Lymphatic System: Structure, Function, and Major Organs

Lymphatic System

Structure and Function

The lymphatic system is a vital component of the human body, closely linked with the circulatory and immune systems. It consists of lymph (fluid), lymphatic vessels, lymphatic tissues, and red bone marrow. The system performs several essential functions to maintain homeostasis and defend against pathogens.

• Drains excess interstitial fluid: Removes fluid from tissue spaces and returns it to the bloodstream, preventing edema.

• Transports dietary lipids: Lymphatic vessels absorb and transport lipids and fat-soluble vitamins (A, D, E, K) from the gastrointestinal tract.

• Carries out immune responses: Provides sites for highly specific immune reactions against microbes and abnormal cells.

Lymphatic vessels contain valves to ensure one-way flow of lymph. Lymph is a clear, pale-yellow fluid containing plasma and white blood cells. An exception is chyle, a lipid-rich lymph from the small intestine.

The main difference between interstitial fluid and lymph is their location: interstitial fluid is found between cells, while lymph is within lymphatic vessels and tissues.

Lymphatic Tissue

Lymphatic tissue is a specialized form of reticular connective tissue rich in lymphocytes. These cells are crucial for adaptive immune responses.

• B cells: Responsible for antibody-mediated immunity.

• T cells: Responsible for cell-mediated immunity.

Lymphatic Capillaries

Lymphatic capillaries are the starting point of the lymphatic vessels, located in spaces between cells. They have unique structural features that facilitate their function.

• Closed at one end.

• Resemble small veins but have thinner walls and more valves.

• Absent in avascular tissues (cartilage, epidermis, cornea, CNS, parts of spleen, red bone marrow).

• Greater permeability than blood capillaries, allowing absorption of large molecules like proteins and lipids.

• Larger diameter than blood capillaries.

• One-way structure: interstitial fluid enters but does not exit.

• Anchoring filaments: Elastic fibers that pull capillary walls apart when interstitial fluid accumulates, increasing fluid entry.

• Lacteals: Specialized capillaries in the small intestine that absorb dietary lipids; the lymph here is called chyle and appears creamy white.

Lymphatic Ducts and Trunks

Lymph flows from capillaries to vessels, then through lymph nodes, and finally into lymph trunks and ducts. The system is organized to efficiently drain different regions of the body.

• Principal lymph trunks:

  • Lumbar trunks: Drain lower limbs, pelvic wall, kidneys.

  • Intestinal trunks: Drain stomach, intestines, pancreas, spleen, liver.

  • Bronchomediastinal trunks: Drain thoracic wall, lungs, heart.

  • Subclavian trunks: Drain upper limbs.

  • Jugular trunks: Drain head and neck.

• Lymphatic ducts:

  • Thoracic duct: Begins at the cisterna chyli (anterior to L2 vertebra), receives lymph from the left side of the head, neck, chest, left upper limb, and all areas below the ribs.

  • Right lymphatic duct: Located at the junction of the right internal jugular and right subclavian veins; receives lymph from the right jugular, right subclavian, and right bronchomediastinal trunks.

• Pumps maintaining lymph flow:

  • Skeletal muscle pump: Muscle contractions compress vessels, moving lymph toward the heart.

  • Respiratory pump: Pressure changes during breathing facilitate lymph movement.

Primary Lymphatic Organs

Primary lymphatic organs are sites where lymphocytes are produced and mature.

• Red bone marrow: Located in flat bones and the epiphysis of long bones; contains pluripotent stem cells that produce mature B cells and pre-T cells (which migrate to the thymus).

• Thymus: Located in the mediastinum; bilobed, divided into lobules by connective tissue capsules.

  • Cortex: Contains many T cells, dendritic cells, epithelial cells, and macrophages. Pre-T cells mature here, assisted by dendritic cells and educated by epithelial cells, which also produce thymic hormones. Only 2% of T cells survive; the rest undergo apoptosis.

  • Medulla: Contains mature T cells, epithelial cells, dendritic cells, and macrophages. Thymic corpuscles (Hassall’s corpuscles) are concentric layers of epithelial cells, possibly involved in T cell death.

Lymph Nodes

Lymph nodes are bean-shaped structures located along lymphatic vessels, with large clusters in the mammary glands, axillae, and groin. They filter lymph and are sites of immune cell activation.

• Capsule: Dense connective tissue with trabeculae dividing the node into compartments.

• Cortex: Outer layer contains B cells; deeper cortex contains T cells and dendritic cells.

• Medulla: Inner region with B cells, plasma cells, and macrophages.

• Lymphatic nodules: Sites for plasma cell and memory B cell formation.

  • Primary nodule: Mainly B cells.

  • Secondary nodule: Forms in response to antigen; B cells proliferate and differentiate into memory B cells or plasma cells.

Spleen

The spleen is the largest mass of lymphatic tissue, located in the left hypochondriac region. It filters blood, removes defective cells, and supports immune functions.

• Capsule: Dense connective tissue with inward trabeculae forming the stroma.

• White pulp: Lymphatic tissue around the splenic artery; B and T cells carry out immune functions.

• Red pulp: Contains venous sinuses and cords of splenic tissue (Billroth’s cords) with macrophages, lymphocytes, and plasma cells. Functions include removal of defective blood cells, storage of platelets, and blood cell production in fetal life.

Lymphatic Nodules (MALT)

Lymphatic nodules, also known as mucosa-associated lymphatic tissue (MALT), are scattered in the lamina propria of mucous membranes lining the gastrointestinal, respiratory, urinary, and reproductive tracts. They provide localized immune protection.

• Tonsils: Five tonsils form a ring at the junctions of the oral cavity/oropharynx and nasal cavity/nasopharynx.

  • Pharyngeal tonsil (adenoid): Located in the posterior wall of the nasopharynx.

  • Palatine tonsils: Located in the posterior region of the oral cavity.

  • Lingual tonsils: Located at the base of the tongue.