Spleen The spleen is located inside the abdominal cavity on the upper left side, weighs about 150 grams and measures about 10 cm in length. It has a rich blood supply and filters high volumes of blood every day. In the process it is able to detect antigens, microorganisms and select damaged and old red blood cells and platelets for destruction. It’s also responsible for recycling iron from degraded hemoglobin.
The spleen consists of a red pulp and a white pulp within a meshwork of fibers enclosed by a dense connective tissue capsule. Blood vessels, lymphatic vessels and nerves enter and leave the spleen through the hilum, which is an opening in the capsule. From the capsule, connective tissue called trabeculae extends into the organ and acts as a support for blood vessels. The trabeculae form a path for branches of the splenic artery, from which smaller arteries (central arteries) branch off to enter the pulp.
The main constituents of the spleen, the white and red pulp, form two functionally and morphologically different units.
The red pulp is composed of splenic sinuses separated by splenic cords (cords of Billroth). The splenic cords consist of a meshwork of cells and fibers, together with dendritic cells, macrophages, a relatively small amount of lymphatic cells and a great number of red blood cells (erythrocytes). The splenic sinuses are engorged with blood which ultimately gives the red pulp its red color. The main task of the red pulp is to filter blood to find defective and old erythrocytes as well as select them for breakdown. The red pulp also acts as a reservoir for monocytes, stored inside the splenic cords, which can rapidly be mobilized to leave the spleen and assist in an immunological response.
The white pulp consists of lymphatic tissue (lymphoid follicles) and monitors incoming blood for pathogens. Aggregations of lymphocytes, mainly T-cells, envelop central arteries throughout the spleen forming so called periarterial lymphatic sheaths (PALS). In certain areas, the white pulp expands into greater spherical aggregations consisting of multiplying B-cells (lymphoid follicles). Surrounding the B-cells is a darker stained area called mantle zone which marks the border between the red and white pulp. The splenic lymphoid follicles have an appearance similar to lymphoid follicles in other lymphoid tissues, except for the presence of a central artery.