What is the spleen?
The spleen is a secondary lymphoid organ. Secondary lymphoid tissues provide an environment in which the cells of the immune system can interact with antigen and with one another to develop an immune response to antigen. The spleen is a major site of immune response to blood borne antigens. In addition, the splenic red pulp contains macrophages that are responsible for clearing the blood of unwanted foreign substances and senescent erythrocytes, even in the absence of specific immunity. Thus, it acts as a filter for the blood.
Location of spleen
The spleen is located within the peritoneum in the left upper quadrant of the abdomen between the fundus of the stomach and the diaphragm. It receives its blood supply from the systemic circulation via the splenic artery, which branches off the celiac trunk, and the left gastroepiploic artery.
Blood supply of spleen
The blood returning from the spleen drains into the portal circulation via the splenic vein. Therefore, the spleen can become congested with blood and increase in size when there is portal vein hypertension. Approximately 10 percent of individuals have one or more accessory spleens. Accessory spleens are usually 1 cm in diameter and resemble lymph nodes. However, they usually are covered with peritoneum, as is the spleen itself. Accessory spleens typically lie along the course of the splenic artery or its gastroepiploic branch, but they may be elsewhere. The commonest location is near the hilus of the spleen, but approximately 1 in 6 accessory spleens can be found embedded in the tail of the pancreas, where they may be occasionally mistaken for a pancreatic mass lesion.
Dimensions of spleen
The average weight of the spleen in the adult human is 135 g (range: 100–250 g). However, when emptied of blood it weighs only approximately 80 g. On autopsy of 539 subjects with normal spleens, there was a positive correlation between the spleen weight and both the degree of acute splenic congestion and the subject’s height and weight, but not with the subject’s sex or age.
The splenic volume can be estimated by computed tomography (CT) of the abdomen. In one study, the splenic volume was calculated from the linear and the maximal cross-sectional area measurements of the spleen, using the following formula: splenic volume = 30 cm3 + 0.58 (the product of the width, length, and thickness of the spleen measured in centimeters).
Using this formula, the mean value of the calculated splenic volume for 47 normal subjects was 214.6 cm3, with a range of 107.2 to 314.5 cm. The calculated splenic volume did not appear to vary significantly with the subject’s age, gender, height, weight, body mass index, or the diameter of the first lumbar vertebra, the latter being considered representative of body habitus on CT.
The splenic volume also can be estimated by sonography, which provides good correlation with volumes measured by helical abdominal CT or actual volume displaced by the excised organ. In one study of 50 patients, the linear measurement by sonography that correlated most closely with CT volume was the spleen width measured on a longitudinal section with the patient in the right lateral decubitus position.
There was also good correlation between splenic length measured in the right lateral decubitus position and CT volume (r = 0.86, p < 0.001). In another study of 32 normal spleens from adult corpses, the ultrasonogram measurements of maximal height, width, and breadth of the spleen were compared with the actual volume displaced by the excised organ.