Lymphatics

The Lymphatic System and Lymphoid Organs and Tissues

Overview of the Lymphatic System

• Function:

  • The lymphatic system returns fluids that leak from blood vessels back to the bloodstream.

  • Primary components:

  • Lymphatic vessels: Transport lymph.

  • Lymph: The fluid collected and transported by lymphatic vessels.

  • Lymph nodes: Act as filters for lymph.

• Lymphoid organs and tissues:

  • Provide structural basis for the immune system.

  • Play a role in the body’s defense mechanisms and resistance to disease.

  • Major lymphoid organs include:

  • Spleen

  • Thymus

  • Tonsils

  • Lymph nodes

  • Other lymphoid tissues

Fluid Dynamics in the Lymphatic System

• Fluid movement in capillaries:

  • Hydrostatic and osmotic pressures exist on capillaries resulting in fluid dynamics:

  • This pressure forces fluid out at the arterial end and reabsorbs most of it at the venous end.

  • The fluid transferred (up to 3 L daily) becomes part of the interstitial fluid.

• Lymphatic vessels:

  • Lymphatic vessels collect excess protein-containing interstitial fluid and return it to the blood.

  • Also responsible for transporting pathogens and absorbed fats.

Distribution and Structure of Lymphatic Vessels

Lymphatic Capillaries

• Description:

  • Blind-ended vessels, absent from bones and teeth.

  • In the brain, astrocytes form channels connecting to lymphatics in meninges: This is part of the glymphatic system.

  • In case of malfunction, it may lead to degenerative diseases.

  • Structure: Similar to blood capillaries but more permeable.

• Features:

  • Overlap loosely to form one-way minivalves allowing for the uptake of proteins.

  • More permeable in inflamed tissues, allowing the uptake of debris and cells.

• Lacteals:

  • Found in the small intestine for absorption and transport of digested fats.

  • Each fingerlike villus of the intestinal mucosa contains one central lacteal.

  • Fatty lymph, or chyle, ultimately combines with other lymph.

Larger Lymphatic Vessels

• Pathway:

  • Lymphatic capillaries → collecting vessels → trunks → ducts.

• Structural features:

  • Collecting lymphatic vessels consist of the same three tunics as veins but have thinner walls, more valves, and anastomoses.

  • Collecting vessels travel with superficial veins in the skin and deep vessels of the trunk travel with deep arteries.

• Lymphatic Trunks:

  • Nine major lymphatic trunks named according to the regions they drain:

  • Paired lumbar

  • Bronchomediastinal

  • Subclavian

  • Jugular trunks

  • Single intestinal trunk

Lymphatic Ducts

• Right Lymphatic Duct:

  • Drains lymph from the right upper limb, and right side of head and thorax.

  • Empties into the blood at the junction of the right internal jugular and subclavian veins.

• Left (Thoracic) Duct:

  • Drains lymph from the rest of the body.

  • Empties into bloodstream at junction of left internal jugular and subclavian veins; starts as cisterna chyli.

Lymphatic Circulation

• Lymph transport:

  • Lymphatic vessels are low-pressure conduits relying on mechanisms that promote flow:

  • Milking action of skeletal muscles

  • Pressure changes in thorax during breathing

  • Pulse pressure waves from adjacent arteries

  • Rhythmic contractions of smooth muscle in walls of lymphatics.

  • Increased physical activity augments lymph flow.

  • Immobilization of a badly infected area restricts outflow of inflammatory materials.

Clinical Considerations

Homeostatic Imbalance

• Lymphedema:

  • Occurs when there is insufficient return of lymph to blood, leading to severe localized edema.

  • Possible causes:

  • Tumors blocking lymphatics.

  • Removal of lymphatics during cancer surgery.

  • Condition can improve if some lymphatic pathways remain viable and enlarge.

Lymphoid Cells

• Types of Lymphoid Cells:

  • Immune system cells found in lymphoid tissues alongside supporting cells forming the structural framework.

• Immune System Cells:

  • Lymphocytes:

  • 2 types:

    • T Lymphocytes (T cells):

    • Activated T cells directly attack and destroy infected cells.

    • B Lymphocytes (B cells):

    • Differentiate into plasma cells that secrete antibodies.

  • Macrophages:

  • Phagocytose pathogens and debris.

  • Dendritic Cells:

  • Capture and deliver antigens to lymph nodes to activate T cells.

Supporting Cells

• Reticular Cells:

  • Fibroblast-like cells that produce reticular fiber stroma in lymphoid organs and tissues.

Lymphoid Tissue

• Definition:

  • Houses and provides sites for the activation and proliferation of lymphocytes.

  • Ideal surveillance locations for lymphocytes and macrophages.

  • Composed of loose reticular connective tissue which dominates all lymphoid organs except the thymus.

Types of Lymphoid Tissue

• Diffuse Lymphoid Tissue:

  • Loose arrangement of lymphoid cells and reticular fibers present in every body organ.

  • Larger collections found in the lamina propria of mucous membranes.

• Lymphoid Follicles (Nodules):

  • Solid spherical bodies formed of tightly packed lymphoid cells and reticular fibers.

  • Have germinal centers containing proliferating B cells.

  • May comprise part of larger lymphoid organs, such as lymph nodes.

  • Examples include Peyer’s patches and the appendix.

Lymphoid Organs

Classification of Lymphoid Organs

• Mucosa-Associated Lymphoid Tissue (MALT):

  • Includes tonsils, Peyer’s patches in the small intestine, and appendix.

• Functional Categories:

  • Primary lymphoid organs: Sites where T and B cells mature.

  • Secondary lymphoid organs: Where mature lymphocytes first encounter their antigens and become activated.

Lymph Nodes

• Function:

  • Clustered along lymphatics, with hundreds throughout the body embedded in connective tissue.

  • Large clusters are present near body surface in inguinal, axillary, and cervical regions.

• Protective Functions:

  • Cleansing the lymph:

  • Act as lymph filters, removing and destroying pathogens via macrophages.

  • Immune system activation:

  • Strategically located for encountering antigens and activating lymphocytes.

Structure of a Lymph Node

• Shape and size:

  • Varies, but most are bean-shaped and small.

  • Surrounded by an external fibrous capsule, with fibers extending inward as trabeculae.

• Regions:

  • Cortex:

  • Contains densely packed lymphoid follicles with germinal centers full of dividing B cells.

  • Deep part houses T cells in transit (circulate among blood, nodes, and lymph) and abundant dendritic cells.

  • Medulla: Consists of medullary cords, thin inward extensions of cortical tissue containing B cells, T cells, and plasma cells.

  • Lymph Sinuses:

  • Large lymphatic capillaries spanned by crisscrossing reticular fibers.

  • Macrophages reside on the fibers, phagocytizing foreign matter in lymph.

Circulation in the Lymph Nodes

• Lymph Entry:

  • Enters lymph node via afferent lymphatic vessels and travels through sinuses.

• Exit:

  • Exits the node at the hilum via efferent lymphatic vessels.

• Flow characteristics:

  • Lymph flow is slow, permitting time for lymphocytes and macrophages to function effectively.

  • Lymph travels through multiple nodes before being entirely cleansed.

Clinical Aspects of Lymph Nodes

• Buboes:

  • Infected lymph nodes (often pus-filled); associated with conditions like the bubonic plague.

• Symptoms:

  • Overwhelmed by bacteria they try to destroy, resulting in inflamed, swollen, and tender nodes (lymphadenopathy).

• Cancer Infiltration:

  • Lymph nodes can be secondary sites for cancer if metastasizing cells become trapped.

  • Cancer-infiltrated lymph nodes usually swollen but not painful, distinguishing them from infected nodes.

The Spleen

• Description:

  • A soft, blood-rich organ about the size of a fist; the largest lymphoid organ.

• Functions:

  • Serves as a site for lymphocyte proliferation and immune surveillance.

  • Cleanses blood of aged and defective red blood cells and platelets.

  • Stores iron and recycles red blood cell breakdown products.

  • Stores blood platelets and monocytes.

  • May function in fetal erythrocyte production.

Structure of the Spleen

• Capsule:

  • Contains a fibrous capsule and trabeculae.

• Components:

  • White Pulp:

  • Where immune function occurs, composed of lymphocytes on reticular fibers, arranged around central arteries (visualized as islands of white in a sea of red).

  • Red Pulp:

  • Responsible for destroying old blood cells and bloodborne pathogens; consists of many R B Cs and macrophages, composed of splenic cords (reticular tissue) separating blood-filled sinusoids.

Clinical Considerations for the Spleen

• Rupture Risks:

  • The spleen has a thin capsule; direct impact or severe infection can result in rupture, spilling blood into the peritoneal cavity.

• Splenectomy:

  • The surgical removal of the spleen was once standard treatment for severe hemorrhage and shock risks.

  • Surgeons discovered the splenic tissue can sometimes repair itself if left undisturbed; emergency splenectomies have decreased in frequency.

  • Other organs, like the liver and bone marrow, typically take over the spleen’s functions if it is removed.

  • In children younger than 12, the spleen may regenerate if a small part remains.

Mucosa-Associated Lymphoid Tissue (MALT)

• Definition:

  • Lymphoid tissues located in mucous membranes throughout the body, notably lining the respiratory, genitourinary, and digestive tracts.

• Major Collections:

  • Include tonsils, Peyer’s patches, and the appendix.

Tonsils

• Function:

  • Form a ring of lymphoid tissue around the entrance to the pharynx.

• Types:

  • Paired Palatine Tonsils: The largest and most frequently infected.

  • Lingual Tonsil: Located at the base of the tongue.

  • Pharyngeal Tonsil (enlarged called adenoid): Found at the upper part of the throat.

  • Tubal Tonsils: Surround openings of auditory tubes into pharynx.

• Histological Features:

  • Contain follicles with distinct germinal centers and diffusely scattered lymphocytes.

  • Only partially encapsulated, with epithelium invaginating into deep tonsillar crypts, which trap debris and bacteria.

Intestinal MALT

• Peyer’s Patches:

  • Clusters of lymphoid follicles in the wall of the distal small intestine, structurally similar to tonsils.

• Appendix:

  • Contains numerous lymphoid follicles, contributing to immune functions. Prevent infection.

The Thymus

• Description:

  • A bilobed lymphoid organ located in the mediastinum, partially covering the heart.

• Function:

  • Main site of T lymphocyte development.

  • Most active and largest during childhood; gradually atrophies post-puberty.

  • In older adults, it is almost entirely replaced by fibrous and fatty tissue but continues to produce immunocompetent cells, albeit at a slower rate.

Thymic Structure

• Lobules:

  • Cortex: Contains rapidly dividing lymphocytes and macrophages.

  • Medulla: Contains lymphocytes and thymic corpuscles (keratinized epithelial cells), involved in the development of regulatory T cells that prevent autoimmunity.

• Unique Features:

  • No follicles present since it lacks B cells.

  • Does not directly combat antigens.

  • A blood-thymus barrier keeps immature T lymphocytes isolated from antigens to prevent premature activation.

  • The stroma primarily consists of epithelial cells.

Summary of Lymphoid Organs and Tissues