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The lymphatic system resorbs fluid and protein from tissues and extravascular spaces as part of the larger fluid homeostasis of the human body. Because of an absence of basement membrane beneath lymphatic endothelial cells, lymphatic channels have a unique permeability, allowing resorption of proteins that are too large to be resorbed by venules.
Lymphatic channels are situated in the deep dermis and subdermal tissues parallel to the veins, with valves (like veins) to ensure one-way flow.
The lymphatic system also performs an immune function. Bacteria, allergens, and cancerous cells from elsewhere in the body that have collected in the nodes stimulate lymphocyte proliferation, thereby greatly enlarging the node.
The lymphatic system and immune system circulate a fluid known as lymph throughout the body in a series of vessels that make up this system.
Lymph is made up of white blood cells (lymphocytes and macrophages) that fight infection. The fluid that is resorbed is recycled into the systemic circulation, as explained below. En route, the lymph is filtered at the lymph nodes where bacteria and other harmful substances are destroyed.
The Lymphatic Ducts
LEFT LYMPHATIC DUCT (thoracic duct):
The thoracic duct is synonymous with the left lymphatic duct.
- both lower extremities,
- the gastrointestinal tract, and
- the left upper body (upper extremity, chest wall, upper back, shoulder, and breast),
and enters the venous circulation through the thoracic duct.
Lymph drains via afferent lymphatics to regional lymph nodes and then by efferent lymphatics to the cisterna chyli (a collection sac that is the beginning of the thoracic duct) and up the thoracic duct into the subclavian vein and venous circulation.
Left Lymphatic Duct Pathway:
Lymph in extracellular fluid → afferent lymphatics → lymph nodes → efferent lymphatics → cisterna chyli → thoracic duct → through the diaphragm → left subclavian/internal jugular vein → venous return to the heart.
RIGHT LYMPHATIC DUCT:
The right thoracic duct drains the
- right thorax,
- right arm,
- head, and
and enters the venous circulation via the right lymphatic duct.
Right Lymphatic Duct Pathway:
Lymph in extracellular fluid → afferent lymphatics → lymph nodes → efferent lymphatics → right lymphatic duct → right internal jugular/right subclavian vein junction → venous return to the heart.
PRIMARY LYMPHOID TISSUES: the bone marrow (makes B-lymphocytes) and thymus (makes T-lymphocytes).
Bone marrow: makes adult B-cells after birth which migrate to secondary lymphoid tissues.
Thymus: the majority of T-cells develop in the thymus, and a specific population develops in intestinal tissue. The thymus, just above the heart, becomes smaller with maturity (by puberty) and interstitial fat replaces a lot of it.
SECONDARY LYMPHOID TISSUES: lymph nodes, spleen, tonsils, and GI and respiratory lymphoid tissue collections.
Lymph nodes: lymph nodes are little nodules of lymphoid tissue dispersed throughout the body that filter the lymph passing to them from the afferent lymphatic vessels; they add lymphocytes to the lymph, helping to defend from disease and infection.
Lymph node clusters: Clusters of lymph nodes are found in the neck, axilla (underarm), chest, abdomen, and groin.
Spleen: located in the left upper quadrant of the abdomen, it participates in cellular and humoral immunity through its lymphoid elements. It is the largest lymphopoietic organ, making up about 1/4 of the total lymphoid mass of the body. The spleen also keeps a reserve of blood in case of hemorrhaging.
Lymphocyte populations increase within the spleen, especially during acute infection when it may enlarge (swell).
- removes aged red blood cells, opsonized bacteria, antibody-coated cells, bacteria, and other particles via the monocyte-macrophage system;
- is a storage depot for 1/3 of circulating platelets.
Tonsils: Made up of the palatine tonsils and the pharyngeal tonsil (adenoid).
The palatine tonsils are any of four sets of lymphoepithelial tissue located in the throat and mouth. They are composed of lymphoid tissue similar to that of lymph nodes but are covered by the same mucosa that covers the mouth and pharynx. They are pitted (have crypts) that can collect particulate material that can contribute to their infection/enlargement. They become swollen or inflamed in the presence of certain infections.
The pharyngeal tonsil, the adenoid, is the uppermost tonsil, behind the nasal cavity.
Common infectious agents that infect the tonsils are
- group A beta-hemolytic Streptococcus and other infectious agents (e.g., diphtheria,
- Epstein-Barr virus).
Lymphangitis and Lymphadenitis
Considering the major role the lymphatic system plays in immunity and anti-infectious mechanisms, it is obvious that any infection will involve its vessels, nodes, and node clusters.
LYMPHANGITIS: Infection can result in inflammation of the lymphatic channels. Pathogens such as bacteria, mycobacteria, viruses, fungi, and even parasites can invade the lymphatic vessels after a cutaneous inoculation. From there, organisms can spread toward the regional lymph nodes.
Clinical manifestations of lymphangitis are variable but may present as erythematous streaks (red streaks) with pain and rapid spread or by nodular swellings along the course of the lymphatic vessels. These red, tender streaks extend to involve the regional lymph nodes (called lymphadenitis).
In people with normal immunity, the most common causative organism is Streptococcus pyogenes; lymphangitis can also occur from a Staphylococcus aureus infection.
In immunocompromised patients, gram-negative organisms are also important causes of lymphangitis following lower limb cellulitis.
LYMPHADENITIS: extension of lymphangitis to involve the lymph node(s), causing them to enlarge and become tender.
LYMPHEDEMA: abnormal accumulation of interstitial fluid and fibroadipose tissues resulting from injury, infection, or congenital abnormalities of the lymphatic system.
Since the lymphatic system is a series of one-way paths for lymph to follow, on its way back into the systemic circulation, any interruption of these vessels can result in extracellular fluid not being drained properly, causing edema. This can frequently be seen in surgical complications such as mastectomy where one arm may be permanently edematous.
In the Field
In the field, the most likely emergency considerations regarding the lymphatic system will probably be:
- AIRWAY OBSTRUCTION: In children, the tonsils/adenoid can be so swollen that they obstruct breathing.
Furthermore, swollen tonsils may only present as part of a larger infectious process, so besides breathing, circulatory support for consequences of sepsis or dehydration also may be necessary.
- SEPSIS: The presence of red streak(s) going up an arm or leg, indicating lymphangitis as part of a larger infectious condition, is a pre-sepsis warning. The reason this is an emergency is that, due to the destination of lymph into the systemic circulation, such streaks may be precursors of a pending sepsis.
Maintenance and support of ABC (airway, breathing, circulation) are part of the support for any child with partial airway obstruction; also, O2, emotional support for the child and parents, and rapid transport to an appropriate ER with pediatric services.
For those with overt lymphangitis/lymphadenitis--(red streak(s)/enlarged, tender lymph nodes--transport is indicated so that effective antimicrobial treatment can be started along with support for possible sepsis.