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Running Head: THE LYMPHATIC SYSTEM
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The Lymphatic System
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1. In our immune systems, the lymphatic system plays a very significant role and is made up
of:
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The lymphatic organs e.g. thymus, bone marrow, lymph nodes and spleen
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Lymphatic vessels-transport the lymph
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Lymphatic fluid/lymph
The following diagram shows the anatomical positions of the lymphatic organs in the system.
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2. Physiological functions and histology of the lymphatic organ system:
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Thymus-a primary epithelial organ in the lymphatic system where the maturation of T
cells occurs. The major lymphocytes in our adaptive immune system are T
lymphocytes/cells. Smaller quantities of other cells like plasma cells and B cells can also
be found here. A lack or loss of the thymus organ would lead to an increased
susceptibility to infections and in turn immunodeficiency. The septum which is made up
of epithelial cells divides the lobules in the thymus while the isthmus divides the two
identical lobes. These lobes can each be subdivided into a peripheral cortex and a central
medulla, which play a role in T cells development. Earlier thymocytes developments like
positive selection and gene rearrangement occur in the cortex while the later stages like
extensive negative selection occur in the medulla. Cells found in the thymus are the
hematopoietic cells (ones whose genesis is the bone marrow thymocytes i.e. developing T
cells) and stromal cells like the epithelial cells in the medulla and cortex. Stromal cells
are critical in acquisition of T cell tolerance.
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Spleen-synthesis of blood cells and antibodies (to fight antigens), the recycling of old red
blood cells as well the filtering of the blood(by removing blood cells and bacteria that
have been coated by antibodies) occurs here. The spleen also acts as storage for platelets,
lymphocytes and other blood cells that can be needed in case of emergency situations.
For the first five months of prenatal development, the spleen also aids in the creation of
red blood cells. Found between the diaphragm and the stomach, the spleen is split into the
red pulp and the white pulp. The white pulp has macrophages and both B and T cells
around it in aggregations while the vascular red pulp has sinusoids (a leaky capillary) and
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parenchyma. Thanks to the slit epithelial cells lining the spleen, blood is filtered when
passing through allowing old or damaged blood vessels to undergo phagocytosis in the
red pulp. Trabeculae act as the connective tissues in the spleen and support it, together
with a dense capsule, as well as take blood vessels (Saladin, 1998).
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Lymph nodes-These are stopover stations for the lymph that are distributed across the
body. The lymph passes through lymph nodes before going into the blood circulation. A
node filters debris and foreign particles from the lymph to aid in defense against
infections and microorganisms. They are bean/kidney like in shape, covered by dense
connective tissue that form a fibrous capsule and are connected through lymphatic
vessels. These capsules have extensions known as trabeculae that support the blood
vessel supply to the node. Each node also has a medulla (inner) and cortex (outer). The
medulla has blood vessels, medullary cords (containing macrophages, B cells and plasma
cells) and sinuses that secrete plasma cells activated by antigens. Afferent lymphatic
vessels allow lymph into the node. Elastin and reticular fibers that are thin form a
reticular network that will allow adherence by macrophages, lymphocytes and dendritic
cells to the surface and provide structural support for the node. The venules around this
area are endothelial and therefore allow exchange of blood. From the sub-capsular
sinuses through to the sinuses in the trabeculae and cortex, filtered lymph is drained so
that it can exit the node and get back into blood circulation through medullary sinuses.
Vessels in the nodes are small and do not allow macrophages to pass through hence
trapping them to function within the node. The outer cortex has follicles that contain B
cells and can become germinal centers in presence of antigen while the paracortex has
accessory cells and most T cells (Rouvière & Tobias, 1938).
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Lymphatic vessels-these are structures that have valves and thin walls and allow the
transport of the lymph. Afferent lymph vessels carry lymph to the node while those that
carry lymph from the node are known as efferent lymph vessels. They contain smooth
muscles, adventitia (attach vessels with surrounding tissue) and endothelial cells.
Lymphatic capillaries are much smaller and lack the adventitia as well as the smooth
muscle layer. Smaller or initial lymphatics collect lymph into the nodes while larger
lymphatics help in propelling the lymph through peristalsis (with the relaxation and
contraction of the smooth muscles), skeletal muscle contraction, arterial pulsation and the
valve system. This makes it a closed system without a centralized pumping organ like in
the cardiovascular system. Lymph is drained through lymph ducts into subclavian veins
so as to return into the general circulation (Swartz, 2001).
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Lymph-this is the fluid circulating in the lymphatic system. Lymph capillaries collect
interstitial fluid, vessels transport it into the lymph nodes and after filtration it rejoins the
blood circulation through subclavian veins. Lymph has components like that of blood
plasma but it also has white blood cells. The fluid can transport bacteria to the nodes,
excess interstitial fluid and proteins back into blood circulation, metastatic cancer cells
and fats (originating from the digestive system in form of chyle).
3. Diseases associated with the lymphatic system
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Lymphadenopathy-occurs due to enlargement of lymph nodes.
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Castleman disease-inflammatory disorders that may cause node enlargement.
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Lymphoma-cancer affecting the lymph nodes and can be Hodgkin or Non
Hodgkin.
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Lymphedema-when lymph node blockage causes swelling.
Fun facts about the lymphatic system:
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It is believed that massage can help improve the flow of the lymph through manual
lymph drainage e.g. when there is a broken bones that has caused swelling.
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Our bodies collect up to 2.5 liters of lymph every day.
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Lymph flows upwards towards the neck and not in a loop like the cardiovascular system.
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Your heartbeat and muscle movement aids in the smooth flow of the lymph.
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References
Rouvière, H., & Tobias, M. J. (1938). Anatomy of the human lymphatic system. Edwards
Brothers, Incorporated.
Saladin, K. S. (1998). Anatomy & physiology. WCB/McGraw-Hill.
Swartz, M. A. (2001). The physiology of the lymphatic system. Advanced drug delivery
reviews, 50(1), 3-20.