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The spleen is the largest lymphoid organ
in the body and plays a vital role in hematologic functions as well
as in the defense of the body against invading pathogens.
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The spleen is first identifiable during the fifth week of embryogenesis.
It weighs about 10 to 11 g at birth and enlarges throughout childhood
until it reaches its maximum size of 100 to 200 g by adulthood.1 The
spleen receives its blood supply via the splenic artery and drains
into the portal system via the splenic vein. The spleen has two
major compartments: the white pulp (which serves the immunologic
functions) and the red pulp (the area where blood is filtered).
A marginal zone separates the red and white pulp and functions primarily
for presentation and processing of antigens.
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Arterioles entering the splenic tissue become surrounded by T
lymphocytes (periarteriolar lymphoid sheaths). B lymphocytes also
lie adjacent to the arterioles within germinal centers. Due to the
angle of the small artery branches leaving the central arteries
and entering the white pulp, the plasma is effectively “skimmed
off” into the white pulp. The remaining erythrocytes then
enter the red pulp.
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The red pulp is the largest area of the spleen. It is composed
of splenic sinuses as well as an open meshwork of passages (the
cords of Billroth). Ninety percent of the blood entering the spleen
flows directly, but slowly, through the cords, allowing the macrophages lining
their walls to remove aged and damaged red blood cells and particulate
matter. In these cords, the red blood cells are forced to squeeze
through narrow fenestrations between endothelial cells before entering
the sinuses and eventually draining into the splenic vein.
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The spleen receives about 6% of the cardiac output,
but at any one time contains only about 50 mL of blood.1 About
30% of the circulating platelet mass is sequestered within the
spleen. In cases of splenomegaly, a higher volume of blood may become
trapped (see “Hypersplenism/Asplenia,” below).
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Throughout the second trimester, the spleen plays an active role
in hematopoiesis. At birth, little to no splenic hematopoiesis continues. However,
in some disease states, such as myelofibrosis, osteopetrosis, or
severe hemolytic anemia, the spleen can resume its hematopoietic
functions.
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Red Blood Cell Clearance
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Due to the unique structure and harsh environment of the spleen’s red
pulp, it plays a key role in the removal or culling of aged or damaged
red cells from the circulation. Young, normal red cells are able
to tolerate the demands of this environment, but as cells age or
are damaged, they become rigid and can no longer maneuver through the
splenic cords and pass through the small slits between endothelial cells
into the sinuses. These ...