Module 20.1 — Structure and Function of the Lymphatic System

Immune and Lymphatic System

  • Immunity: diverse processes protecting body from cellular injury and pathogens; immune system includes leukocytes (WBCs) and immune proteins in plasma.

  • Lymphatic system: group of lymphoid organs/tissues that works with the immune system and supports fluid homeostasis and fat absorption; two main components:

    • Lymphatic vessels - blind-ended tubes

    • Lymphoid tissue and organs (tonsils, lymph nodes, spleen, thymus, mucosa-associated lymphoid tissue)

  • Interaction: lymphatic system aids immune defense and maintains interstitial fluid balance.

Lymphatic System: Structure

  • Lymphatic vessels: network of vessels collecting excess interstitial fluid and returning it to the cardiovascular system.

  • Lymphatic tissue and organs: tonsils, lymph nodes, spleen, thymus.

Functions of the Lymphatic System

  • Regulation of interstitial fluid volume:

    • Net filtration in capillaries causes fluid loss from plasma to interstitial space; about 2−4 L/day2-4 \text{ L/day} must be returned to circulation to maintain blood volume and pressure.

    • Lymphatic vessels pick up excess interstitial fluid and return it to the cardiovascular system.

  • Absorption of dietary fats:

    • Fat breakdown products enter lacteals in the small intestine and are transported via lymph to the blood.

  • Immune functions:

    • Lymphoid organs filter pathogens from lymph and blood; house leukocytes; participate in leukocyte maturation.

Lymphatic Vessels and Circulation

  • Lymph collection and flow:

    • Lymph-collecting vessels merge into lymph trunks; nine trunks drain specific regions: Lumbar (lower limbs/pelvis),  Jugular (head/neck),  Intestinal (fat-rich lymph from small intestine),  Bronchomediastinal,  Subclavian (upper limbs).\text{Lumbar (lower limbs/pelvis)},\; \text{Jugular (head/neck)},\; \text{Intestinal (fat-rich lymph from small intestine)},\; \text{Bronchomediastinal},\; \text{Subclavian (upper limbs)}.

  • Major ducts:

    • Thoracic duct (left side body and lower body): drains into junction of left internal jugular and left subclavian veins.

    • Right lymphatic duct (right side of body): drains into junction of right internal jugular and right subclavian veins.

  • Cisterna chyli: expanded sac that drains into thoracic duct and right lymphatic system; thoracic duct is the largest lymphatic duct.

Lymphatic Vasculature Details

  • Lymphatic capillaries: lacteals in the small intestine; blind-ended, one-way system moving lymph away from tissues.

  • Lymphatic vessels: low-pressure circuit; valves prevent backflow; muscle contractions (skeletal and smooth) aid lymph movement; often situated between muscles.

Lymphoid Tissues and Organs

  • Reticular tissue: loose connective tissue forming a network that traps pathogens; lymphoid tissue is organized into lymphoid organs and clusters.

  • Major leukocytes in lymphoid tissues: macrophages, B and T lymphocytes, dendritic cells, reticular cells.

Lymph Nodes

  • Structure: small clusters along lymphatic vessels with external connective tissue capsule; cortex and medulla regions; afferent vessels enter, efferent vessels exit.

  • Function: trap pathogens; dendritic cells and leukocytes eliminate threats; ~90% of pathogens trapped before entering blood.

Spleen

  • Location: left upper quadrant; largest lymphoid organ.

  • Internal regions:

    • Red pulp: macrophages destroy old erythrocytes.

    • White pulp: leukocytes and dendritic cells filter pathogens from blood.

Thymus

  • Location: superior mediastinum; two lobes.

  • Function: generates a population of functional T cells through thymic hormones.

  • Size and aging: highly active in infants/children; max size around 12−14 years12-14 \text{ years}; after this, atrophy and fat replacement reduce T cell production.

Lymphoid Tissues and Organs (General)

  • Lymphoid tissues pathogens encounter leukocytes and dendritic cells, reticular network traps pathogens.

  • MALT (Mucosa-associated lymphoid tissue): loosely organized lymphoid clusters protecting mucous membranes exposed to pathogens; lacks a connective tissue capsule.

MALT and GI Tract Lymphoid Structures

  • Tonsils: located around oral and nasal cavities; part of oropharyngeal defense.

  • Peyer’s patches: aggregated lymphoid nodules in ileum; defend against bacteria that escape large intestine.

  • Appendix: defends against bacteria in large intestine.

Lymphoid Organization in the GI Tract

  • MALT distribution in intestines: MALT in small intestine and appendix; Peyer’s patches prominent in ileum.

Lymphedema

  • Role of lymphatic system in fluid balance evident in lymphedema: swelling due to excess interstitial fluid.

  • Causes: removal/blockage of lymphatic vessels (e.g., after surgery); pathogens can cause blockage.

  • Consequence: fluid accumulates in tissues, leading to enlarged areas (often disfiguring).