Chapter 20 - Lymphatic System

Functions

• Draining excess interstitial fluid & plasma proteins from tissue spaces
• Transporting dietary lipids & vitamins from GI tract to the blood
• Facilitating immune responses

An Overview

• Consists of two semi-independent parts
  • A meandering network of lymphatic vessels
  • Lymphoid tissues and organs scattered throughout the body
    • The spleen, thymus gland, and tonsils
    • Peyer's patches and bits of lymphatic tissue scattered in connective tissue
    • All are composed of reticular connective tissue and all help protect the body
    • Only lymph nodes filter lymph
  • Returns interstitial fluid and leaked plasma proteins back to the blood
• Lymph - interstitial fluid once it has entered lymphatic vessels
• A one-way system in which lymph flows toward the heart
• Lymph vessels include:
  • Microscopic, permeable, blind-ended capillaries
  • Lymphatic collecting vessels
  • Trunks and ducts

Lymph Capillaries - Similar to blood capillaries, with modifications

• Remarkably permeable
  • Endothelial minivalves function as one-way gates that:
    • Allow interstitial fluid to enter lymph capillaries
    • Do not allow lymph to escape from the capillaries
  • Withstand interstitial pressure and remain open
• During inflammation, lymph capillaries can absorb, cell debris, pathogens, cancer cells
• Cells in the lymph nodes, cleanse and "examine " this debris
• Lacteals - specialized lymph capillaries present in intestinal mucosa - absorb digested fat and deliver chyle to the blood

Lymphatic Collecting Vessels

• Have the same three tunics as veins
• Have thinner walls, with more internal valves
• Anastomose more frequently
• Collecting vessels in the skin travel with superficial veins
• Deep vessels travel with arteries
• Nutrients are supplied from branching vasa vasorum

Lymph Transport

• The lymphatic system lacks an organ that acts as a pump
• Vessels are low-pressure conduits
• Uses the same methods as veins to propel lymph
  • Pulsations of nearby arteries
  • Contractions of smooth muscle in the walls of the lymphatics

Lymphatic Trunks - return fluids to the blood

• Lymphatic trunks are formed by the union of the largest collecting ducts
• Major trunks include:
  • Paired lumbar, bronchomediastinal, subclavian, and jugular trunks
  • A single intestinal trunk
• Lymph is delivered into one of two large trunks
  • Right lymphatic duct
    • drains ___________________
    • returns fluid to ______________
  • Thoracic duct - arises from the cisterna chyli
    • drains ___________________
    • returns fluid to ______________
• Life Cycle of Lymphatic Filariasis (Elephantiasis)

Lymphoid Cells

• Lymphocytes are the main cells involved in the immune response
  • The two main varieties are T cells and B cells
  • T cells and B cells protect the body against antigens (anything the body perceives as foreign)
    • Bacteria and their toxins; viruses
    • Mismatched RBCs or cancer cells
  • T cells
    • Manage the immune response
    • Attack and destroy foreign cells
  • B cells
  • Produce plasma cells, which secrete antibodies
    • Antibodies immobilize antigens
• Macrophages - phagocytize foreign substances & help activate T cells
• Dendritic cells - spiny-looking cells with functions similar to macrophages
• Reticular cells - fibroblastlike cells that produce a stroma, or network, that supports other cell types in lymphoid organs

Lymph Nodes

• Lymph nodes are the principal lymphoid organs of the body
• Nodes are imbedded in connective tissue and clustered along lymphatic vessels
• Aggregations of these nodes occur near the body surface in inguinal, axillary, and cervical regions of the body
• Sentinel Node - is the very first lymph node(s) to receive drainage from a cancer-containing area of the breast.
• Their two basic functions are:
  • Filtration - macrophages destroy microorganisms and debris
  • Immune system activation - monitor for antigens and mount an attack against them
• Structure of a Lymph Node
  • Nodes are bean shaped and surrounded by a fibrous capsule
  • Trabeculae extended inward from the capsule and divide the node into compartments
  • Nodes have two histologically distinct regions: a cortex and a medulla
  • The cortex contains follicles with germinal centers, heavy with dividing B cells
  • Dendritic cells nearly encapsulate the follicles
  • The deep cortex houses T cells in transit
  • T cells circulate continuously among the blood, lymph nodes, and lymphatic stream
  • Medullary cords extend from the cortex and contain B cells, T cells, and plasma cells
  • Throughout the node are lymph sinuses crisscrossed by reticular fibers
  • Macrophages reside on these fibers and phagocytize foreign matter
• Circulation in a Lymph Node
  • Lymph enters via a number of afferent lymphatic vessels
  • It then enters a large subcapsular sinus and travels into a number of smaller sinuses
  • It meanders through these sinuses and exits the node at the hilus via efferent vessels
  • Because there are fewer efferent vessels, lymph stagnates somewhat in the node
  • This allows lymphocytes and macrophages time to carry out their protective functions

Other Lymphoid Organs

• The spleen, thymus gland, and tonsils
• Peyer's patches and bits of lymphatic tissue scattered in connective tissue
• All are composed of reticular connective tissue and all help protect the body
• Only lymph nodes filter lymph
• Spleen
  • Largest lymphoid organ, located on the left side of the abdominal cavity beneath the diaphragm
  • Functions
    • Site of lymphocyte proliferation
    • Immune surveillance and response
    • Cleanses the blood
    • Stores breakdown products of RBCs for later reuse
    • Spleen macrophages salvage & store Fe for use by bone marrow
    • Site of fetal erythrocyte production (normally ceases after birth)
    • Stores blood platelets
  • Structure
    • Surrounded by a fibrous capsule, it has trabeculae that extend inward and contains lymphocytes, macrophages, and huge numbers of erythrocytes
    • Two distinct areas of the spleen are:
      • White pulp - area containing mostly lymphocytes suspended on reticular fibers and involved in immune functions
      • Red pulp - remaining splenic tissue concerned with disposing of worn-out RBCs and bloodborne pathogens
• Thymus
  • A bilobed organ that secrets hormones (thymosin and thymopoietin) that cause T lymphocytes to become immunocompetent
    • In infants, it is found in the inferior neck and extends into the mediastinum where it partially overlies the heart
    • It increases in size and is most active during childhood
    • It stops growing during adolescence and then gradually atrophies
  • Thymic lobes contain an outer cortex and inner medulla
  • The cortex contains densely packed lymphocytes and macrophages
  • The medulla contains fewer lymphocytes and thymic (Hassall's) corpuscles
  • The thymus differs from other lymphoid organs in important ways
    • It functions strictly in T lymphocyte maturation
    • It does not directly fight antigens
  • The stroma of the thymus consists of star-shaped epithelial cells which secrete the hormones that stimulatelymphocytes to become immunocompetent
• Tonsils
  • Lymphoid tissue of tonsils contains follicles with germinal centers
  • Tonsil masses are not fully encapsulated
  • Epithelial tissue overlying tonsil masses invaginates, forming blind-ended crypts
  • Crypts trap and destroy bacteria and particulate matter
  • Simplest lymphoid organs form ring of lymphatic tissue around pharynx
    • Palatine tonsils - either side of the posterior end of the oral cavity
    • Lingual tonsils - lie at the base of the tongue
    • Pharyngeal tonsil - posterior wall of the nasopharynx
    • Tubal tonsils - surround the openings of the auditory tubes into the pharynx
• Aggregates of Lymphoid Follicles
  • Peyer's patches - isolated clusters of lymphoid tissue, similar to tonsils
    • Found in the wall of the distal portion of the small intestine
    • Similar structures are found in the appendix
    • Destroy bacteria, preventing them from breaching the intestinal wall
    • Generate "memory" lymphocytes for long-term immunity
  • MALT
    • MALT - mucosa-associated lymphatic tissue is composed of:
      • Peyer's patches, tonsils, and the appendix (digestive tract)
      • Lymphoid nodules in the walls of the bronchi (respiratory tract)
    • MALT protects the digestive and respiratory systems from foreign matter

Metastasis Through Lymphatic System

• Characteristic of malignant tumors
• Spread of disease from one organ to another
  • cancer cells travel via blood or lymphatic system
  • cells establish new tumors where lodge
• Secondary tumor sites can be predicted by direction of lymphatic flow from primary site
• Cancerous lymph nodes are firm, enlarged and nontender -- infected lymph nodes are not firm and are very tender

Development Aspects

• Beginnings of the lymphatic vessels and main clusters of lymph nodes are apparent by the fifth week of embryonic development
  • These arise from the budding of lymph sacs from developing veins
• Lymphatic organs (except the thymus) arise from mesoderm
• The thymus (endodermal origin) forms as an outgrowth of the pharynx
• Except for the spleen and tonsils, lymphoid organs are poorly developed at birth

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