Lymphatic System: Organs, Cells, and Function

Lymphatic System Overview

• A key part of the lymphatic system includes capillaries, vessels, trunks, and ducts
• The associated organs of the lymphatic system: spleen, liver, thymus, tonsils, and lymph nodes
• The spleen and lymph node anatomy are explained further in the introduction to lymphatic lab
• It's important to understand these cells: Dendritic Cells, T Cells, B Cells, and Macrophages
• The lymphatic system overlaps with the cardiovascular and immune systems

Circulation and Function

• About 20 liters of blood passes through arterioles, but only 17 liters are recollected by venules
• The lymphatic system collects the missing 3L of fluid, looks for foreign pathogens in the fluid, and helps dump large molecules into the bloodstream
• If the fluid that escapes the artery is not taken up by the vein it can cause edema if the lymphatic system does not take it up
• Lymphatic vessels are blind tubes with overlapping endothelial cells, and flaps that can only open inward

Collecting the Missing Fluid

• Internal valves prevent backflow, similarly to veins
• The muscular pump, the respiratory pump, and the smooth muscle cells help push lymph through
• The pulse of proximal arteries is also a contributing factor to lymph flow
• Like the blood vessels, lymphatic vessels start small becoming larger coming together, as: lymphatic capillaries, lymphatic vessels, lymphatic trunks, and two lymphatic ducts
• Fluid is dumped back into the venous system where the subclavian and jugular veins meet, eventually moving into the superior vena cava and into the right atrium
• The right lymphatic duct drains the right-upper quadrant – The thoracic duct drains the rest

Looking for Foreign Pathogens

• Some parts of the immune system reside in the skin, bone marrow, or circulatory system
• Lymphocytes (B and T Cells) are housed, especially in lymphoid structures
• Other immune cells are in the lymphatic system
• Sentinel cells filter through lymph looking for potentially foreign, or non-self, elements
• Lymphoid tissue refers to tissues where lymphocytes (B and T cells) mature and where macrophages and dendritic cells seek pathogens
• Several organs have collections of lymphoid tissue that become lymphoid organs
• Naive B lymphocytes were first seen in the Bursa of Fabricius
• "T" Cells received their name because they are found in the thymus
• Primary lymphoid areas are organs where lymphocytes are produced and mature as the bone marrow and thymus
• Secondary lymphoid organs screen lymph: lymph nodes, spleen, MALTs and Appendix

Mucosa-Associated Lymphatic Tissues

— Mucosa-Associated Lymphatic Tissues are where Lymphatic tissue includes: tonsils, Peyer's patches in the intestine, the eyes, salivary glands, breast, and others

• Dendritic cells are found in tissues exposed to the outside: skin, digestive track, and the lungs
• Dendritic cells are sentinel immune cells and professional antigen-presenting cells
• Once a threat is discovered, dendritic cells moving into the lymphatic system and activating B and T cells that search for that exact antigen
• Macrophages are located in many tissues and are professional antigen-presenter cells (PAPC)
• Phagocytosis is used by macrophages to directly and immediately fight invaders
• Lymph nodes are repositories for immune cells that can recognize foreign threats
• The spleen acts as a giant lymph node that helps filter pathogens in the blood
• White Pulp in the spleen houses lymphocytes
• MALTs, mucosa associated lymphoid tissues, like the tonsils, have deep crypts that trap debris and bacteria
• MALTs contain lots of macrophages that digest what they catch
• Tonsil crypts trap material, where irritation that comes from this leads to calcium buildup

Help With Large Molecules

• Lymphatic vessels help with how how to get large molecules into the blood, which they can't squeeze through in capilaries