Lymphatic System

LYMPHATIC SYSTEM

Introduction

The lymphatic system was first described in the seventeenth century by two scientists, Olaus Rudbeck and Thomas Bartholin, working independently. It is an important system of the human body that works closely with the circulatory and immune systems.

The lymphatic system is mainly made up of lymph, lymphatic capillaries, lymphatic vessels, lymph nodes, and lymphatic organs. It forms a complex network that runs throughout the body.

Lymph is a tissue fluid that is formed when substances from blood capillaries pass into the tissue spaces. This system helps in maintaining fluid balance, defending the body against infections, and absorbing fats and fat-soluble vitamins.

The lymphatic system is considered a subsystem of the circulatory system and is also a major part of the immune system. It produces and transports disease-fighting cells called lymphocytes.

When infection occurs, lymph nodes often swell. This happens due to the accumulation of lymph fluid, bacteria, and immune cells, indicating that the body is actively fighting disease.

Major roles of the lymphatic system include:

• Maintaining fluid balance in the body
• Absorbing fats and fat-soluble vitamins (A, D, E, K)
• Protecting the body against infections and foreign particles

LYMPH

Lymph is a clear to pale white fluid that circulates through the lymphatic system. It is mainly composed of white blood cells, especially lymphocytes, which help in defending the body against bacteria, viruses, and other harmful organisms.

Lymph bathes the body tissues, helps maintain fluid balance, and removes waste materials and microorganisms from tissues. It enters the bloodstream through lymphatic vessels and ducts.

Lymph contains water, proteins, glucose, oxygen, and white blood cells. It is formed from the excess interstitial fluid that is not reabsorbed back into blood capillaries.

Composition of Lymph

Lymph is a clear, colourless fluid whose composition is similar to blood plasma but contains less protein.

Cellular Part

• Lymphocytes
• T-cells
• B-cells

These cells play a vital role in immunity.

Non-Cellular Part

• Water (about 94%)
• Solids (about 6%)

Solid Components

• Proteins
• Fats
• Carbohydrates
• Urea
• Creatinine
• Inorganic and non-protein nitrogenous substances

Chemical Composition of Lymph

FUNCTIONS OF THE LYMPH

Fluid and Protein Balance

During blood circulation, a large amount of fluid filters out of blood capillaries into tissue spaces. About 1–2 litres (nearly 10%) of body fluid remains as interstitial fluid. This fluid contains essential proteins and nutrients.

The lymphatic system collects this excess fluid and proteins and returns them to the bloodstream, preventing tissue swelling and maintaining fluid balance.

Transportation of Nutrients

The lymphatic system works together with the circulatory system to transport nutrients. It plays a major role in carrying:

• Fats
• Fat-soluble vitamins (A, D, E, K)

These substances are absorbed from the intestine and transported to the bloodstream through lymph.

It also helps in distributing oxygen, hormones, and other nutrients to body cells indirectly through blood circulation.

Digestion

The lymphatic system assists digestion by helping in the absorption of dietary fats. Specialized lymphatic vessels present in the intestinal lining absorb fats and transport them as lymph.

Proper functioning of the lymphatic system is essential for fat assimilation. Failure of this system may lead to malnutrition. By ensuring proper fat transport, the lymphatic system also helps prevent unhealthy fat accumulation.

Excretion

The lymphatic system removes:

• Dead blood cells
• Excess tissue fluid
• Waste materials
• Cellular debris

It also helps in eliminating toxins, pathogens, and abnormal cells such as cancer cells from tissues and intercellular spaces.

Protection

The lymphatic system provides immunity through B-cells and T-cells.

• B-cells provide humoral immunity by producing antibodies
• T-cells provide cell-mediated immunity by destroying infected or abnormal cells

When harmful pathogens enter the body, these cells identify them, become activated, and destroy them, thereby protecting the body from diseases.

FORMATION OF LYMPH

The formation of lymph occurs through five important steps:

• Collection of excess fluid
• Entry into lymphatic capillaries
• Transport through lymphatic vessels
• Filtration in lymph nodes
• Return to bloodstream

Collection of Excess Fluid

The first step is the collection of interstitial fluid from tissue spaces. This fluid is formed when water and small molecules leak out of blood capillaries due to pressure differences.

Interstitial fluid supplies nutrients to cells and removes waste products. Excess fluid collects in tissue spaces and resembles plasma but contains less protein.

Entry into Lymphatic Capillaries

Lymphatic capillaries are thin-walled vessels present in most tissues except the central nervous system and bone marrow.

The entry of interstitial fluid into lymphatic capillaries occurs due to:

• Pressure gradients
• Anchoring filaments

Anchoring filaments pull open the capillary walls when tissues move, allowing fluid to enter. Once inside, the fluid is called lymph.

Transport through Lymphatic Vessels

After entering capillaries, lymph flows into larger lymphatic vessels. These vessels resemble veins and contain:

• One-way valves
• Smooth muscle in the walls

Lymph movement is aided by:

• Muscle contraction
• Arterial pulsation
• Body movements

Lymph passes through several lymph nodes during transport.

Filtration in Lymph Nodes

Lymph nodes are small, bean-shaped structures located along lymphatic vessels. They filter lymph by removing:

• Bacteria
• Viruses
• Foreign particles
• Cancer cells

Lymph enters through afferent vessels and exits through efferent vessels. Inside the node, immune cells identify and destroy harmful substances.

Return to Bloodstream

After filtration, lymph enters larger vessels and finally drains into two major ducts:

• Thoracic duct
• Right lymphatic duct

These ducts empty purified lymph into the subclavian veins, returning it to the bloodstream.

LYMPHATIC CAPILLARIES

Lymphatic capillaries are microscopic vessels that form the beginning of the lymphatic system. They are larger in diameter than blood capillaries and have very thin walls, which allow easy entry of tissue fluid.

Lymphatic capillaries are found throughout the body except in the following areas:

Absence of Lymphatic Capillaries

These capillaries contain specialized one-way valves. These valves allow interstitial fluid to enter the capillary but prevent it from flowing back into the tissue space.

Lymphatic capillaries are made up of endothelial cells. When the pressure of interstitial fluid increases, it pushes open the valves, allowing fluid to enter. When pressure normalizes, the valves close, preventing backward flow.

Attached to lymphatic capillaries are anchoring filaments. These are fine collagen fibers that connect endothelial cells to surrounding tissues. When tissues swell or move, anchoring filaments pull the capillaries open, allowing more fluid to enter.

LYMPHATIC VESSELS

Lymphatic vessels transport lymph from tissues toward the heart. They are thin-walled and larger than lymphatic capillaries.

There are two main types of lymphatic vessels:

Types of Lymphatic Vessels

These vessels:

• Are closed at one end
• Contain valves to prevent backflow
• Drain excess interstitial fluid into venous circulation

The walls of lymphatic vessels consist of endothelium and smooth muscle. Movement of lymph is supported by:

• Skeletal muscle contraction
• Pulsation of nearby arteries
• Normal body movements

LYMPH TRUNKS

Lymph from lymphatic vessels passes through lymph nodes and then combines to form lymph trunks. These trunks are large collecting channels.

Major Lymph Trunks

LYMPH DUCTS

Lymph trunks finally drain into two major lymphatic ducts.

Types of Lymphatic Ducts

• Thoracic duct (Left lymphatic duct)
• Right lymphatic duct

Drainage of Lymphatic Ducts

THORACIC DUCT (LEFT LYMPHATIC DUCT)

The thoracic duct is the largest lymphatic duct, measuring about 38–45 cm in length. It begins in the abdomen as a dilated sac called cisterna chyli.

The cisterna chyli receives lymph from:

• Left lumbar trunk
• Right lumbar trunk
• Intestinal trunk

In the neck region, the thoracic duct also receives lymph from:

• Left jugular trunk
• Left subclavian trunk
• Left broncho-mediastinal trunk

The thoracic duct drains lymph from the entire left side of the body and lower half of the body, which is why it is called the left lymphatic duct.

RIGHT LYMPHATIC DUCT

The right lymphatic duct is short, measuring about 1–2 cm.

It receives lymph from:

• Right jugular trunk
• Right subclavian trunk
• Right bronchomediastinal trunk

It drains lymph from the right side of the head, neck, thorax, and right upper limb into the right subclavian and internal jugular veins.

LYMPHATIC TISSUES AND ORGANS

Lymphatic tissues and organs are classified into two types:

Primary Lymphatic Organs

Primary lymphatic organs are responsible for the formation and maturation of lymphocytes.

Primary Lymphatic Organs

Stem cells in bone marrow produce B-cells and immature T-cells. Immature T-cells migrate to the thymus for maturation.

Thymus

The thymus is a lymphoid organ located in the thoracic cavity behind the sternum and in front of the heart.

Key features:

• Large in infants
• Maximum size at 10–12 years (about 40 g)
• Gradually shrinks after puberty

The thymus has two lobes, each covered by a connective tissue capsule. The capsule extends inward as trabeculae, dividing the organ into lobules.

Each lobule has:

• Outer dark cortex
• Inner light medulla

The thymus is made of glandular epithelium and hematopoietic connective tissue. It is the site where T-cells mature, multiply, and gain immune competence.

Secondary Lymphatic Organs

Secondary lymphatic organs are sites where lymphocytes become activated.

Secondary Lymphatic Organs

• Lymph nodes
• Spleen

These organs are surrounded by a capsule.

Lymphatic nodules are not considered secondary organs because they lack a capsule and are found along mucous membranes, protecting respiratory, digestive, urinary, and reproductive tracts.

Lymph Node

Lymph nodes are small, bean-shaped structures, measuring 1–25 mm in length. They are found in the neck, armpits, groin, and other regions.

Each lymph node is covered by a dense connective tissue capsule. Lymph nodes often swell during infections, indicating immune activity.

Structure of Lymph Node

• Outer capsule of fibrous tissue
• Internal stroma divided into cortex and medulla
• Trabeculae extend inward
• Cortex contains lymph follicles
• Germinal centers are sites of B-cell proliferation
• Medulla contains plasma cells and macrophages
• Hilum allows entry and exit of vessels

Afferent vessels bring lymph into the node, and efferent vessels carry filtered lymph away.

Functions of Lymph Node

• Filters lymph and removes pathogens
• Exposes antigens to immune cells
• Activates B-cells and T-cells
• Produces lymphocytes
• Removes dead cells and cancer cells
• Produces plasma proteins like globulins

Spleen

The spleen is a dark purple, bean-shaped organ, about 12 cm long and weighing approximately 200 g. It lies in the left upper abdomen, beneath the rib cage.

It filters blood, stores blood cells, and produces immune cells. Damage to the spleen can be life-threatening.

Structure of Spleen

The spleen has:

• Diaphragmatic surface (contacts diaphragm)
• Visceral surface (contacts stomach)

It is covered by peritoneum and a fibrous capsule. Trabeculae divide the organ internally.

The splenic tissue contains Malpighian corpuscles, which are lymphoid structures.

The spleen is supplied by the splenic artery and drained by the splenic vein.

White Pulp

White pulp consists of lymphatic tissue surrounding arteries. It contains lymphoid follicles rich in B-cells and follicular dendritic cells.

White pulp appears as white dots on a cut surface of the spleen and is responsible for immune functions.

Red Pulp

Red pulp consists of blood-filled sinusoids and splenic cords. It contains macrophages that destroy old and damaged red blood cells.

Functions of Spleen

• Produces blood cells during foetal life
• Filters blood and removes old RBCs
• Produces antibodies and WBCs
• Destroys pathogens by phagocytosis
• Maintains fluid balance
• Acts as a blood reservoir