The Lymphatic System: Function and Composition

• Blood capillaries supply the body's tissues with oxygen-rich blood while removing carbon dioxide-rich blood.
• Approximately 20 liters of fluid exit the arterial capillaries daily.
• Only around 17 liters of fluid returns to the venous capillaries.
• Interstitial fluid, similar to blood plasma, leaks from blood vessels into the surrounding tissue.
• Lymphatic vessels drain excess fluid from tissues as lymph.
• Lymphatic vessels then return this fluid to the blood.
• Lymph returns to the big veins at the venous angle. This is the junction between the subclavian and internal jugular veins.
• Lymph returns via the thoracic duct and right lymphatic duct.

Lymphatic System Composition

• Consists of lymphatic fluid, lymphatic vessels, lymphatic tissue, and lymphatic organs.
• These elements are located throughout the tissues of the body.

Lymphatic System Functions

• Drains excess interstitial fluid from the tissues and returns it to the bloodstream.
• Initiates an immune response against disease by producing and transporting lymphocytes.
• Transports dietary lipids absorbed by the gastrointestinal tract into the blood.

Lymphatic Trunks and Ducts

• Lymphatic vessels merge into larger trunks, which eventually converge to form the right lymphatic duct and the thoracic duct.
• The thoracic duct is larger and drains lymph from the rest of the body.
• It originates in the abdomen as the cisterna chyli.

Cisterna Chyli

• Dilated sac-like structure.
• Located at the lower end of the thoracic duct, anterior to the bodies of L1 and L2.
• Formed by the merging of the right and left lumbar trunks and the intestinal trunk.
• Passes through the diaphragm at the aortic aperture.

Lymphatic Ducts

• The thoracic duct empties into the junction where the left internal jugular vein joins the left subclavian vein (left venous angle).
• The left bronchomediastinal trunk drains the left half of the thoracic wall, the diaphragm's anterior part, the left lung, and the heart's left half.
• Right lymphatic duct is formed by the right jugular and right subclavian trunks.
• Right lymphatic duct drains lymph from the body's upper right quadrant.
• Right lymphatic duct empties into the junction where the right internal jugular vein joins the right subclavian vein (right venous angle).

Lymphatic Organs

• Lymphatic organs are divided based on their function.
• Primary lymphatic organs are bone marrow and the thymus gland. These are sites of lymphocyte production, maturation, and selection.
• Secondary lymphatic organs include diffuse lymphatic tissue, the spleen, and lymph nodes.
• Secondary organs are sites for encountering pathogens and becoming activated.
• Lymphatic organs are also divided according to their structure. Can be encapsulated or not.
• Encapsulated lymphatic organs include the thymus, lymph nodes, and spleen. These have a dense capsule and stroma made of fibers/reticular cells
• Not encapsulated lymphatic organs include diffuse lymphatic system and mucosa-associated lymph tissue

Lymph Nodes

• Kidney-shaped and can measure up to 3 cm in length.
• Located along the course of lymphatic vessels, with about 600 lymph nodes in the body.
• Small encapsulated bodies.
• Depression called the hilum where arteries and nerves enter, and veins exit.
• Lymph nodes are covered by a capsule of dense connective tissue.
• Extensions of the capsule, called trabeculae, form the partitions within the lymph nodes. Reticular tissue forms the stroma of the lymph node.
• Immunocompetent B cells and T cells exist throughout the lymph node.
• Lymph Nodes are divided in two sections, the outer cortex and inner medulla, with paracortex in the middle.
• The cortex contains lymphatic follicles, and medulla does not.
• The cortex receives lymph from afferent vessels, while the medulla forms sinuses that lead to efferent vessels at the hilum.
• The outer cortex contains lymphatic follicles that mostly contain B-cells.
• The inner cortex (paracortex) contains mostly T-cells.
• Medullary cords contain mostly plasma cells.
• Other cells in the lymph node includes macrophages, dendritic cells, follicular dendritic cells, and reticular cells.
• Macrophages and dendritic cells trap antigens and present them on their surfaces
• When B cells in lymphatic follicles are stimulated, they differentiate into plasma cells.
• Plasma cells move to the medulla (medullary cords).
• Large tortuous lymph sinuses surrounded by lymphoid cells are linked to form the medullary cords.
• The medullary cords contain B lymphocytes and some plasma cells.
• Lymph nodes filter the lymph, removing foreign material and microorganisms.
• All lymph is filtered by at least one lymph node before returning to the blood. Antibody-mediated and cell-mediated immune responses occur in the lymph nodes.
• They serve as antibody and lymphocyte production sites.
• Responsible for antigen recognition.

Lymph Node Vessels

• Afferent lymphatic vessels enter the lymph node at different points and have valves that prevent backflow of lymph
• Each afferent vessel empties into space known as the subcapsular sinus
• The trabecular sinuses are a continuation of the subcapsular sinuses
• They follow the trabeculae and drain into the medullary sinuses.
• Medullary Sinuses are found separating the cords. They converge at the hilum into the efferent lymphatic vessel
• Lymph is removed from the medullary sinus via one or two efferent lymphatic vessels that leave the lymph node at the hilum
• Valves in the efferent vessels prevent lymph from flowing in the wrong direction.

Lymphoid Nodules

• The outer cortex houses primary and secondary lymphoid follicle groups
• Primary lymphoid nodules do not have a germinal center
• Spherical aggregates of naive B lymphocytes (virgin B cells and memory cells)
• Secondary lymphoid follicles have a germinal center or pale center
• Germinal centers are the forming result of B cells responding to outside antigens
• Outer region contains Small, migrating lymphocytes with some B cells from the origin site
• Antigen Presenting cells are also called as APCs