In this note, we are going to discuss the anatomy and function of the lymphatic system.

The lymphatic, or lymphoid, system is part of both the circulatory and immune systems.

It plays a crucial role in regulating the body’s fluid balance and protecting it against pathogens.

The lymphatic system consists of the following components:

• Lymphatic vessels
• Primary lymphoid organs: The bone marrow and thymus, where lymphocytes are produced and mature;
• Secondary lymphoid organs: These include the spleen, tonsils, appendix, lymph nodes, and mucosa-associated lymphoid tissue (MALT). In these sites, lymphocytes are activated and proliferate to fight infections.

Functions

• The primary function of the lymphatic system is to produce, maintain, and distribute blood cells called lymphocytes.
• Additionally, it plays a role in maintaining normal blood volume and balancing the chemical composition of interstitial fluid.
• Another key function is to provide an alternative pathway for transporting hormones, nutrients, and waste products throughout the body.

Lymph

Lymphatic vessels originate in peripheral tissues and transport a fluid connective tissue called lymph to the venous system.

Lymph is formed through the filtration of plasma and consists of:

• Interstitial fluid: Similar to blood plasma but with a lower concentration of proteins.
• Lymphocytes: The cells responsible for the immune response.
• Macrophages: Various types of phagocytic cells.

In most lymphatic vessels, lymph is clear and colorless. However, lymph produced in the gastrointestinal system is opaque and milky due to the presence of chylomicrons, and it is sometimes referred to as chyle.

Lymphatic vessels

As previously mentioned, lymphatic vessels originate in peripheral tissues and transport lymph to the venous system.

The smallest of these vessels are the lymphatic capillaries, which collect interstitial fluid from the tissues.

These capillaries are arranged into networks called lymphatic plexuses.

Lymphatic capillaries are present in most tissues, except for the central nervous system. In the small intestine, specialized lymphatic capillaries known as lacteals transport lipids absorbed during the digestive process. However, lymphatic capillaries are absent in avascular tissues such as epithelia and cartilage.

Lymphatic plexuses converge to form larger lymphatic vessels responsible for transporting lymph away from the tissues. These vessels are grouped into two categories:
superficial and deep lymphatic vessels.

The movement of lymph through these vessels is primarily driven by the indirect action of adjacent structures, like skeletal muscle contractions and arterial pulsation. Valves within the vessels provide unidirectional flow of lymph.

Both superficial and deep lymphatic vessels pass through lymph nodes, which monitor and filter the lymph.

Vessels that bring lymph to the lymph node are called afferent lymphatic vessels,

while those that carry lymph away from the lymph node are called efferent lymphatic vessels.

Efferent lymphatic vessels eventually drain lymph into larger structures called lymphatic trunks, which include:

• Lumbar trunks

• Intestinal trunk

• Bronchomediastinal trunks

• Subclavian trunks

• Jugular trunks

These lymphatic trunks converge into two main lymphatic ducts:

• The right lymphatic duct drains lymph from the right upper limb, right side of the head, and right side of the chest.

• The thoracic duct is a larger vessel that collects lymph from the rest of the body.

It begins as the so-called cisterna chyli, an expanded saclike chamber.

These ducts then drain lymph into the venous circulation: the thoracic duct empties into the left subclavian vein, while the right lymphatic duct – near the junction of the right internal jugular and right subclavian veins.

Lymphocytes

The most important cells of the immune system – lymphocytes, are derived from stem cells located in the primary lymphoid organs, namely the bone marrow and thymus. These organs are discussed in more detail in the corresponding videos.

Upon maturation, lymphocytes are primarily distributed to the secondary lymphoid organs.

Lymphocytes are categorized into three primary classes based on their histological and functional characteristics:

• B lymphocytes (bone marrow–derived)
• T lymphocytes (thymus-dependent)
• NK cells (natural killer cells)

The primary function of lymphocytes is to initiate an immune response against foreign substances called antigens.

B lymphocytes support this response indirectly by producing antibodies that bind to specific antigens. T lymphocytes and NK cells directly target and eliminate infected or cancerous cells.

Lymph nodes

Lymph nodes are secondary lymphoid organs distributed throughout the body and organized according to their respective regions (e.g., axillary, cervical, abdominal).

Each lymph node is surrounded by a dense, fibrous connective tissue capsule.

Fibrous extensions from the capsule, known as trabeculae, divide the node into functional areas.

Nerves and blood vessels enter and exit the lymph node at the hilum.

As previously mentioned, lymph nodes have both afferent

and efferent lymphatic vessels.

Three regions can be distinguished inside the lymph node: an outer cortex,

an inner medulla,

and an intermediate area called the paracortex.

Lymph enters the lymph node through a subcapsular space that contains branching reticular fibers, macrophages, and dendritic cells. From there, it flows into the cortex.

Within the cortex, lymphocytes (primarily B cells, with some supporting T cells) aggregate in regions known as lymphoid follicles.

Lymph then moves through the lymph sinuses into the paracortex, a region dominated by T cells. Here, lymphocytes exit the bloodstream and enter the lymph node by crossing blood vessel walls.

After passing through the sinuses of the paracortex, lymph flows into the medulla. The medulla contains B cells and macrophages. In the medulla, elongated masses of densely packed lymphoid tissue, known as medullary cords, are situated between the sinuses.

Lymph continues through a network of sinuses in the medulla before exiting the lymph node via efferent lymphatic vessels at the hilum.

As lymph flows through a lymph node, more than 99% of the antigens are eliminated, effectively filtering the lymph.

Golosary

Introduction to the lymphatic system

Lymphatic system

systema lymphaticum

Lymphatic capillaries

vasa lymphocapillaria

Lymphatic plexus

plexus lymphaticus

Superficial lymphatic vessels

vasa lymphatica superficialia

Deep lymphatic vessels

vasa lymphatica profunda

Lymphatic valves

valvulae lymphaticae

Lymph nodes

nodi lymphatici

Afferent vessels

vasa afferentia

Efferent vessels

vasa efferentia

Lumbar trunk

truncus lumbalis

Intestinal trunk

truncus intestinalis

Bronchomediastinal trunk

truncus bronchomediastinalis

Subclavian trunk

truncus subclavius

Jugular trunk

truncus jugularis

Right lymphatic duct

ductus lymphaticus dexter

Thoracic duct

ductus thoracicus

Cisterna chyli

cisterna chyli

Capsule of the lymph node

capsula nodi lymphoidei

Hilum of the lymph node

hilum nodi lymphoidei

Cortex of the lymph node

cortex nodi lymphoidei

Medulla of the lymph node

medulla nodi lymphoidei

Subcapsular sinus

sinus subcapsularis

Medullary cords

chordae medullares

Terminal sinus

sinus terminalis