Anatomy and Physiology of the Spleen

Embryology

The splenic primordium appears during the fifth week of development as a mesodermal proliferation between the two leaves of the dorsal mesogastrium. In the early stages of development the splenic mesenchyme is also adherent to the dorsal pancreatic bud. As the stomach rotates around an anteroposterior axis, with its caudal portion moving upward and to the right and its cephalic portion moving downward and to the left, a portion of the dorsal mesogastrium eventually fuses with the peritoneum of the posterior abdominal wall. The splenic mesenchyme then separates from the pancreas, and the spleen remains intraperitoneal. The splenic primordium is eventually infiltrated by lymphoid cells. Hematopoiesis is prominent in the spleen from the third to the fifth months of embryonic life. By the fourth month, the red pulp structure begins to appear.

Anatomy

The spleen lies underneath the ninth, tenth, and eleventh ribs on the left, measures 7 to 13 cm in length, and weighs an average of 150 g, although normal weights range from 70 to 250 g and may decrease with age. Splenomegaly is usually considered if splenic weight is greater than 500 g or length greater than 15 cm; massive splenomegaly is defined as splenic weight exceeding 1500 g. The spleen becomes palpable underneath the left costal margin in instances where its size is at least twice normal. The spleen is asymmetric in shape with a smooth convex portion abutting the diaphragm and a concave surface medially ( Fig. 136.1 ).

Externally, the spleen is enveloped almost entirely by peritoneum, which is adherent to the splenic capsule and forms several ligaments to surrounding structures. The surgically significant ligaments are the gastrosplenic ligament, containing the short gastric vessels, and the splenocolic and splenorenal ligaments that tether the spleen to the colon and kidney, respectively ( Fig. 136.2 ). The splenic ligaments develop collateral vessels in cases of portal hypertension. Knowledge of these ligaments is critical because they need to be carefully divided when mobilizing the spleen. The gastrosplenic ligament is particularly important because it contains the splenic vessels, which are also often accompanied by the tails of the pancreas. Knowledge of the location of the tail of the pancreas is clinically relevant during a splenectomy to help avoid pancreatic injury. Computed tomography (CT) image analysis has shown that the distance between the pancreatic tail and the splenic hilum averages 3.4 ± 1.5 cm and is typically at least 1 cm. Therefore surgeons need to stay within 1 cm of the splenic hilum during a splenectomy to avoid injury to the pancreas.

Approximately 20% of the population has one or more accessory spleens, usually located within the splenic hilar region. Accessory spleens may also be found in the pancreas, omentum, and even in the pelvis and reproductive glands ( Fig. 136.3 ). A technetium 99m ( ^99m Tc)-labeled red blood cell scan can be used to help localize accessory spleens if complete splenectomy is mandatory, as in surgical management of immune thrombocytopenic purpura (ITP). The incidence of accessory spleens may be as high as 30% in individuals with hematologic pathology.

Blood Supply, Lymphatic Drainage, and Innervation

The spleen receives approximately 5% of the cardiac output, via the splenic artery, the largest of the three branches of the celiac trunk ( Fig. 136.4 ). However, the spleen also receives some accessory supply from branches of the left gastroepiploic artery. The splenic artery is a tortuous artery that lies posterior to the superior border of the body of the pancreas, forming multiple coils, and eventually divides into two or three main branches that penetrate through the hilum of the spleen. There are two main patterns of splenic artery anatomy, the magistral type, in which a long splenic artery trunk reaches close to the splenic hilum before dividing into branches, and the distributed type, in which there is a short splenic artery trunk with branching far from the splenic hilum. The distributive type is the more common variation.

The splenic artery branches in turn divide into segmental arteries that enter along the splenic trabeculae ( Fig. 136.5 ). There is little collateral circulation at this level, and occlusion of one of these arteries usually is associated with infarction of the corresponding region of the spleen, a phenomenon seen in embolic diseases. Segmental arteries give rise to trabecular arteries, which in turn, and by means of perpendicular branches, give origin to central arteries. There is an ongoing debate regarding the paths of blood flow after it enters the spleen. In general, it is thought that the blood takes two paths: a fast (closed) circulation that takes the blood directly from the arterioles to venules and has a predominance of plasma, and a slower (open) circulation that takes the blood through the pulp. The majority (90%) of flow is in fact of the slow (open) type, which exposes the circulating cells and erythrocytes to splenic macrophages in the red pulp.

Irrespective of the circulation in the spleen, veins leave the spleen through fibrous bands, or trabeculae, attached to the capsule, and coalesce to form the splenic vein. The splenic vein joins the superior mesenteric vein behind the neck of the pancreas to give origin to the portal vein (see Fig. 136.4 ).

Lymphatic drainage follows the vasculature. Drainage is into the splenic hilar and celiac nodes via the pancreaticosplenic lymph nodes.

The splenic nervous plexus is formed by branches of the celiac plexus, left celiac ganglion, and right vagus. It runs together with the splenic artery and is composed mainly of sympathetic fibers that reach blood vessels and nonstriated muscle of the capsule and trabeculae. Referred pain from the spleen to the left shoulder, commonly referred to as Kehr sign, is observed, particularly after splenic rupture.