Anatomy and Physiology of the Lymphatic System

                TL:DR: The components of the Lymphatic system consist of lymphatic collectors, vessels and ducts, lymph nodes, peyer’s patches, tonsils, adenoids, spleen, thymus and the appendix.  All of these have some impact on the lymphatic system and the production of lymph fluid, which in itself may also be considered a part of lymphatic anatomy.  Lymphatic collectors, vessels and ducts are responsible for the collection of lymph fluid into the vascular system that is responsible for transporting it back into the circulatory system.  The Lymphatic vessels create a one-directional flow of lymphatic fluid that is returned to the circulatory system through the lymphatic ducts near the heart.  The lymph nodes process and filter lymph fluid and are, in part, responsible for immune response to disease, which accounts for the volume of white blood cells within the lymph nodes.  The spleen is responsible for filtering and storing blood and creating white blood cells.  The Thymus is responsible for the maturation process of certain types of white blood cells.  The tonsils and Adenoids are responsible for defense against harmful pathogens received through the oral and nasal passages.  Peyer’s patches are responsible for eliminating harmful bacteria in the small intestine.  And lastly, the appendix is responsible for mediating the destruction of bacteria in the intestine, though its full purpose is not believed to be completely understood at this time.

 

Lymph collectors, vessels and ducts

                Lymph collectors are filled by the lymph capillaries.  The lymph capillaries are essentially made of a form of skin cell with a valve between it and the lymph collectors.  Typically, the lymph capillary is compressed flat.  However, as the tissues that surround the lymph capillary swell with fluid, the capillary is pulled open and it fills with the surrounding fluid, thus draining the tissue of the fluid.  Once the outside pressure, that pulled the capillary open, has dropped, the lymph capillary closes itself off, the valve between the capillary and the lymph collector then opens, and the lymph fluid is drained out of the capillary.  After this, the process repeats itself.

                The Lymph collectors transport the fluid to deeper vessels.  These collectors do not close off like the capillaries, though they have their own valves to prevent the fluid from moving backward in the system. The same is true of the deeper lymph vessels.  Each valve segment is known as a lymphangion.  These valve segments consist of skin cells as the inner layer, smooth muscle as the middle layer and connective tissue as the outer layer.  Each lymphangion is responsible for the movement of the lymph that enters its segment.  Unlike the circulatory system, there is no pressure from behind pushing lymph forward continuously.  When lymph fluid fills a valve segment, the valve closes off and the lymph pump activates, pushing the fluid from one valve segment to the next by the contraction of the smooth muscle fibers.  This pumping action can be assisted by surrounding skeletal muscles and by mechanical pressure, such as massaging.  The lymph collectors and deep collectors pump the lymphatic fluid to lymph ducts which are responsible for returning the fluid to the circulatory system.  The primary duct responsible for most of the body’s lymphatic return is the Thoracic Duct, which drains into the left subclavian vein, near the heart.

Lymph Nodes

                There are around 600-700 lymph nodes throughout the human body.  They typically are found in clusters in certain regions, such as in the groin or armpit.  The size and number of lymph nodes found in a cluster will vary, with the size of lymph nodes increasing as the number decreases.  Lymph nodes are kidney shaped and are responsible for filtering out bacteria and dead cells, producing white blood cells and returning water to the circulatory system.  Because of how lymph nodes work, there are more lymph vessels entering a lymph node than leaving a lymph node, and the amount of lymph fluid entering a lymph node is greater than the amount leaving.  Lymph nodes in a cluster are also chained together, so lymph fluid is filtered and concentrated more and more the further it passes through the lymphatic system.  The first cluster of nodes that lymph fluid enters are known as the primary lymph nodes.

 

Mucosa-Associated Lymphoid Tissue (MALT), and Gut-Associated Lymphatic Tissue

                This type of tissue is not specific organs or vessels, but rather it is epithelial tissue that is dense with lymphatic follicles that are capable of monitoring the surface with which they are in contact.  This type of tissue can be found in various locations throughout the body, but the most commonly known structures associated with this type of tissue would be the tonsils and adenoids.  Similar features found in the gut are called Peyer’s patches.

                The MALT found in the nasopharyngeal (nose and throat) portion of the respiratory tract are located at the surface of the tonsils and the adenoids.  This tissue is capable of monitoring the air that we breath and the food that we eat and mounting an immune response to it.  As we eat and breath, food and air pass over these surfaces (food and air for the tonsils, air for the adenoids).  This tissue begins to absorb nutrients and samples the material and checks it for bacteria and viruses.  If there are infective cells then the immune system response can begin immediately and locally.

                This same process is present in the gut as well.  In the walls of the small intestine there is GALT that monitors the digesting food that passes through.  This tissue also absorbs nutrients, monitoring it for infective material, and can mount an immune response, if necessary.  This type of tissue is called a peyer’s patch.

 

Lymphatic Organs

                The primary lymphatic organs consist of Bone Marrow and the Thymus.  These two are considered the primary lymphatic organs because they are the source of development and proliferation of lymphocytes.

                All lymphocytes (white blood cells, or WBC) originate in bone marrow.  Some cells leave the bone marrow early and move to the Thymus.  Other cells remain in the bone marrow to differentiate.  These are called B-lymphocytes.  After differentiation in the bone marrow, these WBCs move to lymph nodes where they will mature as they come in contact with infective cells. 

                The Thymus is responsible for proliferation and maturation of T-lymphocytes.  When differentiation of these occurs, they become antigen-specific cells.  That is, they develop the recognition of certain harmful foreign invaders and mount a quick defense against them.  Forms of these differentiated cells always remain in the body, which helps us protect against the same bacteria and virus causing illness every time we encounter it.  After these leave the Thymus, they settle in specific regions of the body to monitor for those harmful foreign invaders. 

 

Spleen and Appendix

                The spleen is the largest lymphatic organ.  It is responsible for the breakdown of old red blood cells as well as protecting against infection.  Old and new red blood cells are differentiated in the spleen.  Old cells are marked for destruction while new cells are returned to the circulatory system.  Certain cells then consume and breakdown the old red blood cells.  Any infected cells are destroyed and the lymphatic tissue in the spleen has a similar response to that of the MALT and GALT mentioned earlier. 

                The appendix helps with many functions throughout life.  It helps regulate and control several systems in fetal development and helps with immune response in adults.  Throughout adult life it appears to function similarly to peyer’s patches, as mentioned earlier.  It helps mediate immune response to harmful cells in the gut.  It also helps with maturation and differentiation of B-lymphocytes, just like lymph nodes.  It has also been discovered that it produces molecules that help direct WBCs to certain parts of the body. 

 

Conclusion

                This is not a comprehensive or exhaustive list of the anatomy and physiology of the human lymphatic system.  It is a thorough overview, but to go in any further depth at this time would be at the cost of many more readers than I have lost to this point.  Much of the details of this will put even the most avid reader right to sleep.  However, I find this material very interesting, so researching and writing about this has been very stimulating for me, and I hope for some of you as well.  The lymphatic system is very complex.  It is a body-wide system that is responsible for more than just moving lymphatic fluid from one place to another.  It is the primary reason that we are able to fight off illness.  It is the reason that we survive the flu and cold season each year and the reason why we don’t get sick with the flu and cold every year.  However, as we will discuss in the next article on edema and lymphedema, it can be harmed and it can be overwhelmed.  When these happen, we can get very sick in ways that we may have never thought.  Until next time…

 

All the Best!

 

Disclaimer: I attempted to be as thorough with my research for this topic as possible.  If I have made any mistakes or you have any questions, please feel free to contact me through my contact information located elsewhere on this website. 

 

Bibliography

1. Földi Michael, Földi Ethel, K. Strössenreuther Roman H, & Kubik, S. (2012). Lymphostatic Diseases. In Földi's textbook of lymphology: For physicians and lymphedema therapists. essay, Elsevier Urban & Fischer.