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. To learn more
about lymphatic organs, click on one of the topics listed
below.
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