10. LYMPHATIC SYSTEM

INTRODUCTION TO LYMPHATIC SYSTEM

The lymphatic system forms a closed network of lymph vessels through which lymph circulates all over the body. The flow of lymphatic fluid is unidirectional, from tissue spaces to the blood. The lymphatic system protects the body against harmful agents through body fluid circulation. Blood plasma components filter through the walls of blood capillaries, producing interstitial fluid that moves into lymphatic vessels as lymph.

COMPOSITION OF LYMPHATIC SYSTEM

• Lymph nodes (through which lymph passes before returning to blood)
• Lymphatic vessels (transport lymph)
• Structures and organs containing lymphatic tissue (spleen, thymus)
• Red bone marrow (stem cells develop into blood cells, including lymphocytes)
• Diffuse lymphoid tissue (e.g., tonsils)
• Lymph (fluid)

FUNCTIONS OF LYMPHATIC SYSTEM

1. Tissue Drainage: About 21 litres of plasma fluid travel to tissues every 24 hours; 3-4 litres are drained by lymphatic vessels, preventing tissue saturation and CVS failure.
2. Absorption in the Small Intestine: Central lacteals (lymphatic vessels) of villi absorb fat and fat-soluble vitamins (A, D, E, K).
3. Immunity: Lymphatic organs produce and mature lymphocytes.
   • B-cells: Participate in humoral immune response; produce antibodies against antigens.
   • T-cells: Responsible for cell-mediated immunity; mature in thymus; have T-Cell Receptors (TCR).
4. Transporting Dietary Lipids: Lipids and lipid-soluble vitamins are absorbed by GIT through lymphatic vessels.
5. Defensive: Lymphocytes and monocytes remove bacteria from tissues.

LYMPH NODES

Lymph nodes are small, glandular structures present along lymph vessels. Around 500-600 lymph nodes are present in the human body in groups in the neck, collarbone, underarms, and groin. Lymphadenitis is the medical term for enlargement of lymph nodes due to infection.

Structure of Lymph Nodes

• Cortex: Contains primary and secondary lymphoid follicles with germinal centres.
• Paracortex: Tissue between cortex and medulla; comprises T-lymphocytes.
• Medulla: Contains macrophages, B-cells, and antibody-producing plasma cells.

Types of Lymph Nodes (Location-wise)

• Cervical Lymph Nodes: Head and neck region (drain scalp, face, nasal cavity, pharynx)
• Supraclavicular Lymph Nodes: Along collarbone (drain clavicle and upper chest)
• Axillary Lymph Nodes: Armpit area (drain arms, thorax, breast)
• Mediastinal Lymph Nodes: Central chest between lungs (drain chest cavity, upper abdomen, lungs)
• Supratrochlear Lymph Nodes: Above elbow joint (drain fingers, arms, forearm)
• Mesenteric Lymph Nodes: Lower abdomen along small intestine (drain cecum, jejunum, ileum, colon, rectum)
• Inguinal Lymph Nodes: Groin area (drain genital areas, large intestine, abdominal wall, legs)
• Femoral Lymph Nodes: Upper thigh along femoral veins (drain genital parts, buttock, thighs, leg)
• Popliteal Lymph Nodes: Knee area (popliteal fossa) (drain knee, thighs, calf, feet)

Functions of Lymph Nodes

1. Lymphocyte formation (B-lymphocytes and T-lymphocytes)
2. Killing bacteria and particulate matter by phagocytosis (macrophages)
3. Preventing spread of cancer cells (temporarily halt cancer cells)
4. Preventing entry of poisonous substances into circulation
5. Production of γ-globulin

LYMPH VESSELS

Lymph vessels are thin-walled, endothelial-lined channels that arise near capillary beds and return interstitial tissue fluid and inflammatory cells to the blood.

Two Main Lymphatic Ducts

• Thoracic Duct: Originates from cisterna chyli (anterior to first two lumbar vertebrae); ~40cm long; drains into left subclavian vein. Drains lymph from head and neck, left half of thorax, left arm, pelvic and abdominal cavities, and legs.
• Right Lymphatic Duct: ~1cm long; located at root of neck; drains into right subclavian vein. Drains lymph from head and neck, right arm, and right half of thorax.

Functions of Lymph Vessels

1. Fluid transport
2. Homeostasis
3. Immunosurveillance and immunomodulation on pathogen attack
4. Transport of dietary fat
5. Drainage of CSF and aqueous humor
6. Contribution to neurodegenerative and neuroinflammatory diseases
7. Response to anticancer therapies

LYMPHATIC ORGANS AND TISSUES

Primary Lymphatic Organs

• Red Bone Marrow: Site of blood cell production; B-lymphocytes mature here; T-lymphocytes mature in thymus.
• Thymus Gland: Located in upper thoracic cavity (posterior to sternum, anterior to ascending aorta). Produces thymic hormones (e.g., thymosin).

Secondary Lymphatic Organs

• Spleen: Ductless gland in upper left abdominal cavity.
• Lymph Nodes: Oval-shaped nodes along lymphatic vessels; rich in lymphocytes; filter and destroy bacteria and viruses.
• Tonsils: Located in throat area; contain white blood cells; keep bacteria out of nose and mouth.
• Peyer’s Patches: Present in intestinal walls and appendix; intercept pathogens entering through intestinal tract.

SPLEEN

Spleen is a ductless gland found in the upper left area of the abdominal cavity. It is closely related to the circulatory system. One major function is destruction of older RBCs.

Structure of Spleen

Spleen is made up of red and white pulp units centered on central arteries. White pulp is lymphoid (B cell follicles, T cell areas). Red pulp includes pulp sinuses and pulp cords.

Functions of Spleen

1. Phagocytosis: Splenic macrophages engulf worn-out RBCs, WBCs, platelets, cell debris, and microorganisms.
2. Haemopoiesis: In foetus – produces all blood cells; in adult – produces only lymphocytes.
3. Immune Response: Centre for multiplication of B-lymphocytes and T-lymphocytes; B-lymphocytes convert to plasma cells.
4. Storage of Erythrocytes: Known as “blood bank” – stores RBCs and releases them when body requires more oxygen.

LYMPH

Lymph is a clear watery fluid with composition similar to plasma and interstitial fluid. It is created as a result of tissue filtration and drainage of fluids.

Composition of Lymph

• Water (major component)
• Cellular components: Lymphocytes (1×10³ – 2×10³ per mm³), monocytes, macrophages, plasma cells
• Organic components: Proteins (2-6% of solid – albumin, enzymes, globulin, antibodies), lipids (5-15% of solid), carbohydrates (120 mg%), amino acids, nitrogenous waste products
• Inorganic substances: Na⁺, Ca²⁺, K⁺, Cl⁻, bicarbonates

Formation of Lymph

1. Capillary filtration
2. Interstitial fluid formation
3. Lymphatic capillaries collect interstitial fluid (now called lymph)
4. Lymphatic vessels combine to form larger vessels (with one-way valves)
5. Lymph nodes filter and clean lymph
6. Muscle contractions assist lymph flow
7. Return to circulation via subclavian veins

Functions of Lymph

1. Absorption of fatty acids and transport of fat and chyle
2. Production of immune cells (lymphocytes, monocytes, plasma cells)
3. Maintenance of body cell moisture
4. Carrier for oxygen, hormones, nutrients; eliminates metabolic wastes
5. Supplies antibodies and lymphocytes to blood
6. Maintains tissue fluid composition and blood volume
7. Prevents entry of microbes and foreign materials into lymph nodes