The lymphatic system has three primary functions. First of all, it returns excess interstitial fluid to the blood. Of the fluid that
leaves the capillary, about 90 percent is returned. The 10 percent that does not return becomes part of the interstitial fluid that surrounds the tissue cells. Small protein molecules may "leak" through the capillary wall and increase the osmotic pressure of the interstitial fluid. This further inhibits the return of fluid into the capillaries, and fluid tends to accumulate in the tissue spaces. If this continues, blood volume and blood pressure decrease significantly and the volume of tissue fluid increases, which results in edema (swelling). Lymph capillaries pick up the excess interstitial fluid and proteins and return them to the venous blood. After the fluid enters the lymph capillaries, it is called lymph.
The second function of the lymphatic system is the absorption of fats and fat-soluble vitamins from the digestive system and the subsequent transport of these substances to the venous circulation. The mucosa that lines the small intestine is covered with fingerlike projections called villi. There are blood capillaries and special lymph capillaries, called lacteals, in the center of each villus. The blood capillaries absorb most nutrients, but the fats and fat-soluble vitamins are absorbed by the lacteals. The lymph in the lacteals has a milky appearance due to its high fat content and is called chyle.
The third and probably most well known function of the lymphatic system is defense against invading microorganisms and disease. Lymph nodes and other lymphatic organs filter the lymph to remove microorganisms and other foreign particles. Lymphatic organs contain lymphocytes that destroy invading organisms.
The lymphatic system consists of a fluid (lymph), vessels that transport the
lymph, and organs that contain lymphoid tissue.
Lymph
Lymph is a fluid similar in composition to blood plasma. It is derived from
blood plasma as fluids pass through capillary walls at the arterial end. As
the interstitial fluid begins to accumulate, it is picked up and removed by
tiny lymphatic vessels and returned to the blood. As soon as the interstitial
fluid enters the lymph capillaries, it is called lymph. Returning the fluid to
the blood prevents edema and helps to maintain normal blood volume and
pressure.
Lymphatic Vessels
Lymphatic vessels, unlike blood vessels, only carry fluid away from the
tissues. The smallest lymphatic vessels are the lymph capillaries, which begin
in the tissue spaces as blind-ended sacs. Lymph capillaries are found in all
regions of the body except the bone marrow, central nervous system, and
tissues, such as the epidermis, that lack blood vessels. The wall of the lymph
capillary is composed of endothelium
in which the simple squamous cells overlap to form a simple one-way valve.
This arrangement permits fluid to enter the capillary but prevents lymph from
leaving the vessel.
The microscopic lymph capillaries merge to form lymphatic vessels. Small
lymphatic vessels join to form larger tributaries, called lymphatic trunks,
which drain large regions. Lymphatic trunks merge until the lymph enters the
two lymphatic ducts. The right lymphatic duct drains lymph from the upper
right quadrant of the body. The thoracic duct drains all the rest.
Like veins, the lymphatic tributaries have thin walls and have valves to
prevent backflow of blood. There is no pump in the lymphatic system like the
heart in the cardiovascular system. The pressure gradients to move lymph
through the vessels come from the skeletal muscle action, respiratory
movement, and contraction of smooth muscle in vessel walls.
Lymphatic Organs
Lymphatic organs are characterized by clusters of lymphocytes and other
cells, such as macrophages, enmeshed in a framework of short, branching
connective tissue fibers. The lymphocytes originate in the red bone marrow
with other types of blood cells and are carried in the blood from the bone
marrow to the lymphatic organs. When the body is exposed to microorganisms and
other foreign substances, the lymphocytes proliferate within the lymphatic
organs and are sent in the blood to the site of the invasion. This is part of
the immune response that attempts to destroy the invading agent. The four
types of lymphatic organs are described below.
Lymph Nodes
Lymph nodes are small bean-shaped structures that are usually less
than 2.5 cm in length. They are widely distributed throughout the
body along the lymphatic pathways where they filter the lymph before
it is returned to the blood. Lymph nodes are not present in the
central nervous system. There are three superficial regions on each
side of the body where lymph nodes tend to cluster. These areas are the inguinal nodes in the groin, the axillary
nodes in the armpit, and the cervical nodes in the neck.
The typical lymph node is surrounded by a connective tissue capsule and
divided into compartments called lymph nodules. The lymph nodules are dense
masses of lymphocytes
and macrophages and are separated by spaces called lymph sinuses. Several
afferent lymphatic vessels, which carry lymph into the node, enter the node on
the convex side. The lymph moves through the lymph sinuses and enters an
efferent lymphatic vessel, which carries the lymph away from the node. Because
there are more afferent vessels than efferent vessels, the passage of lymph
through the sinuses is slowed down, which allow time for the cleansing
process. The efferent vessel leaves the node at an indented region called the hilum.
Tonsils
Tonsils are clusters of lymphatic tissue just under the mucous membranes that
line the nose, mouth, and throat (pharynx). There are three groups of tonsils.
The pharyngeal tonsils are located near the opening of the nasal cavity into
the pharynx. When these tonsils become enlarged they may interfere with
breathing and are called adenoids. The palatine tonsils are the
ones that are located near the opening of the oral cavity into the
pharynx. Lingual tonsils are located on the posterior surface of the
tongue, which also places them near the opening of the oral cavity
into the pharynx. Lymphocytes and macrophages in the tonsils provide
protection against harmful substances and pathogens
that may enter the body through the nose or mouth.
Spleen
The spleen is located in the upper left abdominal cavity, just beneath the
diaphragm, and posterior to the stomach. It is similar to a lymph node in
shape and structure but it is much larger. The spleen is the largest lymphatic
organ in the body. Surrounded by a connective tissue capsule, which extends
inward to divide the organ into lobules, the spleen consists of two types of
tissue called white pulp and red pulp. The white pulp is lymphatic tissue
consisting mainly of lymphocytes around arteries. The red pulp consists of venous sinuses filled with blood and cords of lymphatic cells,
such as lymphocytes and macrophages. Blood enters the spleen through
the splenic artery, moves through the sinuses where it is filtered,
then leaves through the splenic vein.
The spleen filters blood in much the way that the lymph nodes
filter lymph. Lymphocytes in the spleen react to pathogens in the
blood and attempt to destroy them. Macrophages then engulf the
resulting debris, the damaged cells, and the other
large particles. The
spleen, along with the liver, removes old and damaged erythrocytes
from the circulating blood. Like other lymphatic tissue, it produces
lymphocytes, especially in response to invading pathogens. The sinuses in the
spleen are a reservoir for blood. In emergencies such as hemorrhage,
smooth muscle in the vessel walls and in the capsule of the spleen contracts.
This squeezes the blood out of the spleen into the general circulation.
Thymus
The thymus is a soft organ with two lobes that is located anterior to
the ascending aorta and posterior to the sternum. It is relatively large in
infants and children but after puberty it begins to decrease in size so that
in older adults it is quite small.
The primary function of the thymus is the processing and maturation of
special lymphocytes called T-lymphocytes or T-cells. While in the thymus, the
lymphocytes do not respond to pathogens and foreign agents. After the
lymphocytes have matured, they enter the blood and go to other lymphatic
organs where they help provide defense against disease. The thymus also
produces a hormone, thymosin, which stimulates the maturation of lymphocytes
in other lymphatic organs.
Source:
SEER's Training Website.
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