Spleen
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Introduction
The spleen is one of the most common sites of embolus lodging, along with the brain and kidney. The resulting infarcts can be small or large, single or multiple, or even involve the whole organ.
Anatomy
The spleen is normally about 150 gm, measuring 12 cm by 7cm by 3 cm. It is enclosed in a thin connective tissue capsule.
The spleen receives its blood from the splenic artery, a main offshoot of the celiac trunk. The splenic vein joins the portal artery. Portal hypertension can thus lead to fluid backup and splenomegaly.
The white pulp of the spleen contains lymphoid nodules and their germinal centres. Blood vessels called central arteries leave the main trabecular arteries and become surrounded by lymphatic sheaths, comprised primarily by T lympohcytes. The sheath expands to form lymphoid nodules, which contains B lymphocytes. On antigenic stimulation, germinal centres form within the nodules and allow for clonal expansion.
As vessels enter the red pulp, they lose their lymphoid sheath and become thin-walled vascular sinusoids, separated by cords of losely connected macrophages. A portion of blood slowly seeps through this filter.
Function
The spleen filters all foreign matter from the blood stream, including 50% of old or damaged red blood cells, equalling ~2x1011 cells daily.
Red blood cells undergo extreme deformation when passing from cords into sinusoids.
It also recycles iron.
It also is involved in RBC pitting: Howell-Jolly bodies and Pappenhemier bodies
It is also acts as a reservoir for blood loss
- 30-40% of the body's platelets are in the spleen, and this number can increase to 80-90% in people with splenomegaly.
Many reticular cells mature in the spleen.
The spleen is a major secondary organ of the immune system. Dendritic cells in the periarterial sheath trap antigens and present them to T lymphocytes. T and B cells interact at the edges of the follicles, leading to the generation of plasma cells which migrate to the sinuses of the red pulp.
The spleen is a source of hematopoiesis before birth and again in cases of extreme anemia.
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