Lymphatic System: Structure and Function

Lymphatic System Composition

• The lymphatic system is a network of lymph vessels and lymphatic organs distributed throughout the body.
• Lymph is the fluid transported within this system.

Lymphatic System Functions

• It transports and houses lymphocytes along with other immune cells.
• Returns excess fluid from body tissues to the blood to maintain blood volume.

Primary Lymphatic Structures

• Involved in the formation and maturation of lymphocytes.
• These structures are red bone marrow and the thymus.

Secondary Lymphatic Structures

• Do not form lymphocytes
• House lymphocytes and other immune cells
• Sites of immune response initiation.
• Includes lymph nodes, spleen, tonsils, and lymphatic nodules, as well as MALT (mucosa-associated lymphatic tissue).

Characteristics of Lymph

• Some fluid leaves blood capillaries but is not reabsorbed.
• The fluid between/around cells is known as interstitial fluid.
• Interstitial fluid enters lymphatic capillaries and becomes lymph.
• Lymphatic capillaries transport the lymph back to the blood.

Components of Lymph

• Primarily water with dissolved solutes and a small amount of protein.
• It may also contain cell debris, pathogens, or cancer cells.

Lymphatic Capillaries

• These are small, closed-ended vessels that absorb interstitial fluid.
• They are interspersed around most blood capillaries but absent in avascular tissues, red marrow, spleen, and CNS.
• Slightly larger than blood capillaries; possess no basement membrane.
• Walls consist of overlapping endothelial cells that act as flaps, allowing fluid to enter but not exit.
• Lacteals are specialized lymphatic capillaries in the GI tract, absorbing lipid-soluble substances.

Lymphatic Vessels

• Lymphatic vessels are fed by lymphatic capillaries.
• Located adjacent to arteries and veins.
• Have valves that prevent pooling and backflow of lymph.
• The lymphatic system lacks a pump, so it moves lymph through skeletal muscle and respiratory pumps.
• Pulsatile movement of blood in nearby arteries and rhythmic contraction of smooth muscle in larger lymph vessel walls help.
• Some vessels connect to lymph nodes for lymph filtration.

Edema and Lymphedema

• Edema is swelling.
• Lymphedema is swelling caused by blockage of the lymph system, often due to obstruction, surgical removal of lymph nodes, spread of malignant tumors, scarring from radiation therapy, or trauma/infection.
• Elephantiasis is extreme lymphedema; in Southeast Asia and Africa, it may be caused by filarial worms infecting the lymphatic system.

Red Bone Marrow

• Located between trabeculae of spongy bone, especially in flat bones of the skull, ribs, sternum, vertebrae, ossa coxae, and heads of the humerus and femur.
• Site of hemopoiesis: production of blood's formed elements, including T-lymphocytes and B-lymphocytes.
• T-lymphocytes migrate to the thymus to complete maturation.

Thymus

• Involved in T-lymphocyte maturation
• Located in the mediastinum.
• Larger in children than adults; grows until puberty and then regresses (replaced by adipose tissue).
• It has an outer cortex containing immature T-lymphocytes and an inner medulla with mature T-lymphocytes.

Lymph Nodes

• Filter lymph and remove unwanted substances.
• Small, oval, encapsulated structures located along deep and superficial lymph vessel pathways.
• Possess an outer cortex (with a mantle zone of T-cells and dendritic cells, and a germinal center with B-cells and macrophages) and an inner medulla.
• Occur in clusters receiving lymph from body regions.
• Cervical lymph nodes receive lymph from the head and neck.
• Axillary lymph nodes receive lymph from the breast, axilla, and upper limb.
• Inguinal lymph nodes in the groin receive lymph from the lower limb and pelvis.
• Lymph enters through several afferent vessels, creating pressure to push it through the node to the single exit (efferent) vessel.
• Macrophages remove foreign debris.
• Lymphocytes may initiate an immune response, potentially causing enlarged nodes.

Spleen

• The largest lymphatic organ, located in the left upper quadrant of the abdomen.
• Subdivided into two regions: white pulp and red pulp.
• White pulp contains clusters of T- and B-lymphocytes and macrophages around a central artery; monitors blood for foreign materials.
• Red pulp contains erythrocytes, platelets, macrophages, and B-lymphocytes; monitors and recycles old red blood cells and platelets; serves as a storage site for erythrocytes and platelets.
• Splenectomy is the surgical removal of the spleen.
• May be performed due to ruptured spleen from abdominal injury, infection, cyst, tumor, lymphoma or other cancer, or blood disorders.
• Individuals who undergo splenectomy may be more prone to life-threatening infection.

Tonsils

• Secondary lymphoid organs in the throat and oral cavity help prevent against foreign substances that are either ingested or inhaled.
• Pharyngeal tonsils are located on the posterior wall of the nasopharynx; when enlarged, they are called adenoids.
• Palatine tonsils are located on the posterolateral portion of the oral cavity near the soft palate.
• Lingual tonsils are located on the posterior 1/3 of the tongue.

Lymphatic Nodules

• Clusters of lymphatic cells with some extracellular matrix that are not completely surrounded by a connective tissue capsule.
• Scattered nodules are termed diffuse lymphatic tissue.
• Found in every body organ, helping defend against infection.
• In some areas, they group together to form larger structures, such as MALT.

MALT (Mucosa-Associated Lymphatic Tissue)

• Located in gastrointestinal, respiratory, genital, and urinary tracts, helping defend against foreign substances.
• Prominent in the small intestines, especially the ileum.
• Peyer Patches are large collections of lymphatic nodules within the wall of the GI tract.