Lymphatic system
Lymphatic System: Facts, Functions & Diseases
Kim Ann Zimmermann, LiveScience Contributor | February 08, 2013 03:22pm ET
The primary function of the lymphatic system is to transport lymph, a clear, colorless fluid
containing white blood cells that helps rid the body of toxins, waste and other unwanted
materials.
Lymphatic comes from the Latin word lymphaticus, meaning "connected to water," as lymph is
clear.
The lymphatic system, which is a subset of the circulatory system, has a number of functions,
including the removal of interstitial fluid, the extracellular fluid that bathes most tissue. It also
acts as a highway, transporting white blood cells to and from the lymph nodes into the bones,
and antigen-presenting cells to the lymph nodes.
Description of the lymphatic system
The lymphatic system is a network of tissues and organs that primarily consists of lymph vessels, lymph
nodes and lymph. The tonsils, adenoids, spleen and thymus are all part of the lymphatic system.
There are 600 to 700 lymph nodes in the human body that filter the lymph before it returns to the
circulatory system.
The spleen, which is largest lymphatic organ, is located on the left side of the body just above the kidney.
Humans can live without a spleen, although people who have lost their spleen to disease or injury are
more prone to infections.
The thymus, which stores immature lymphocytes and prepares them to become active T cells, is located
in the chest just above the heart.
Tonsils are large clusters of lymphatic cells found in the pharynx. Although tonsillectomies occur much
less frequently today then they did in the 1950s, it is still among the most common operations performed
and typically follows frequent throat infections.
When bacteria are recognized in the lymph fluid, the lymph nodes make more infection-fighting white
blood cells, which can cause swelling. The swollen nodes can sometimes be felt in the neck, underarms
and groin.
Unlike blood, which flows throughout the body in a continue loop, lymph flows in only one direction —
upward toward the neck — within its own system. It flows into the venous blood stream through the
subclavien veins, which are located on either sides of the neck near the collarbones.
Plasma leaves the cells once it has delivered its nutrients and removed debris. Most of this fluid returns to
the venous circulation through the venules and continues as venous blood. The remainder becomes
lymph.
Lymph leaves the tissue and enters the lymphatic system through specialized lymphatic capillaries. About
three-quarters of these capillaries are superficial capillaries that are located near the surface of the skin.
There are also deep lymphatic capillaries that surround most of the body’s organs.
There are two drainage areas that make up the lymphatic system. The right drainage area handles the right
arm and chest. The left drainage area clears all of the other areas of the body, including both legs, the
lower trunk, the upper left portion of the chest, and the left arm.
Lymph
Lymph is a fluid derived from blood plasma. It is pushed out through the capillary wall by pressure
exerted by the heart or by osmotic pressure at the cellular level. Lymph contains nutrients, oxygen, and
hormones, as well as toxins and cellular waste products generated by the cells. As the interstitial fluid
accumulates, it is picked up and removed by lymphatic vessels that pass through lymph nodes, which
return the fluid to the venous system. As the lymph passes through the lymph nodes, lymphocytes and
monocytes enter it.
At the level of the gastrointestinal (GI) tract, lymph has a milky consistency that is attributable to fatty
acids, glycerol, and rich fat content. Lacteals are lymph vessels that transport intestinal fat and are
localized to the GI tract.[1, 5, 3]
Lymphatic vessels
Lymphatic capillaries are blind-ended tubes with thin endothelial walls (only a single cell in thickness).
They are arranged in an overlapping pattern, so that pressure from the surrounding capillary forces at
these cells allows fluid to enter the capillary (see the image below). The lymphatic capillaries coalesce to
form larger meshlike networks of tubes that are located deeper in the body; these are known as lymphatic
vessels.
Lymph capillaries in spaces. Blind-ended lymphatic capillaries arise
within interstitial spaces of cells near arterioles and venules.
The lymphatic vessels grow progressively larger and form 2 lymphatic ducts: the right lymphatic duct,
which drains the upper right quadrant, and the thoracic duct, which drains the remaining lymphatic
tributaries. Like veins, lymphatic vessels have 1-way valves to prevent any backflow (see the image
below). The pressure gradients that move lymph through the vessels come from skeletal muscle action,
smooth muscle contraction within the smooth muscle wall, and respiratory movement.[1, 6, 2, 5, 4]
Lymphatic 1-way valves.
Lymph nodes
Lymph nodes are bean-shaped structures that are widely distributed throughout the lymphatic pathway,
providing a filtration mechanism for the lymph before it rejoins the blood stream. The average human
body contains approximately 600-700 of them, predominantly concentrated in the neck, axillae, groin,
thoracic mediastinum, and mesenteries of the GI tract. Lymph nodes constitute a main line of defense by
hosting 2 types of immunoprotective cell lines, T lymphocytes and B lymphocytes.
Lymph nodes have 2 distinct regions, the cortex and the medulla. The cortex contains follicles, which are
collections of lymphocytes. At the center of the follicles is an area called germinal centers that
predominantly host B-lymphocytes while the remaining cells of the cortex are T-lymphocytes. Vessels
entering the lymph nodes are called afferent lymphatic vessels and, likewise, those exiting are called
efferent lymphatic vessels (see the image below).
Lymph node structure.
Extending from the collagenous capsule inward throughout the lymph node are connective tissue
trabeculae that incompletely divide the space into compartments. Deep in the node, in the medullary
portion, the trabeculae divide repeatedly and blend into the connective tissue of the hilum of the node.
Thus the capsule, the trabeculae, and the hilum make up the framework of the node. Within this
framework, a delicate arrangement of connective tissue forms the lymph sinuses, within which lymph and
free lymphoid elements circulate.
A subcapsular or marginal sinus exists between the capsule and the cortex of the lymph node. Lymph
passes from the subcapsular sinus into the cortical sinus toward the medulla of the lymph node. Medullary
sinuses represent a broad network of lymph channels that drain toward the hilum of the node; from there,
lymph is collected into several efferent vessels that run to other lymph nodes and eventually drain into
their respective lymphatic ducts (see the image below).[1, 6]
Lymph drainage flow; lymphatic duct anatomy.
Thymus
The thymus is a bilobed lymphoid organ located in the superior mediastinum of the thorax, posterior to
the sternum. After puberty, it begins to decrease in size; it is small and fatty in adults after degeneration.
The primary function of the thymus is the processing and maturation of T lymphocytes. While in the
thymus, T lymphocytes do not respond to pathogens and foreign organisms. After maturation, they enter
the blood and go to other lymphatic organs, where they help provide defense. Structurally, the thymus is
similar to the spleen and lymph nodes, with numerous lobules and cortical and medullary elements. It also
produces thymosin, a hormone that helps stimulate maturation of T lymphocytes in other lymphatic
organs.[2, 5, 3, 4]
Spleen
The spleen, the largest lymphatic organ, is a convex lymphoid structure located below the diaphragm and
behind the stomach. It is surrounded by a connective tissue capsule that extends inward to divide the
organ into lobules consisting of cells, small blood vessels, and 2 types of tissue known as red and white
pulp. Red pulp consists of venous sinuses filled with blood and cords of lymphocytes and macrophages;
white pulp is lymphatic tissue consisting of lymphocytes around the arteries. Lymphocytes are densely
packed within the cortex of the spleen.
The spleen filters blood in much the same way that lymph nodes filter lymph. Lymphocytes in the spleen
react to pathogens in the blood and attempt to destroy them. Macrophages then engulf and phagocytose
damaged cells and cellular debris. The spleen, along with the liver, eradicates damaged and old
erythrocytes from the blood circulation. Like other lymphatic tissue, it produces lymphocytes in an
immunologic response to offending pathogens.[5, 3, 4]
Therefore, the spleen conducts several important functions, as follows:
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It serves as a reservoir of lymphocytes for the body
It filters blood
It plays an important role in red blood cell and iron metabolism through macrophage phagocytosis of
old and damaged red blood cells
 It recycles iron by sending it to the liver
 It serves as a storage reservoir for blood
 It contains T lymphocytes and B lymphocytes for immunologic response
Tonsils
Tonsils are aggregates of lymph node tissue located under the epithelial lining of the oral and pharyngeal
areas. The main areas are the palatine tonsils (on the sides of the oropharynx), the pharyngeal tonsils (on
the roof of the nasopharynx; also known as adenoids), and the lingual tonsils (on the base of the posterior
surface of the tongue).
Because these tonsils are so closely related to the oral and pharyngeal airways, they may interfere with
breathing when they become enlarged. The predominance of lymphocytes and macrophages in these
tonsillar tissues offers protection against harmful pathogens and substances that may enter through the
oral cavity or airway.[1, 2]
Diseases of the lymphatic system include lymphedema, lymphoma, lymphadenopathy, lymphadenitis,
filariasis, splenomegaly, and tonsillitis.
Lymphedema
Lymphedema results when the lymphatic system cannot adequately drain lymph, resulting in an
accumulation of fluid that causes swelling. It may be either primary or secondary.

Primary lymphedema is an inherited condition that occurs as a result of impaired or missing lymphatic
vessels; it may be present at birth, may develop with the onset of puberty, or may occur in adulthood,
with no apparent causes
 Secondary lymphedema is basically acquired regional lymphatic insufficiency, which may occur as a
consequence of any trauma, infection, or surgical procedure that disrupts the lymphatic vessels or
results in the loss of lymph nodes
Treatment consists of compression bandages or pneumatic stockings to alleviate the swelling after
appropriate diagnosis is made.[7, 4]
Lymphoma
Lymphoma is a medical term used for a group of cancers that originate in the lymphatic system.
Lymphomas usually begin with malignant transformation of the lymphocytes in lymph nodes or bunches
of lymphatic tissue in organs like the stomach or intestines. Hodgkin lymphoma and non-Hodgkin
lymphoma are the 2 major categories of lymphoma, characterized by enlargement of lymph nodes,
usually present in the neck. Symptoms of lymphoma include chronic fatigue, weak immune function,
weight loss, and night sweats.[4]
Lymphadenopathy
Lymphadenopathy is a lymphatic disorder in which the lymph nodes become swollen or enlarged as a
consequence of an infection. For example, swollen lymph nodes in the neck may occur as a result of a
throat infection or sinus infection.[3]
Lymphadenitis
Lymphadenitis is an inflammation of the lymph node that is due to a bacterial infection of the tissue in the
node, which causes swelling, reddening, and tenderness of the skin overlying the lymph node.[3]
Filariasis
Filariasis is a lymphatic system disorder that results from a parasitic infection that causes lymphatic
insufficiency.[3]
Splenomegaly
Splenomegaly, or enlarged spleen, is a lymphatic system disorder that develops as a result of a viral
infection, such as mononucleosis.
Tonsillitis
Tonsillitis is caused by an infection of the tonsils (the lymphoid tissues present in the back of the oral
cavity). The tonsils help filter out bacteria; when infected, they become swollen and inflamed, leading to
sore throat, fever, and difficulty and pain while swallowing.[3]
FUNCTIONS of the LYMPHATIC SYSTEM
The main functions of the lymphatic system are as follows:
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the main function of the lymphatic system is to collect and transport tissue
fluids from the intercellular spaces in all the tissues of the body, back to the
veins in the blood system;
it plays an important role in returning plasma proteins to the bloodstream;
digested fats are absorbed and then transported from the villi in the small
intestine to the bloodstream via the lacteals and lymph vessels.
new lymphocytes are manufactured in the lymph nodes;
antibodies and anti (manufactures in the lymph nodes) assist the body to
build up an effective immunity to infectious diseases;
lymph nodes play an important role in the defence mechanism of the body.
They filter out micro-organisms (such as bacteria) and foreign substances such
as toxins, etc.
it transports large molecular compounds (such as enzymes and hormones)
from their manufactured sites to the bloodstream.