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spleen,soft, purplish-red organ that lies under the diaphragm on the left side of the abdominal cavity. The spleen acts as a filter against foreign organisms that infect the bloodstream, and also filters out old red blood cells from the bloodstream and decomposes them. These functions are performed by phagocytic cells that are capable of engulfing and destroying bacteria, parasites, and debris. Ordinarily, the spleen manufactures red blood cells only toward the end of fetal life, and after birth that function is taken over by the bone marrow. However, in cases of bone marrow breakdown, the spleen reverts to its fetal function. The spleen also acts as a blood reservoir; during stress or at other times when additional blood is needed, the spleen contracts, forcing stored blood into circulation (see circulatory systemcirculatory system,
group of organs that transport blood and the substances it carries to and from all parts of the body. The circulatory system can be considered as composed of two parts: the systemic circulation, which serves the body as a whole except for the lungs, and the
..... Click the link for more information. ). It is sometimes necessary to remove the spleen entirely, particularly in trauma cases, although recent studies have shown the spleen to be far more important than initially suspected in the fight against infection.
An organ of the circulatory system present in most vertebrates, lying in the abdominal cavity usually in close proximity to the left border of the stomach.
In humans the spleen normally measures about 1 by 3 by 5 in. (2.5 × 7.5 × 12.5 cm) and weighs less than ½ lb (230 g). It is a firm organ with an oval shape and is indented on its inner surface to form the hilum, or stalk of attachment to the peritoneum. This mesentery fold also carries the splenic artery and vein to the organ.
The spleen is an important part of the blood-forming, or hematopoietic, system; it is also one of the largest lymphoid organs in the body and as such is involved in the defenses against disease attributed to the reticuloendothelial system. Although the chief functions of the spleen appear to be the production of lymphocytes, the probable formation of antibodies, and the destruction of worn-out red blood cells, other less well-understood activities are known. For example, in some animals it may act as a reservoir for red blood cells, contracting from time to time to return these cells to the bloodstream as they are needed. In the fetus and sometimes in later life, the spleen may be a primary center for the formation of red blood cells. Another function of the spleen is its role in biligenesis. Because the spleen destroys erythrocytes, it is one of the sites where extrahepatic bilirubin is formed. See Bilirubin
in vertebrates (including man), an unpaired parenchymatous organ in the abdominal cavity that participates in the formation, disintegration, and redistribution of blood, in metabolic processes, and in the protective reactions of the organism.
All vertebrates have a spleen. In cyclostomes and some dip-noans (barramundas) accumulations of lymphoid tissue that serve as a homologue of the spleen are found in the wall of the intestine, stomach, or spiral valve. In other vertebrates the spleen is apart from the wall of the intestinal canal and is situated in the dorsal mesentery of the intestine or stomach or at the beginning of the rectum. In all reptiles except tuataras and in birds the spleen is rudimentary and small in size. The spleen may be lobular or laciniate. It is trilobate in monotremes, and traces of lobulation are found in marsupials and primates. The spleen varies in shape from flat and ribbonlike, for example, in rodents and carnivores, to round, for example, in salientians, testudinates, and cetaceans. In mammals the spleen also varies in size.
In man the spleen starts forming at the end of the first month of fetal life in the mesenchyma, under the dorsal mesentery. The germ of the spleen is an accumulation of mesenchymal cells penetrated by blood vessels. Some mesenchymal cells subsequently differentiate to form reticular tissue, whereas others become round and give rise to the hematopoietic elements of the lymphocytic and myelocytic series. At birth, the human spleen is characterized by the virtual cessation of myelopoiesis and the intensification of lymphopoiesis.
The formed spleen is shaped like a coffee bean, with one surface convex (diaphragmatic) and the other concave (visceral). It is situated in the left hypochondriac region, between the diaphragm and the stomach and at the level of the ninth, tenth and 11th ribs. It is 12 cm long and 7-8 cm wide and weighs 150–200 g. The size and weight of the spleen vary from individual to individual and with the physiological condition of the organism; for example, the spleen is enlarged at rest but becomes contracted after blood loss.
The spleen is externally covered by a serous membrane and by a fibrous capsule containing smooth-muscle cells. Bands, or trabeculae, pass from within the capsule into the spleen, where they join and with the capsule form the spleen’s supporting framework. The trabeculae divide the spleen into separate lobules filled with red and white pulp. Splenic red pulp primarily consists of reticular tissue whose loops contain macrophages, granular and nongranular leukocytes, giant cells similar to megakaryocytes, normal and disintegrating erythrocytes, and blood vessels similar to venous sinuses.
The middle of the lobules is interspersed with white pulp, which consists of lymphoid tissue that produces lymphocytes. The lymphoid tissue is situated along the arteries in the form of spherical follicles and elongated lymphatic sheaths, both of which are called the lymphatic follicles of the spleen, or malpi-ghian corpuscles. The centers of the corpuscles are filled with reticular cells in different stages of maturity, macrophages, plasma cells, lymphoblasts, and large- and medium-size lymphocytes. The peripheral portions of the corpuscles contain small lymphocytes backed by a layer of macrophages.
The functioning of the elements of the red and white pulp is closely related to blood circulation. Blood is brought to the spleen by way of the splenic artery, whose segmental branches penetrate first into the trabeculae and then into the pulp. Near the trabeculae, the arteries of the pulp are invested with lymphatic sheaths and white-pulp follicles. The central artery, which is a part of the artery of the pulp that passes through the follicles, separates into several tiny brushlike blood vessels in the lymphatic follicle or at the site where it exits the lymphatic follicle. The proximal segments of the tiny blood vessels are furnished with cuff’s or cases that regulate the size of the lumens of the blood vessels.
The brushlike blood vessels ramify into capillaries, which nourish the pulp. Some capillaries are in contact with the lacunae of the reticular tissue, whereas other cappillaries transport blood directly to the venous sinuses, which hold a substantial volume of blood, enabling the spleen to function as a depot. An increase in the volume of blood in the sinuses results in the formation of wide interendothelial openings in the sinal walls. Circulating blood flows through the openings and interacts with red-pulp elements.
The sinal endothelium is highly phagocytic. Sphincters are found where the sinuses turn into venules. Sphincteral contraction improves the interaction between the blood and splenic pulp. Blood leaves the pulp through a system of venules that transport the blood via the splenic vein to the portal vein. The size of the spleen and the structure and interrelationships between the elements of the red and white pulp are constantly changing as a result of alterations in the volume of blood filling the spleen and the reactions of splenic lymphoid tissue.
The structure of the spleen is responsible for the various functions the organ performs in hematopoiesis, metabolism (for example, of iron), hemolysis, blood deposition and purification, and the protection of the body against injurious agents. The spleen acts as a biological filter for blood, from which it removes damaged erythrocytes and other foreign elements, including dyes and bacteria. The organ also performs an immu-nobiological function by producing antibodies and regenerating phagocytizing cells.
IA. L. KARAGANOV
Diseases of the spleen, or splenopathies, are caused mainly by the organ’s enlargement. Hypoplasia or the absence of the organ do not significantly impair the functioning of the body. Primary diseases of the spleen include tumors, cysts, abscesses, and traumas. The spleen may become impaired as a result of such infectious diseases as sepsis, tuberculosis, malaria, syphilis, and infectious mononucleosis and such blood diseases as leukemia, Hodgkin’s disease, and polycythemia rubra. Circulatory disturbances in the portal system may also cause the spleen to become diseased. The intensified disintegration of blood cells in the spleen with splenomegaly causes a decrease in the number of erythrocytes, neutrophils, and platelets in the blood (hypersplenism) or a decrease only in the number of erythrocytes (hemolytic anemia) or platelets (thrombocytopenic purpura). The common venous circulation in the spleen and liver causes the latter to degenerate when the spleen is made to overflow with blood under experimental conditions. An increase in pressure in the portal vein with cirrhosis of the liver results in the enlargement of the spleen, a condition known as Banti’s syndrome.
Important methods used in diagnosing splenopathies include palpation of the spleen, splenic puncture, roentgenography of the spleen after the injection of a contrast medium into splenic blood vessels, the measurement of pressure in splenic blood vessels, scanning, and laparoscopy. Hypersplenism with hemolytic anemia, thrombocytopenic purpura (especially in children), and Banti’s syndrome are treated surgically by removing the spleen.
REFERENCEGeller, L. I. Fiziologiia i pathologiia selezenki. Moscow, 1964.
A. N. SMIRNOV [23–571–]