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.
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. The spleen receives its blood supply via the splenic artery and drains into the portal system via the splenic vein. The spleen has 2 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.
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.
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.
The spleen receives about 6% of the cardiac output, but at any time contains only about 50 mL of blood. 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” below).
Throughout the second trimester, the spleen plays an active role in hematopoiesis. At birth, little to no splenic hematopoiesis continues. However, the spleen can resume its hematopoietic functions in some disease states such as myelofibrosis, osteopetrosis, or severe hemolytic anemia.
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, ...