Note: this page is adapted from the Wikipedia article at http://en.wikipedia.org/wiki/Lymphatic_system
The lymphatic system is part of the circulatory system; its job is to carry a clear fluid called lymph (from Latin lympha, “water”) toward the heart.
This system provides an accessory return route to the blood for the approximately 3 liters of plasma per day that do not get reabsorbed into blood vessels (out of 20 liters per day circulated through the body’s capillary filtration system)
It also aids in the defense of the immune system. Lymph carries lymphocytes (a subset of white blood cells); it also contains waste products and debris of cells, as well as bacteria and protein.
There are three major components to the lymphatic system:
- Lymphatic organs
- A conducting network of lymphatic vessels
- The lymph fluid itself
Organs such as the spleen, thymus, and tonsils are part of the lymphatic system. The lymphatic system also includes the bone marrow, and the lymphoid tissue associated with the digestive system. Bone marrow and the thymus are the two primary lymphoid organs involved in the production of lymphocyte tissues.
Bone marrow both creates T cells (a subset of white blood cells that mature in the thymus) and produces and matures B cells (white blood cells that make antibodies against antigens).
The central or primary lymphoid organs generate lymphocytes from immature progenitor cells (similar to stem cells). The thymus is the main organ in this category.
Secondary or peripheral lymphoid organs maintain mature but naive lymphocytes, and initiate an adaptive immune response when required. These organs include the spleen, the lymph nodes, tonsils, and adenoids.
The tertiary lymphoid tissue assumes an immune role only when challenged with antigens that result in inflammation. In these cases it imports lymphocytes from blood and lymph.
The spleen creates red blood cells during the first five months of prenatal development, after which time that function moves to the bone marrow. The spleen however retains the ability to produce lymphocytes, and it stores red blood cells as well. It can store enough blood cells to help in an emergency, and up to 25% of lymphocytes can be stored at any one time.
A lymph node is an organized collection of lymphoid tissue, through which the lymph passes on its way back to the blood. These are located at various points along the lymphatic system, and there are between 500-600 such nodes in the body. Many of these are clustered under the arms and in the abdomen. These nodes are major sites of B cells, T cells, and other immune cells.
Other Lymphoid Tissue
Consisting of connective tissue with various lymphocytes and other white blood cells, lymphoid tissue is concerned with immune functions related to defending the body against infections and the spread of tumors.
Lymphatic vessels carry lymph between different parts of the body. They transport lymph back to the blood, replacing that 3 liters of plasma that does not get returned to the blood through capillaries.
The lymphatic vessels maintain the balance of the body fluids by draining and transporting fluid back to the cardiovascular system, along with proteins and antigens.
The lymphatic system performs many interrelated functions.
- It removes interstitial fluid (fluid that provides nutrients to cells and allows waste removal) from tissues
- It absorbs and transports fatty acids from the digestive system
- It transports white blood cells to and from the lymph nodes and into the bones
- It transports antigen-presenting cells to the lymph nodes where an immune response has been stimulated
Most nutrients are absorbed by the small intestine and passed to the liver; however, fats are passed on to the lymphatic system to be transported into the blood circulation.
The lymphatic system plays a major role in the spread of cancer (metastasis): it is responsible for carrying cancerous cells between various parts of the body.
Lymphadenopathy is one or more enlarged lymph nodes. This may be caused by infections such as mononucleosis, tuberculosis, and HIV, as well as rheumatoid arthritis and cancer.
Lymphedema is swelling due to the accumulation of lymph.
Various cancers can occur in the lymphatic system. These include lymphoma, lymphangiosarcoma (a malignant soft tissue tumor), lymphangioma, and Lymphangioleiomyomatosis (benign tumors).
Hippocrates in the 5th century BC was one of the earliest people to mention the lymph system. Herophilos in the 3rd century BC and Rufus of Ephesus in the 2nd century AD made further studies into the lymph system, as well as the Greek physician Galen.
The 1500s and 1600s saw further inquiries into the functions of the lymph system. In 1628, William Harvey speculated that blood circulates through the body rather than being produced anew by the liver and heart. Olaus Rudbeck in 1652 discovered that the liver contained lymphatic vessels, and Thomas Bartholin found such vessels throughout the entire body.
Finally, Alexander Monro in the late 1700s was the first to describe the lymphatic system in detail.