Lymphatic System: Structure and Function – Study Notes

Lymphatic System

Introduction

The lymphatic system is a vital component of the circulatory and immune systems. It consists of a network of vessels, tissues, and organs that help maintain fluid balance, defend against pathogens, and transport dietary fats.

• Transport excess interstitial fluid (lymph) from tissues to the veins

• Produce and house lymphocytes for the immune response

• Transport absorbed fats from intestines to blood

The lymphatic system is made up of:

• Lymph Vessels

• Lymph

• Lymphoid Tissues

• Lymphoid Organs

Lymphatic Vessel Network

Lymphatic vessels transport excess fluid back to the circulatory system. As lymph flows through the network, it passes through progressively larger diameter vessels:

1. Lymphatic capillaries: closed-ended tubes interspersed among most blood capillary beds. They have overlapping endothelial cells that act as one-way flaps, allowing interstitial fluid to enter but not exit.

2. Lymphatic vessels: formed from merging lymphatic capillaries, possess valves to prevent backflow.

3. Lymphatic trunks: formed from merging lymphatic vessels, drain lymph from large regions of the body.

4. Lymphatic ducts: only two, which empty lymph into the veins.

Movement of Lymph

The lymphatic system lacks a central pump, so lymph is moved by:

• Skeletal muscle and respiratory pumps

• Pulsation from nearby arteries

• Smooth muscle contraction in larger lymph vessel walls

Lacteals: specialized lymphatic capillaries located in the GI tract that collect interstitial fluid, lipids, and lipid-soluble vitamins. ONLY seen in the small intestine.

Hydrostatic pressure of the interstitial fluid drives movement of lymph into lymphatic capillaries.

Lymphatic Trunks and Ducts

There are five main lymphatic trunks throughout the body:

• Jugular trunk: drains lymph fluid from head and neck

• Subclavian trunk: drains lymph from arms/upper limbs

• Bronchomediastinal trunk: drains lymph from inside of thoracic cavity

• Intestinal trunk: drains lymph fluid from intestines, abdominal cavity

• Lumbar trunks: drain lymph fluid from legs

Lymphatic ducts are formed by the fusion of trunks:

• Right lymphatic duct: smaller, deep to the clavicle, returns lymph at the junction of the right subclavian and internal jugular veins

• Thoracic duct: largest, collects lymph from most of the body, returns lymph into the junction between the left subclavian and internal jugular veins

Cisterna chyli: rounded, saclike structure located just inferior to the diaphragm that marks the beginning of the thoracic duct.

Lymphoid Structures

Classification of Lymphoid Structures

Lymphoid structures are divided into two main categories based on their function:

• Primary lymphoid structures

• Secondary lymphoid structures

Primary Lymphoid Structures

Primary lymphoid structures are the sites where lymphocytes are formed and mature.

• Red bone marrow: made up of reticular connective tissue, developing blood cells, and adipocytes. Location varies based on age (e.g., ilium).

• Thymus: bilobed organ located superficial to the heart, site of T-lymphocyte differentiation and maturation. Gets smaller with age and eventually disappears.

Hematopoiesis: production of blood cells that occurs in the red bone marrow of certain bones, regulated by erythropoietin (EPO) from the kidneys.

Secondary Lymphoid Structures

Secondary lymphoid structures are sites of immune response initiation and include:

• Lymph nodes

• Spleen

• Tonsils

• MALT (mucosa-associated lymphatic tissue)

Lymph Nodes

Lymph nodes are small, oval structures located along the pathway of lymph vessels. They function to recognize and filter foreign material in the lymph and initiate an immune response.

• Afferent lymphatic vessels: bring lymph into the node

• Efferent lymphatic vessels: carry filtered lymph away from the node

Lymph nodes are typically found in clusters, such as:

• Axillary lymph nodes

• Inguinal lymph nodes

• Cervical lymph nodes

Spleen

The spleen is the largest lymphatic organ located in the left upper quadrant just lateral to the stomach. In addition to its primary function, the spleen has three additional functions:

• Serves as a reservoir for erythrocytes and platelets

• Phagocytizes old, defective erythrocytes and platelets

• Phagocytizes bacteria and other foreign materials

The spleen is divided into two main regions:

• Red pulp: red blood cell function or storage/breakdown

• White pulp: immune response

Tonsils

Tonsils are large clusters of lymphatic cells and matrix located mainly in the pharynx. Their primary function is to recognize foreign material in inhaled and ingested substances and initiate an immune response.

There are three main tonsils:

• Pharyngeal tonsil: adenoids, posteriosuperior wall of the nasopharynx

• Palatine tonsils: posterolateral wall of the oral cavity

• Lingual tonsils: located along the posterior 1/3 of the tongue

MALT (Mucosa-Associated Lymphatic Tissue)

MALT consists of lymphatic nodules within the mucosa of the GI, respiratory, genital, and urinary tracts. Its primary function is to recognize foreign material in these tracts and initiate an immune response.

Summary Table: Lymphatic Structures and Functions

Structure	Location	Primary Function
Red Bone Marrow	Medullary cavity of bones (e.g., ilium)	Production of blood cells (hematopoiesis)
Thymus	Superior to the heart	T-lymphocyte maturation
Lymph Nodes	Along lymphatic vessels (axillary, inguinal, cervical)	Filter lymph, initiate immune response
Spleen	Left upper quadrant, lateral to stomach	Filter blood, immune response, RBC/platelet reservoir
Tonsils	Pharynx (nasopharynx, oral cavity, tongue)	Recognize foreign material in inhaled/ingested substances
MALT	Mucosa of GI, respiratory, genital, urinary tracts	Immune response in mucosal tissues

Key Terms and Definitions

• Lymph: Interstitial fluid that has entered lymphatic vessels.

• Lymphocyte: A type of white blood cell involved in immune responses.

• Lacteal: Specialized lymphatic capillary in the small intestine that absorbs dietary fats.

• Hematopoiesis: Formation of blood cellular components.

• MALT: Mucosa-associated lymphatic tissue, found in mucosal linings.

Relevant Equations

Hydrostatic pressure drives lymph movement:

$P_{lymph} = P_{interstitial} - P_{lymphatic}$

Where $P_{lymph}$ is the pressure in the lymphatic capillary, $P_{interstitial}$ is the pressure in the interstitial fluid, and $P_{lymphatic}$ is the pressure inside the lymphatic vessel.

Summary

• The lymphatic system maintains fluid balance, supports immune function, and transports dietary fats.

• It consists of a network of vessels, primary and secondary lymphoid structures, and specialized tissues.

• Understanding the structure and function of the lymphatic system is essential for comprehending immune responses and fluid homeostasis.