Lymphatic SystemLymphatic System

► Composed of lymph, lymphatic vessels, lymph Composed of lymph, lymphatic vessels, lymph nodes, tonsils, spleen, and thymusnodes, tonsils, spleen, and thymus

► Functions: Functions: ► Drain interstitial fluid from tissue spacesDrain interstitial fluid from tissue spaces► Transport fats from GI tract to bloodTransport fats from GI tract to blood► Surveillance and defense: nodes and organs Surveillance and defense: nodes and organs

house and produce lymphocytes and house and produce lymphocytes and monocytes monocytes

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►Lymphatic trunks: drain lymph rom Lymphatic trunks: drain lymph rom large regions of the body and are large regions of the body and are named for the regions they servenamed for the regions they serve

►Ex: lumbar trunk, intestinal trunk, Ex: lumbar trunk, intestinal trunk, bronchomediastinal trunk, subclavian bronchomediastinal trunk, subclavian trunk, jugular trunktrunk, jugular trunk

►Merges to form either the thoracic Merges to form either the thoracic duct or right lymphatic ductduct or right lymphatic duct

►Lymphatic capillaries:Lymphatic capillaries: microscopic microscopic closed end tubes with walls composed closed end tubes with walls composed of one layer of simple squamous of one layer of simple squamous epitheliumepithelium

►Lymphatic vesselsLymphatic vessels: formed by the : formed by the merging of lymphatic capillaries. merging of lymphatic capillaries. Similar to veins because they have Similar to veins because they have valves and wall composed of three valves and wall composed of three layers, but lymph vessels have more layers, but lymph vessels have more valves and thinner wallsvalves and thinner walls

Lymph CirculationLymph Circulation

►The lymphatic system begins as The lymphatic system begins as lymphatic capillaries merge to form lymphatic capillaries merge to form larger lymphatic vessels, which merge larger lymphatic vessels, which merge to form lymphatic trunks, which merge to form lymphatic trunks, which merge to form lymphatic ducts which empty to form lymphatic ducts which empty into the subclavian veininto the subclavian vein

► Filtration forces water and dissolved Filtration forces water and dissolved substances from the capillaries into the substances from the capillaries into the interstitial fluid. interstitial fluid.

► Not all of this water is returned to the blood Not all of this water is returned to the blood by osmosis, and excess fluid is picked up by by osmosis, and excess fluid is picked up by lymph capillaries to become lymph. lymph capillaries to become lymph.

► From lymph capillaries fluid flows into lymph From lymph capillaries fluid flows into lymph veins (lymphatic vessels) which virtually veins (lymphatic vessels) which virtually parallel the circulatory veins and are parallel the circulatory veins and are structurally very similar to them, including structurally very similar to them, including the presence of semilunar valves.the presence of semilunar valves.

► The lymphatic veins flow into one of two The lymphatic veins flow into one of two lymph ducts. lymph ducts.

► The right lymph duct drains the right arm, The right lymph duct drains the right arm, shoulder area, and the right side of the head shoulder area, and the right side of the head and neck. and neck.

► The left lymph duct, or thoracic duct, drains The left lymph duct, or thoracic duct, drains everything else, including the legs, GI tract everything else, including the legs, GI tract and other abdominal organs, thoracic and other abdominal organs, thoracic organs, and the left side of the head and organs, and the left side of the head and neck and left arm and shoulder. neck and left arm and shoulder.

► These ducts then drain into the subclavian These ducts then drain into the subclavian veins on each side where they join the veins on each side where they join the internal jugular veins to form the internal jugular veins to form the brachiocephalic veins. brachiocephalic veins.

►Lymph nodes lie along the lymph veins Lymph nodes lie along the lymph veins successively filtering lymph. Afferent successively filtering lymph. Afferent lymph veins enter each node, efferent lymph veins enter each node, efferent veins lead to the next node becoming veins lead to the next node becoming afferent veins upon reaching it.afferent veins upon reaching it.

Lymphokinetic motion (flow of Lymphokinetic motion (flow of the lymph)the lymph)

► Is due to: 1) Lymph flows down the Is due to: 1) Lymph flows down the pressure gradient. pressure gradient.

► Muscular and respiratory pumps push lymph forward due to function of the semilunar valves

                                                                

Other lymphoid Other lymphoid tissue: tissue:       

1. 1. Lymph nodes:Lymph nodes:

Lymph nodes are small encapsulated organs Lymph nodes are small encapsulated organs located along the pathway of lymphatic vessels. located along the pathway of lymphatic vessels.

They vary from about 1 mm to 1 to 2 cm in They vary from about 1 mm to 1 to 2 cm in diameter and are widely distributed throughout diameter and are widely distributed throughout the body, with large concentrations occurring in the body, with large concentrations occurring in the areas of convergence of lymph vessels. the areas of convergence of lymph vessels.

They serve as filters through which lymph They serve as filters through which lymph percolates on its way to the blood.percolates on its way to the blood.

Antigen-activated lymphocytes differentiate and Antigen-activated lymphocytes differentiate and proliferate by cloning in the lymph nodesproliferate by cloning in the lymph nodes

2. 2. Diffuse Lymphatic Tissue and Diffuse Lymphatic Tissue and Lymphatic nodules:Lymphatic nodules:

►The alimentary canal, respiratory passages, and genitourinary tract are guarded by accumulations of lymphatic tissue that are not enclosed by a capsule (i.e. they are diffuse) and are found in  connective tissue beneath the epithelial mucosa.

►These cells intercept foreign antigens and then travel to lymph nodes to undergo differentiation and proliferation.

► Local concentrations of lymphocytes in these systems and other areas are called lymphatic nodules.

►In general these are single and random but are more concentrated in the GI tract in the ileum, appendix, cecum, and tonsils.

► These are collectively called the Gut Associated Lymphatic Tissue (GALT). MALT (Mucosa Associated Lymphatic Tissue) includes these plus the diffuse lymph tissue in the respiratory tract. 

  

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3. 3. The thymusThe thymus

► The thymus is where immature lymphocytes The thymus is where immature lymphocytes differentiate into T-lymphocytes. differentiate into T-lymphocytes.

► The thymus is fully formed and functional at The thymus is fully formed and functional at birth. birth.

► Characteristic features of thymic structure Characteristic features of thymic structure persist until about puberty, when persist until about puberty, when lymphocyte processing and proliferation are lymphocyte processing and proliferation are dramatically reduced and eventually dramatically reduced and eventually eliminated and the thymic tissue is largely eliminated and the thymic tissue is largely replaced by adipose tissue. replaced by adipose tissue.

►The lymphocytes released by the The lymphocytes released by the thymus are carried to lymph nodes, thymus are carried to lymph nodes, spleen, and other lymphatic tissue spleen, and other lymphatic tissue where they form colonies.where they form colonies.

► These colonies form the basis of T-These colonies form the basis of T-lymphocyte proliferation in the specific lymphocyte proliferation in the specific immune response. T-lymphocytes immune response. T-lymphocytes survive for long periods and recirculate survive for long periods and recirculate through lymphatic tissues.through lymphatic tissues.

► The transformation of primitive or immature lymphocytes into T-lymphocytes and their proliferation in the lymph nodes is promoted by a thymic hormone called thymosin. 

► Ocassionally the thymus persists and may become cancerous after puberty and and the continued secretion of thymosin and the production of abnormal T-cells may contribute to some autoimmune disorders. 

► Conversely, lack of thymosin may also allow inadequate immunologic surveillance and thymosin has been used experimentally to stimulate T-lymphocyte proliferation to fight lymphoma and other cancers. 

4. 4. The spleen:The spleen:► The spleen filters the blood and reacts The spleen filters the blood and reacts

immunologically to blood-borne antigens.immunologically to blood-borne antigens.

This is both a morphologic (physical) and This is both a morphologic (physical) and physiologic process. physiologic process.

In addition to large numbers of lymphocytes the In addition to large numbers of lymphocytes the spleen contains specialized vascular spaces, a spleen contains specialized vascular spaces, a meshwork of reticular cells and fibers, and a rich meshwork of reticular cells and fibers, and a rich supply of macrophages which monitor the blood. supply of macrophages which monitor the blood. 

Connective tissue forms a capsule and trabeculae Connective tissue forms a capsule and trabeculae which contain myofibroblasts, which are contractile. which contain myofibroblasts, which are contractile. 

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► The human spleen holds relatively little blood The human spleen holds relatively little blood compared to other mammals, but it has the compared to other mammals, but it has the capacity for contraction to release this blood capacity for contraction to release this blood into the circulation during anoxic stress. into the circulation during anoxic stress.

► White pulp in the spleen contains lymphocytes White pulp in the spleen contains lymphocytes and is equivalent to other lymph tissue,  while and is equivalent to other lymph tissue,  while red pulp contains large numbers of red blood red pulp contains large numbers of red blood cells that it filters and degrades. cells that it filters and degrades.

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Spleen functionsSpleen functions

► The spleen functions in bothThe spleen functions in both immune and immune and hematopoietic systems. hematopoietic systems.

► Immune functionsImmune functions include: proliferation of include: proliferation of lymphocytes, production of antibodies, removal of lymphocytes, production of antibodies, removal of antigens from the blood. antigens from the blood.

► Hematopoietic functionsHematopoietic functions include: formation of include: formation of blood cells during fetal life, removal and destruction blood cells during fetal life, removal and destruction of aged, damaged and abnormal red cells and of aged, damaged and abnormal red cells and platelets, retrieval of iron from hemoglobin platelets, retrieval of iron from hemoglobin degradation, storage of red blood cells. degradation, storage of red blood cells.