruction (e.g., oedema).
  • Describe the histology, blood/lymph flow and major functions of a lymph node.
  • Locate the major chains / groups of lymph nodes on the body surface.
  • Detail the position and physiological roles of the thymus and spleen.

Introduction & Core Principles

• The lymphatic system is a vast collection of cells + biochemicals transported in lymphatic vessels.
• Closely integrated with the cardiovascular system; shares capillary beds and returns fluid to venous blood.
• Key transport roles:
  • Removes excess interstitial fluid and returns it to blood.
  • Absorbs & transports dietary fats from the small intestine to the bloodstream.
  • Acts as an auxiliary route for large proteins that are unable to enter blood directly.
• Key defensive roles:
  • Houses lymphocytes & macrophages for immune surveillance.
  • Filters lymph to remove pathogens, toxins, and cell debris.

Major Lymphatic Pathways (Macro-anatomy)

• Blood capillary (systemic ⟷ pulmonary) → Interstitial space → Lymphatic capillary → Lymphatic vessel → Lymph node(s) → Larger lymphatic vessels → Lymphatic trunk → Collecting duct → Subclavian vein.
• The path is unidirectional (from tissues back to venous angles) because of valves & pressure gradients.

Lymphatic Capillaries

• Microscopic, blind-ended tubes interwoven with blood capillaries.
• Structure:
  • Thin single layer of overlapping endothelial cells; act as one-way micro-valves.
  • Filaments anchor endothelial cells to surrounding connective tissue; prevent collapse.
• Function:
  • Admit interstitial fluid once local hydrostatic pressure rises.
  • Entry portal for large proteins, dietary lipids, cell debris & pathogens.

Lymphatic Vessels

• Resemble veins but walls are thinner.
• Wall layers (from lumen outward):
  1. Endothelial lining (inner).
  2. Smooth muscle layer (middle) – contributes to intrinsic pumping.
  3. Connective tissue (outer).
• Possess numerous semilunar valves → prevent back-flow and segment lymph column.
• Larger vessels drain through one or more lymph nodes before converging into trunks.

Lymphatic Trunks & Collecting Ducts

• Named according to the region they drain:
  • Lumbar, Intestinal, Intercostal, Bronchomediastinal, Subclavian, Jugular.
• Two terminal ducts:
  1. Right Lymphatic Duct – drains right side of head/neck, right upper limb, right thorax.
  2. Thoracic Duct – larger; drains the remainder of the body, begins at the cisterna chyli.
• Both ducts empty into their respective subclavian veins at the venous angles.

Tissue Fluid Formation

• Blood hydrostatic pressure at arterial capillary end filters water & small solutes from plasma into interstitial space.
• Filtrate composition ≈ plasma but with lower protein content.
• Remaining small plasma proteins generate plasma colloid osmotic pressure that favours re-absorption at venous end.

Lymph Formation

• Filtration > Re-absorption ⟶ net tissue fluid accumulation.
• Rising interstitial hydrostatic pressure forces fluid into lymphatic capillaries where it becomes lymph.
• Continuous drainage prevents oedema (tissue swelling).

Functions of Lymph

• \text{Absorption of dietary fats} via intestinal lacteals; delivers chylomicrons → blood.
• \text{Return of excess interstitial fluid & plasma proteins} to the bloodstream.
• \text{Transport of foreign particles} (antigens, cell debris) to regional lymph nodes for immune processing.

Movement of Lymph (Physiological Pumps)

• Primary driving force: Hydrostatic pressure of newly formed tissue fluid.
• Auxiliary pumps:
  • Skeletal muscle contraction ("muscle pump").
  • Respiratory movements (pressure changes in thorax & abdomen).
  • Rhythmic contraction of smooth muscle in large lymphatic vessel walls.
  • One-way valves ensuring cranial flow.

Obstruction of Lymph Flow

• Causes: Tumour, parasitic infection (e.g., filariasis), surgical removal of nodes, congenital malformation.
• Consequence: Lymphoedema – local accumulation of interstitial fluid, impaired immunity, tissue fibrosis over time.

Lymph Nodes (Microscopic & Macroscopic Anatomy)

• Bean-shaped organs strategically placed along lymphatic vessels.
• Hilum: Indentation for artery, vein, and efferent lymphatic vessel.
• Capsule → Trabeculae → Cortex → Medulla.
  • Cortex: Lymphoid follicles with germinal centers (B cells).
  • Medulla: Medullary cords + sinuses (T cells, macrophages).
• Flow pattern: Afferent vessel → Subcapsular sinus → Cortical sinuses → Medullary sinuses → Efferent vessel at hilum.

Functions of Lymph Nodes

• 1. Filtration: Remove potentially harmful particles from lymph via macrophages.
• 2. Immune Surveillance: Provide an environment for lymphocyte activation & proliferation (both B & T cells).
• Serve as lymphocyte production centers (with red bone marrow).

Location of Major Node Chains

• Cervical – drain head & neck.
• Axillary – upper limbs, breast, superficial thoracic wall.
• Inguinal – lower limbs, external genitalia, lower abdominal wall.
• Abdominal – abdominal viscera.
• Pelvic – pelvic viscera.
• Thoracic – thoracic viscera & deep chest wall.
• Supratrochlear – distal upper limb (often involved in skin infections).

Thymus

• Bilobed gland in superior mediastinum, posterior to upper sternum.
• Size is greatest in infancy & puberty; involutes thereafter, largely replaced by fat & connective tissue in elderly.
• Histology:
  • Lobules separated by septa; each with cortex (densely packed lymphocytes) and medulla (Hassall’s corpuscles).
• Functions:
  • Site of T-lymphocyte maturation (positive & negative selection).
  • Secretes thymosin hormones that promote T-cell differentiation.

Spleen

• Largest lymphatic organ; lies in upper left abdominal quadrant against diaphragm.
• Classical mnemonic dimensions: (1 \times 3 \times 5, 7, 9–11)
  • 1 inch thick, 3 inches wide, 5 inches long; lies between ribs 9–11 and weighs about 7 oz.
• Blood supply: Splenic artery (from coeliac trunk) & splenic vein (to portal vein).
• Internal structure:
  • White pulp: Lymphocytes around central arteries (immune functions).
  • Red pulp: Blood-filled sinusoids + cords (RBC recycling, macrophages).
• Functions:
  • Filters blood, removes aged RBCs & platelets.
  • Stores platelets & acts as blood reservoir.
  • Produces lymphocytes; participates in immune responses to blood-borne antigens.

Summary of Major Lymphatic Organs & Their Functions

• Lymph Nodes: Filter lymph; immune surveillance; lymphocyte production.
• Thymus: Matures T cells; secretes thymosins; most active in childhood.
• Spleen: Filters blood; recycles RBCs; stores blood; houses lymphocytes & macrophages.

Lymphocyte Origins & Differentiation Pathway

1. Stem cells in red bone marrow yield lymphocyte precursors.
2. Precursors migrating to the thymus mature into T cells.
3. Precursors maturing within the bone marrow become B cells.
4. Both mature T & B lymphocytes circulate via blood to secondary lymphatic organs (lymph nodes, spleen, MALT).

Clinical & Practical Notes

• Lymphadenopathy: Enlarged, tender nodes indicate active immune response or malignancy.
• Splenectomy: Increases susceptibility to blood-borne infections; patients need vaccines (e.g., pneumococcal).
• Thymic aplasia (DiGeorge): Failure of T-cell production → severe immunodeficiency.

Key Numbers & Equations (LaTeX Format)

• Layers in a lymphatic vessel wall: 3 (Endothelium, Smooth muscle, Connective tissue).
• Spleen mnemonic dimensions: (1 \times 3 \times 5, 7, 9–11).
• Two primary functions of lymph nodes: 1 Filter lymph, 2 Immune surveillance.
• Two tissue types in spleen: 2 (White pulp, Red pulp).

Study Tips & Concept Integration

• Relate lymph formation to Starling forces; compare with oedema pathophysiology.
• Map lymph node chains on your body; palpate cervical & axillary nodes to appreciate position.
• Draw the lymphatic pathway from a peripheral tissue to the venous circulation — practice labelling every structure.
• Link thymic hormones to endocrine system concepts.
• Remember that any impairment in lymph flow can be clinically expressed as swelling, infection risk or immune deficiency.