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![](https://webarchive.library.unt.edu/eot2008/20080916054738im_/http://www.ncbi.nlm.nih.gov/books/bookres.fcgi/gnd/blood.jpg) As most of the cells in the human body are not in direct contact with the external
environment, the circulatory system acts as a transport system for these cells. Two
distinct fluids move through the circulatory system: blood and lymph. Blood carries
oxygen and nutrients to the body's cells, and carries waste materials away. Blood
also carries hormones, which control body processes, and antibodies,
to fight invading germs. The heart is the pump that keeps this transport system
moving. Together, the blood, heart, and blood vessels form the circulatory system. The lymphatic system (lymph, lymph nodes and lymph vessels) supports the circulatory
system by draining excess fluids and proteins from tissues back into the bloodstream,
thereby preventing tissue swelling. It also serves as a defense system for the body,
filtering out organisms that cause disease, producing white blood cells, and generating
antibodies. The biochemical make up of lymph the fluid found in the lymphatic vessels varies
with the site of origin. For example, lymph from bone marrow, spleen, and thymus have
high concentrations of white blood cells for fighting infection, while lymph from
intestines is high in fat that has been absorbed during digestion. Damage to the
lymphatic and circulatory systems leaves the body more susceptible to sickness and
infection, as well as to serious conditions such as cancer.![top link](https://webarchive.library.unt.edu/eot2008/20080916054738im_/http://www.ncbi.nlm.nih.gov/books/bookres.fcgi/system_default/top.gif)
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"Elephantiasis" occurs after years of infection of the lymph vessels with parasites. Lymph fluid accumulates causing massive swelling of the legs. |
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Scientists once thought that all blood was the same, leading to deadly blood transfusion reactions. |
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