Lymphatic System Comprehensive Notes

Overall Outcomes

• Learning objectives explicitly stated in the slides:
  • Describe the features & general functions of the lymphatic system.
  • Identify & describe all parts of the major lymphatic pathways (capillaries → vessels → nodes → trunks → ducts → venous angles).
  • Explain tissue-fluid vs. lymph formation and the role of plasma colloid osmotic pressure.
  • Explain maintenance of lymph circulation and the consequences of obstruction (e.g. oedema, lymphedema).
  • Describe the structure, location & major functions of a typical lymph node.
  • Locate the major regional chains/groups of lymph nodes (axillary, cervical, inguinal, abdominal, pelvic, etc.).
  • Describe the locations & functions of the thymus and the spleen.

Introduction to the Lymphatic System

• Vast collection of cells (lymphocytes, macrophages) & biochemicals (cytokines, antibodies) that travel in lymphatic vessels.
• Forms a network of vessels closely associated with the cardiovascular system.
• Key transport roles:
  • Moves excess interstitial fluid away from tissues → ultimately returns to bloodstream.
  • Transports dietary fats (chylomicrons) from intestinal lacteals → blood.
• Key defensive roles:
  • Produces, houses & transports immune cells.
  • Filters lymph to help defend the body against infections.

Lymphatic Pathways (Macro-overview)

• Sequence of flow (from tissue back to blood):
  • Lymphatic capillaries → lymphatic vessels → lymph nodes (multiple) → larger lymphatic trunks → collecting ducts (thoracic or right lymphatic) → subclavian veins (venous angles) → general circulation.
• Two parallel circulations highlighted in the slides:
  • Pulmonary capillary network (blood) vs. lymph return.
  • Systemic capillary network (blood) vs. lymph flow.

Lymphatic Capillaries

• Microscopic, closed-ended tubes that closely parallel blood capillaries.
• Structure:
  • Simple endothelial wall with overlapping cells acting as one-way mini-valves.
  • Anchoring filaments attach to surrounding connective tissue; when interstitial pressure rises the flaps open.
• Functional points:
  • First site where tissue fluid becomes lymph.
  • Prevents accumulation of excess fluid (oedema).

Lymphatic Vessels

• Walls resemble veins but are thinner.
• Three tunics (inside → out):
  • Endothelial lining (inner).
  • Smooth-muscle layer (middle).
  • Connective-tissue layer (outer).
• Contain valves similar to those in veins → ensure one-way flow.
• Larger vessels lead to lymph nodes, then exit as efferent vessels that converge into trunks.

Lymphatic Trunks & Collecting Ducts

• Trunks drain lymph from large body regions; named for regions served:
  • Lumbar, intestinal, intercostal, bronchomediastinal, subclavian, jugular, etc.
• Collecting ducts:
  • Thoracic duct (left) – drains \approx 3/4 of body; begins at cisterna chyli; empties into left subclavian vein.
  • Right lymphatic duct – drains right head/neck, right upper limb & right thorax; empties into right subclavian vein.
• Diagrammatic slide shows regions: area drained by right lymphatic duct shaded, remainder by thoracic duct.

Tissue Fluid & Lymph Formation

• Tissue-fluid formation (Starling forces reflected):
  • Capillary blood pressure forces water & small solutes out of plasma.
  • Resulting fluid resembles plasma but lacks most plasma proteins; does contain some smaller proteins that confer plasma colloid osmotic pressure.
• Lymph formation:
  • Filtration exceeds reabsorption → net tissue-fluid accumulation.
  • Rising interstitial hydrostatic pressure pushes fluid into lymphatic capillaries → now termed lymph.
  • Process prevents excess tissue-fluid accumulation (oedema).
  • Summary equation conceptually: \text{Net filtration} = (Pc - Pi) - (\pic - \pii) (pressure vs. oncotic terms).

Functions of Lymph

• Absorption of dietary fats from intestines (lacteals) → delivery to bloodstream.
• Return of excess interstitial fluid & leaked plasma proteins to blood.
• Removal of foreign particles → transport to lymph nodes for immune processing.
• Provides a medium for immune cell trafficking.

Movement & Flow of Lymph

• Driving force: hydrostatic pressure of tissue fluid.
• Enhancing mechanisms (muscle activity):
  • Skeletal-muscle contractions ("muscle pump").
  • Respiratory movements (changes in thoracic pressure during inspiration/expiration).
  • Rhythmical contraction of smooth muscle in larger lymphatic vessels.
  • Valves prevent back-flow.
• Flow pattern through a node (slide depiction): afferent vessel → subcapsular sinus → cortical nodules → medullary sinuses → efferent vessel at hilum.

Obstruction of Lymph Movement

• Causes: tumors, parasites (e.g. Wuchereria bancrofti in filariasis), surgical removal/trauma, congenital malformations.
• Consequences: localized oedema / lymphedema; impaired immune surveillance; compromised dietary fat transport.

Lymph Nodes (Structure & Histology)

• Bean-shaped organs along lymphatic vessels; possess both afferent & efferent vessels (more afferent → slowed flow).
• Microscopic compartments:
  • Capsule – dense connective tissue.
  • Cortex – lymphoid follicles (nodules) with germinal centers (B-cell proliferation).
  • Medulla – medullary cords & sinuses (T cells, macrophages).
  • Hilum – indentation where blood vessels & efferent lymphatic exit.
• Contain abundant lymphocytes & macrophages → filter lymph & mount immune responses.

Location of Major Lymph-Node Groups

• Axillary – underarm; drains upper limbs, thoracic wall, mammary glands.
• Cervical – neck; drains scalp, face, nasal & pharyngeal regions.
• Inguinal – groin; drains lower limbs, external genitalia, abdominal wall inferior.
• Abdominal – along abdominal aorta, branches; drains abdominal organs.
• Pelvic – within pelvic cavity; drains pelvic viscera.
• Other named groups in slides: thoracic, supratrochlear (elbow area).

Functions of Lymph Nodes (Detailed)

• Primary filters of potentially harmful particles (microbes, mutated cells, debris) from lymph.
• Provide immune surveillance:
  • Macrophages engulf & destroy particles.
  • Lymphocytes (B & T cells) recognize antigens → initiate adaptive immune responses.
• Sites of lymphocyte production (with red bone marrow).

Thymus

• Location: mediastinum, posterior to upper sternum.
• Size variation:
  • Prominent in infancy & puberty → involutes (regresses) after; replaced by fat & connective tissue in elderly.
• Structure: encapsulated; subdivided into lobules; each lobule has outer cortex (densely packed thymocytes) & inner medulla (Hassall’s corpuscles).
• Functions:
  • Site of T-lymphocyte maturation & selection (self-tolerance).
  • Secretes thymosins → hormones that stimulate T-cell development.

Spleen

• Largest lymphatic organ; located upper-left abdominal quadrant (posterolateral to stomach, inferior to diaphragm).
• Mnemonic in slide: 1 \times 3 \times 5,\ 7,\ 9\text{–}11 (≈ 1 inch thick, 3 inches wide, 5 inches long; lies between the 7th–11th ribs).
• Structural components:
  • White pulp – lymphocytes around central arteries (immune function).
  • Red pulp – blood-filled sinusoids containing RBCs, lymphocytes & macrophages (filtration).
• Functions:
  • Blood reservoir; releases blood during demand.
  • Filters blood, removing aged/damaged RBCs & pathogens via macrophages.
  • Houses lymphocytes, initiating immune responses to blood-borne antigens.

Major Organs of the Lymphatic System (Consolidated)

• Lymph nodes – filter lymph; immune surveillance; lymphocyte production.
• Thymus – T-cell maturation; secretes thymosins.
• Spleen – filters blood; blood reservoir; immune surveillance of blood.

Lymphocyte Origins & Development

• Stem cells in red bone marrow give rise to lymphocyte precursors.
• Pathways:
  1. Precursors migrate to thymus → become T cells.
  2. Precursors that remain in bone marrow → become B cells.
• Both mature T & B cells enter blood → populate secondary lymphatic organs (nodes, spleen, tonsils).

Clinical & Practical Implications / Connections

• Edema management: understanding lymph formation & flow critical for treating fluid imbalances.
• Cancer metastasis: tumor cells often spread via lymphatic vessels; knowledge of node chains guides sentinel-node biopsy.
• Immunodeficiency: thymic dysfunction in childhood affects T-cell immunity (e.g. DiGeorge syndrome).
• Splenectomy risks: loss of splenic filtration → increased susceptibility to encapsulated bacteria (vaccination protocols).
• Lymphatic obstruction (elephantiasis): highlights necessity of intact lymph flow for fluid balance & immune defense.