The lymphatic system has three primary
functions. First of all, it returns excess interstitial
fluid to the blood. Of the fluid that
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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.
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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.
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