Lymphatic System

Chapter 21 – The Lymphatic System

21.1 Lymph and Lymph Vessels

Lymphatic System Functions

  • Transport and House Lymphocytes and Other Immune Cells

  • Return Excess Fluid in Body Tissues to Blood to Maintain Blood Volume

Lymphatic System Components

  • Lymph Vessels

  • Lymphoid Tissues and Organs

  • Lymph: The fluid transported within lymphatic vessels

21.1a Lymph and Lymphatic Capillaries

Characteristics of Lymph

  • Interstitial Fluid: Some fluid leaves blood capillaries and is not reabsorbed by them. This interstitial fluid moves into lymphatic capillaries where it is then called lymph.

Components of Lymph

  • Water

  • Dissolved Solutes

  • Small Amount of Protein

  • Cell Debris, Pathogens, or Cancer Cells: Sometimes present

Lymphatic Capillaries

  • Structure: Small, closed-ended vessels that absorb interstitial fluid.

  • Size: Slightly larger than blood capillaries; lack a basement membrane.

  • Walls: Composed of overlapping endothelial cells.

  • Fluid Entry: Have flaps between cells through which fluid enters but cannot exit.

  • Anchoring Filaments: Hold endothelial cells to nearby structures.

  • Lacteals: Specialized lymphatic capillaries in the gastrointestinal (GI) tract that absorb lipid-soluble substances.

Movement of Lymph into Lymphatic Capillaries

  • Hydrostatic Pressure: Interstitial fluid pushes lymph into the capillary.

  • Lymph Pressure: Pressure within the lymph vessels forces the flaps of the capillary wall to close, trapping lymph inside.

  • Lymph Flow: Lymph moves through progressively larger vessels: lymphatic capillaries → lymphatic vessels → lymphatic trunks → lymphatic ducts. Ultimately, fluid is returned to blood circulation.

Clinical View: Metastasis

  • Definition: Wandering cancerous cells establishing secondary tumors in different locations of the body.

  • Example: Breast cancer may metastasize to the lungs. Cancerous cells break free from the primary tumor and are transported in the lymph.

21.1b Lymphatic Vessels, Trunks, and Ducts

Structure of Lymphatic Vessels

  • Feeding: Lymphatic vessels are fed by lymphatic capillaries.

  • Location: Found adjacent to arteries and veins.

  • Vessel Tunics: Comprise three tunics: intima, media, and externa.

  • Valves: Prevent pooling and backflow of lymph.

  • Movement Mechanisms: The lymphatic system lacks a pump; moves lymph using:

    • Skeletal Muscles

    • Respiratory Pumps

    • Pulsatile Movement of Blood in nearby arteries

    • Rhythmic Contraction of smooth muscle in larger lymph vessel walls

  • Connection to Lymph Nodes: Some vessels connect to lymph nodes for the filtration of lymph.

Lymphatic Trunks

  • Feeding: Lymphatic trunks are fed by lymphatic vessels.

  • Types of Trunks:

    • Jugular Trunks: Drain lymph from the head and neck.

    • Subclavian Trunks: Drain upper limbs, breast, and the superficial thoracic wall.

    • Bronchomediastinal Trunks: Drain deep thoracic structures.

    • Intestinal Trunks: Drain most abdominal structures.

    • Lumbar Trunks: Drain the lower limbs, abdominopelvic wall, and pelvic organs.

Lymphatic Ducts

  • Feeding: Lymphatic ducts are fed by lymphatic trunks.

  • Size: They are the largest lymphatic vessels.

  • Function: Bring lymph to venous blood circulation.

  • Types of Ducts:

    • Right Lymphatic Duct:

    • Location: Near right clavicle.

    • Function: Drains upper right quadrant of the body, including the right side of the head and neck, right upper limb, and right side of the thorax. Delivers lymph to the junction of right subclavian and right internal jugular veins.

    • Thoracic Duct:

    • Function: Largest lymphatic vessel.

    • Path: Runs from diaphragm to junction of left subclavian and left jugular veins.

    • Features: Contains a saclike cisterna chyli at its base, receives lipid-rich chyle from the GI tract, and drains lymph from the left side of the head and neck, left upper limb, left side of thorax, abdomen, and both lower limbs.

Clinical View: Lymphedema

  • Definition: Accumulation of interstitial fluid due to interference with lymphatic drainage.

  • Symptoms: Causes swelling and pain in the affected area, interferes with wound healing, and may contribute to infection.

  • Causes: Blockage of lymph vessels, which may result from:

    • Trauma or infection of lymph vessels

    • Tumors

    • Radiation therapy scarring or surgical removal of lymph nodes

  • Management: Controlled with compression garments, exercise, and massage.

  • Extreme Cases: Known as elephantiasis, caused by infection with parasitic worms.

21.2 Overview of Lymphoid Structures

Primary Lymphoid Structures

  • Function: Involved in the formation and maturation of lymphocytes.

  • Includes:

    • Red Bone Marrow: Location of hematopoiesis, the production of blood’s formed elements, including T-lymphocytes and B-lymphocytes. T-lymphocytes migrate to the thymus to complete maturation.

    • Thymus: Involved specifically in T-lymphocyte maturation.

Secondary Lymphoid Structures

  • Function: Do not form lymphocytes but house them and other immune cells, serving as sites of immune response initiation.

  • Includes:

    • Lymph Nodes

    • Spleen

    • Tonsils

    • Lymphatic Nodules

    • MALT (Mucosa-associated lymphoid tissue)

21.3a Red Bone Marrow

  • Location: Found between trabeculae of spongy bone in flat bones of the skull, ribs, sternum, vertebrae, ossa coxae, and heads of the humerus and femur.

  • Function: Site of hematopoiesis; production of blood's formed elements, specifically T-lymphocytes and B-lymphocytes, with T-lymphocytes migrating to the thymus for maturation.

21.3b Thymus

  • Location: Situated in the mediastinum; larger in children than adults.

  • Structure: Consists of two thymic lobes, each surrounded by a connective tissue capsule. Trabeculae in the capsule subdivide lobes into lobules.

  • Cortex: Contains immature T-lymphocytes.

  • Medulla: Contains mature T-lymphocytes.

21.4 Secondary Lymphoid Structures

Overview

  • Function: Secondary lymphoid structures house lymphocytes and other immune cells within a reticular connective tissue matrix.

  • Organization: Organized into lymphoid organs (complete capsule of dense irregular connective tissue) and aggregates of lymphoid nodules.

  • Organs with Complete Capsule: Spleen and lymph nodes.

  • Structures with Incomplete Capsule or No Capsule: Tonsils, MALT, and diffuse lymphoid nodules.

21.4a Lymph Nodes

  • Function: Filter lymph and remove unwanted substances.

  • Structure: Small, oval, encapsulated structures located along deep and superficial pathways of lymph vessels, often found in clusters that receive lymph from specific body regions.

  • Types of Lymph Nodes:

    • Cervical Lymph Nodes: Receive lymph from head and neck.

    • Axillary Lymph Nodes: Receive lymph from breast, axilla, and upper limb.

    • Inguinal Lymph Nodes: Receive lymph from lower limb and pelvis.

Lymph Node Components

  • Afferent Lymphatic Vessels: Bring lymph to the node.

  • Efferent Vessel: Drains the lymph node, located at the hilum (involuted portion of the node).

  • Dense Connective Tissue Capsule: Encloses the node, with trabeculae subdividing the node into compartments, including an outer cortex and inner medulla.

Cortex and Medulla of Lymph Node

  • Cortex: Contains lymphoid nodules supported by reticular fibers. The germinal center houses proliferating B lymphocytes and some macrophages. The surrounding mantle zone comprises T-lymphocytes, macrophages, and dendritic cells. Cortical sinuses are tiny open channels lined by macrophages.

  • Medulla: Contains medullary cords (connective tissue fibers supporting B-lymphocytes, T-lymphocytes, and macrophages) and medullary sinuses (tiny open channels lined with macrophages).

Lymph Flow Through Nodes

  • Lymph enters through several afferent vessels, creating pressure that pushes it through the node to a single exit vessel. Lymph is monitored for foreign material, with macrophages removing foreign debris and lymphocytes potentially initiating an immune response, particularly in germinal centers.

  • Palpable Nodes: Can become enlarged due to proliferation of lymphocytes, especially in the neck and axilla region. Lymph exits the node through the efferent vessel and may then pass to neighboring lymph nodes.

Clinical View: Lymphoma

  • Definition: Malignant neoplasm from lymphoid structures.

  • Symptoms: Nontender, enlarged lymph node; possible accompanying symptoms include night sweats, fever, and weight loss.

  • Types of Lymphoma:

    • Hodgkin Lymphoma: Typically affects young adults and people over 60; if caught early, it can be cured by excision, radiation, and chemotherapy.

    • Non-Hodgkin Lymphoma: More common, with some types being aggressive and others slow-growing.

21.4b Spleen

  • Function: Largest lymphoid organ located in the left upper abdominal quadrant.

  • Hilum: The indentation where blood vessels and nerves enter.

  • Blood Supply: Supplied by the splenic artery and drained by the splenic vein.

  • Structure: Surrounded by a connective tissue capsule, with trabeculae extending into the spleen dividing it into red and white pulp.

Tissues of the Spleen

  • White Pulp: Contains clusters of T- and B-lymphocytes and macrophages around the central artery.

  • Red Pulp: Contains erythrocytes, platelets, macrophages, and B-lymphocytes.

    • Splenic Cords: Composed of cells and reticular connective tissue in red pulp.

    • Splenic Sinusoids: Very permeable capillaries that serve as filtration sites.

Monitoring Blood Flow Through the Spleen

  • Filter and Monitor Function: Filters and monitors blood (not lymph). The white pulp monitors for foreign material and bacteria. Macrophages in the sinusoids of the red pulp remove particles through phagocytosis, clearing defective erythrocytes and platelets.

  • Path of Flow: Splenic artery → central artery → sinusoids → venules → splenic vein.

  • Summary of Functions: Ingest foreign particles, clear defective erythrocytes and platelets, and store erythrocytes and platelets. The spleen generates blood cells during the first five months of fetal life.

Clinical View: Splenectomy

  • Definition: Surgical removal of the spleen, commonly performed due to:

    • Ruptured spleen from abdominal injury (most common reason).

    • Infection, cysts, or tumors.

    • Lymphoma or other types of cancer.

    • Blood disorders such as sickle cell anemia.

  • Risks: May increase susceptibility to life-threatening infections.

21.4c Tonsils

  • Types of Tonsils:

    • Pharyngeal Tonsil: Located in nasopharynx, called adenoids when enlarged.

    • Palatine Tonsils: Situated in the posterolateral oral cavity.

    • Lingual Tonsils: Found along the posterior one-third of the tongue.

Tonsil Functions

  • Function: Secondary lymphoid structures participate in immune surveillance of inhaled and ingested substances.

  • Tonsillar Crypts: Invaginations that trap material, containing lymphoid nodules, some with germinal centers.

Clinical View: Tonsillitis and Tonsillectomy

  • Definition: Acute tonsillitis is inflammation and infection of the tonsils, with palatine tonsils most commonly affected.

  • Symptoms: Red and enlarged tonsils, possible obstruction of the pharynx, fever, chills, sore throat, and difficulty swallowing.

  • Causes: Infection by viruses or bacteria. Persistent recurrent infections or chronic tonsillitis may require tonsillectomy, the surgical removal of the tonsils.

21.4d Lymphoid Nodules and MALT
Overview of Lymphoid Nodules

  • Definition: Clusters of lymphoid cells with some extracellular matrix, not completely surrounded by a connective tissue capsule.

  • Diffuse Lymphoid Tissue: Scattered nodules found in every organ, helping to defend against infections. Larger structures can form in certain areas, known as MALT.

MALT (Mucosa-associated Lymphoid Tissue)

  • Location: Located in gastrointestinal, respiratory, genital, and urinary tracts, found in the lamina propria of the mucosa.

  • Function: Help defend against foreign substances and are especially prominent in the small intestine, particularly in the ileum.

  • Peyer Patches: Large collections of lymphoid nodules within the GI tract wall.