Biomedical Engineering Reference
In-Depth Information
blood cells, and activated platelets. The spleen can also initiate the immune response, by
bringing B-cells and T-cells in close contact with any antigen present in the bloodstream.
Lastly, there is a small storage of iron in the spleen obtained from the recycling of dam-
aged red blood cells. This iron store could be used to produce new hemoglobin. The
spleen consists of two major parts: the red pulp and the white pulp. The red pulp contains
a significant quantity of blood cells and gets its color from red blood cells. The white pulp
is similar to lymphatic tissues and also functions as a site for lymphocyte production. It is
the red pulp that acts as a blood filter, and it is in that region where an inflammatory
response can be initiated. Mature lymphocytes from the white pulp can migrate into the
red pulp and enter the blood to circulate throughout the body.
The thymus is located just beneath the sternum and wraps around the trachea. The thy-
mus is divided into a right and left lobe, each consisting of many lobules. The exterior por-
tion of each lobule is composed of dividing/maturing lymphocytes. As the lymphocytes
fully mature (primarily T lymphocytes), they migrate into the interior portion of each lob-
ule, where they exit through blood vessels that permeate the entire thymus. In this way,
the thymus acts as a lymphatic tissue by producing mature lymphocytes. The thymus also
produces hormones that are critical for the immune system development. Upon release,
these hormones promote the development and maturation of lymphocytes within other
lymphatic tissues/organs. Therefore, the thymus is primarily responsible for the matura-
tion of T-cells and the production of hormones which aid in the lymphocyte production in
other lymphatic tissues.
The last major lymphatic organs are the lymph nodes. Lymph nodes are relatively small
organs (typically do not exceed 25 mm) that resemble the shape of a kidney bean. Each
lymph node is innervated and supplied by its own blood vessel. Every lymph node is also
directly connected to lymph flow via lymphatic vessels. The lymphatic vessels that bring
lymph to the nodes are termed afferent lymphatic vessels, while those that take the flow
away from the nodes are termed efferent lymphatic vessels. Lymph nodes function to filter
the lymph before it is returned back into the cardiovascular system via the lymphatic
ducts. Close to 100% of the antigens within lymph are removed within the lymph nodes
by macrophages that are fixed on the lymph node wall. These macrophages engulf and
digest the antigens and then present them to lymphocytes within the node. This acts to ini-
tiate an immune response and is a warning system for the rest of the body that an antigen
has been found within the blood. When there is a major inflammatory process initiated
within the body, these glands become swollen because of an increase in the production of
lymphocytes, which act to destroy any invading particles found within the blood.
Section 8.3
will discuss in more detail the flow of lymph through the lymph nodes, as well
as the rest of the lymphatic system.
The last component of the lymphatic system, which has not been discussed previously,
is the fluid that circulates throughout the lymphatic system termed lymph. Lymph has a
very similar composition to interstitial fluid. One of the primary functions of lymph is to
return proteins that have been transported out of the blood vessel or the cells back into
the cardiovascular system. In this way, there is a very small loss of protein concentration
within the cardiovascular system. Lymph derived from the gastrointestinal system pro-
vides a second rapid mechanism for nutrient absorption for any nutrients that do not
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