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In a nutshell, the lymphatic system is a drainage
system that removes excess fluid from body
tissues and returns it to the bloodstream.
It is actually a subsystem of both the circulatory
and immune system.
The major purpose of the circulatory system
is to bring oxygen and nutrients to body tissues
and remove wastes.
This exchange happens in the smallest blood
vessels called the capillaries.
Blood plasma containing nutrients moves out
of capillaries at the arterial end of capillary
beds, while tissue fluid containing wastes
reabsorbs back in at the venous end.
However, not all of the fluid is drawn back
to the bloodstream at this point.
About 15% of it is left in the tissues and
would cause swelling if accumulated.
This is where the lymphatic system comes into
play, it picks up the excess fluid and returns
it to the circulatory system.
Unlike the blood circulatory system, which
is a closed loop, the lymphatic system is
a one-direction, open-ended network of vessels.
Lymphatic vessels begin as lymphatic capillaries
made of overlapping endothelial cells.
The overlapping flaps function as a one-way
valve.
When fluid accumulates in the tissue, interstitial
pressure increases pushing the flaps inward,
opening the gaps between cells, allowing fluid
to flow in.
As pressure inside the capillary increases,
the endothelial cells are pressed outward,
closing the gaps, thus preventing backflow.
Unlike blood capillaries, the gaps in lymphatic
capillaries are so large that they allow bacteria,
immune cells such as macrophages, and other
large particles to enter.
This makes the lymphatic system a useful way
for large particles to reach the bloodstream.
It is used, for example, for dietary fat absorption
in the intestine.
Once inside lymphatic vessels, the recovered
fluid is called lymph.
Lymph flow is enabled by the same forces that
facilitate blood flow in the veins.
It goes from lymphatic capillaries to larger
and larger lymphatic vessels and eventually
drains into the bloodstream via the subclavian
veins.
On the way, it passes through a number of
lymph nodes, which serve as filters, cleansing
the fluid before it reaches the bloodstream.
Lymph nodes are small bean-shaped structures
scattered throughout the lymphatic network.
They are most prominent in the areas where
the vessels converge.
Lymph nodes contain macrophages and dendritic
cells that directly “swallow up” any pathogens,
such as bacteria or viruses, that may have
been taken up from an infected tissue.
They also contain lymphocytes: T-cells and
B-cells, which are involved in adaptive immune
response, a process that produces activated
lymphocytes and antibodies specific to the
invading pathogen.
These are then carried by the lymph to the
bloodstream to be distributed wherever they
are needed.
The lymphatic system also includes lymphoid
organs.
Primary lymphoid organs – the thymus and
bone marrow, are the sites of lymphocyte production,
maturation and selection.
Selection is the process in which lymphocytes
learn to distinguish between self and non-self,
so they can recognize and destroy pathogens
without attacking the body's own cells.
Mature lymphocytes then leave the primary
for the secondary lymphoid organs – the
lymph nodes, spleen, and lymphoid nodules
- where they encounter pathogens and become
activated.
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The Lymphatic System Overview, Animation

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Amy.Lin 2019 年 3 月 18 日 に公開

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