Ch 20 - Lymphatic System

Introduction to the Lymphatic System

• The lymphatic system plays a crucial role in returning leaked fluids from blood vessels back to the circulatory system.

• Comprises three main components:

  • Lymphatic vessels (or lymphatics)

  • Lymph, which is the fluid in these vessels

  • Lymph nodes, which cleanse lymph.

• Lymphoid organs and tissues provide the structural basis for the immune system by housing phagocytic cells and lymphocytes, including the spleen, thymus, tonsils, and lymph nodes.

Structure and Function of the Lymphatic System

20.1 Overview of the Lymphatic System

• Returns interstitial fluid and leaked plasma proteins to the bloodstream via lymphatic vessels, circulating approximately 3L of interstitial fluid per day.

• Upon entering the lymphatic system, interstitial fluid is referred to as lymph.

Distribution and Structure of Lymphatic Vessels

General Characteristics

• Lymphatic vessels operate on a one-way system, directing lymph only towards the heart.

• Composed of:

  • Lymphatic capillaries: Small, blind-ended vessels interspersed between tissue cells and blood capillaries.

    • Lacked in bones, teeth, and bone marrow but found in the CNS in limited areas to help drain interstitial fluid and CSF.

• Larger lymphatic vessels include collecting lymphatic vessels, trunks, and ducts, resembling veins with thinner walls and more valves.

Lymphatic Capillaries

Structure and Features

• Lymphatic capillaries are more permeable compared to blood capillaries:

  • They can absorb larger molecules like proteins, cell debris, and pathogens unreachable by blood capillaries.

  • Serve as pathways for pathogens and cancer cells to travel throughout the body.

• Specialized structures such as minivalves, formed by overlapping endothelial cells and anchored to the surrounding matrix by collagen filaments, enhance permeability, responding to fluid volume changes.

Lacteals

• Lacteals are specialized lymph capillaries located in the intestinal mucosa that absorb digested fats and transport fatty lymph (chyle) to the bloodstream.

Larger Lymphatic Vessels

• Collecting lymphatic vessels coalesce into lymphatic trunks, each named for the body regions they drain, including:

  • Paired lumbar trunks

  • Paired bronchomediastinal trunks

  • Paired subclavian trunks

  • Paired jugular trunks

  • Single intestinal trunk

• These trunks ultimately drain lymph into one of two major lymphatic ducts:

  • Right lymphatic duct: Drains the right upper limb and right side of the head and thorax.

  • Thoracic duct: Drains the rest of the body and begins as an enlarged sac (cisterna chyli) in approximately half of individuals.

Lymph Transport

• The lymphatic system is a low-pressure system, utilizing:

  • The pumping action of skeletal muscles

  • Pressure changes during respiration

  • Valves to prevent backflow

  • Pulsations from nearby arteries

  • Rhythmic contractions of smooth muscle in vessel walls.

• Increased physical activity enhances lymph flow; immobilization helps to contain inflammatory materials for quicker healing.

Lymphoid Cells, Tissues, and Organs

Overview

• Lymphoid cells consist of immune system cells found in the lymphoid tissue and supporting cells.

  • Lymphocytes: Primary cells of the adaptive immune system, comprising:

    • T cells (T lymphocytes): Manage the immune response and attack infected cells.

    • B cells (B lymphocytes): Produce plasma cells that secrete antibodies to mark antigens for destruction.

• Other immune cells include macrophages and dendritic cells that help activate T cells.

Structure of Lymphoid Tissue

• Lymphoid tissue mainly consists of reticular connective tissue, providing sites for lymphocyte proliferation and immune surveillance.

• Cells are organized into follicles, providing structural support and function to immune cells.

Types of Lymphoid Organs

Primary Lymphoid Organs

• Red bone marrow and thymus where T and B cells mature:

  • Only B cells mature in the bone marrow, while T cells mature in the thymus.

Secondary Lymphoid Organs

• Including lymph nodes, spleen, and mucosa-associated lymphoid tissue (MALT) where mature lymphocytes encounter antigens.

Functions of Lymph Nodes

• Cleansing lymph: Act as filters to remove microorganisms and debris.

• Immune activation: Provide a site for lymphocytes to become activated.

Spleen

Overview

• The spleen is a blood-rich organ about the size of a fist located in the abdominal cavity.

• It serves several functions:

  • Lymphocyte proliferation and immune function.

  • Cleansing of blood by removing aged blood cells and debris.

  • Storage of blood platelets and monocytes for release when necessary.

Structure of Spleen

• Composed of:

  • White pulp: Areas of immune function mainly filled with lymphocytes.

  • Red pulp: Areas for the destruction of aging blood cells and pathogens, rich in RBCs and macrophages that engulf debris.

Clinical Note

• Splenectomy: surgical removal of the spleen, often done to avoid hemorrhage, with the liver and bone marrow taking over some functions. The spleen can often regenerate if a small part remains.

Tonsils

• Simplest lymphoid organs forming a ring around the pharynx, also involved in gathering and removing pathogens from air or food.

• Composed of follicles and crypts to trap and destroy bacteria, facilitating immune responses.

Peyer's Patches and Appendix

• Peyer's Patches: Clusters of lymphoid follicles in the small intestine wall focused on immune surveillance and memory generation against pathogens.

• The appendix serves a similar function with its lymphoid follicles, acting to prevent intestinal infections.

Thymus

• A bilobed organ situated in the anterior mediastinum, vital for T cell maturation.

• Functions uniquely as it lacks B cells and does not directly combat antigens.

• Possesses a blood-thymus barrier to prevent premature activation of developing T lymphocytes.