Lymphatic system
The lymphatic system and the
immune system are terms that are used interchangeably to refer to the body's ability to defend against
pathogens.The lymphatic system is comprised of three interrelated functions: (1) Removal of excess fluids, lymph, from body tissues, (2) Absorption of fatty acids and subsequent transport of fat,
chyle, to the circulatory system and (3) Formation of white blood cells (WBCs), and initiation of
immunity through the formation of
antibodies, lending specific resistance to
pathogens.
Lymph originates as
blood plasma that leaks from the capillaries of the
circulatory system, becoming
interstitial fluid, filling the space between individual cells of tissue. Plasma is forced out of the capillaries by
hydrostatic pressure, and as it mixes with the
interstitial fluid, the volume of fluid accumulates slowly. Most of the fluid is returned to the capillaries by
osmosis. The proportion of interstitial fluid that is returned to the circulatory system by osmosis is about 90% of the former plasma, with about 10% accumulating as overfill. The excess interstitial fluid is collected by the
lymphatic system by
diffusion into lymph
capillaries, and is processed by
lymph nodes prior to being returned to the circulatory system. Once within the lymphatic system the fluid is called
lymph, and has almost the same composition as the original
interstitial fluid.
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The human lymphatic system |
The lymphatic system acts as a secondary circulatory system, except it collaborates with white blood cells in lymph nodes to protect the body from being infected by cancer cells, fungi, viruses or bacteria. Unlike the circulatory system, the lymphatic system is not closed and has no central pump; the lymph moves slowly and under low pressure due to
peristalsis, the operation of
semilunar valves in the lymph veins, and the milking action of skeletal muscles. Like
veins, lymph vessels have one-way, semilunar valves and depend mainly on the movement of skeletal muscles to squeeze fluid through them. Rhythmic contraction of the vessel walls may also help draw fluid into the lymphatic
capillaries. This fluid is then transported to progressively larger lymphatic vessels culminating in the right lymphatic duct (for lymph from the right upper body) and the
thoracic duct (for the rest of the body); these ducts drain into the circulatory system at the right and left
subclavian veins.
Lymph vessels, called
lacteals, are present in the lining of the
gastrointestinal tract. While most other nutrients absorbed by the
small intestine are passed on to the portal venous system to drain, via the
portal vein, into the
liver for processing, fats are passed on to the lymphatic system, to be transported to the blood circulation via the
thoracic duct. The enriched lymph originating in the lymphatics of the
small intestine is called
chyle (not
chyme). The nutrients that are released to the circulatory system are processed by the
liver, having passed through the systemic circulation. The lymph system is a one-way system, transporting interstitial fluid back to blood.
The
thymus,
spleen,
lymph nodes,
peyer's patches,
tonsils,
vermiform appendix, and red
bone marrow are accessory lymphoid tissues that comprise the lymphoid organs. These organs contain a scaffolding that support circulating B- and T-lymphocytes and other immune cells like
macrophages and
dendritic cells. Another sub-component of the lymphatic system is the
reticuloendothelial system. When micro-organisms invade the body or the body encounters other
antigens (such as
pollen), the antigens are transported from the tissue to the lymph. The lymph is carried in the lymph vessels to regional lymph nodes. In the lymph nodes, the macrophages and dendritic cells
phagocytose the antigens, process them, and present the antigens to
lymphocytes, which can then start producing antibodies or serve as memory cells to recognize the antigens again in the future.
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Lymphatic System*
The Lymphatic System (MedicalEngineer.co.uk)
*
Lymphatic System Overview (innerbody.com)
*
Thomas Bartholin and
Olaus Rudbeckius, the discoverers of the lymphatic system in humans.
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History of anatomy in the 17th and 18th centuries *
Lymph Node*
Lymphedema, a condition of localized fluid retention caused by a compromised lymphatic system
*
American Society of Lymphology
