The spleen is an ovoid, usually purplish, pulpy mass about the size and shape of one’s fist. It is relatively delicate and considered the most vulnerable abdominal organ. The spleen is located in the superolateral part of the left upper quadrant (LUQ), or hypochondrium of the abdomen, where it enjoys protection of the inferior thoracic cage (Fig. 2.58A & B). As the largest of the lymphatic organs, it participates in the body’s defense system as a site of lymphocyte (white blood cell) proliferation and of immune surveillance and response.
Figure 2.58. The spleen.
A and B.Surface anatomy of spleen relative to the rib cage, anterior abdominal organs, and thoracic viscera and costophrenic pleural recess. C.Surface anatomy of the spleen and pancreas relative to the diaphragm and posterior abdominal viscera. D, duodenum, LK, left kidney, LS, left suprarenal gland, P, pancreas, S, stomach. D.The visceral surface of the spleen. Notches are characteristic of the superior border. Concavities on the visceral surface are impressions formed by the structures in contact with the spleen. E.The internal structure of the spleen.
Prenatally, the spleen is a hematopoietic (blood-forming) organ, but after birth it is involved primarily in identifying, removing, and destroying expended red blood cells (RBCs) and broken-down platelets, and in recycling iron and globin. The spleen serves as a blood reservoir, storing RBCs and platelets, and, to a limited degree, can provide a sort of “self-transfusion” as a response to the stress imposed by hemorrhage. In spite of its size and the many useful and important functions it provides, it is not a vital organ (not necessary to sustain life).
To accommodate these functions, the spleen is a soft, vascular (sinusoidal) mass with a relatively delicate fibroelastic capsule (Fig. 2.58E). The thin capsule is covered with a layer of visceral peritoneum that entirely surrounds the spleen except at the splenic hilum, where the splenic branches of the splenic artery and vein enter and leave (Fig. 2.58D). Consequently, it is capable of marked expansion and some relatively rapid contraction.
The spleen is a mobile organ although it normally does not descend inferior to the costal (rib) region; it rests on the left colic flexure (Fig. 2.58A & B). It is associated posteriorly with the left 9th–11th ribs (its long axis is roughly parallel to the 10th rib) and separated from them by the diaphragm and the costodiaphragmatic recess—the cleft-like extension of the pleural cavity between the diaphragm and the lower part of the thoracic cage. The relations of the spleen are:
- Anteriorly, the stomach
- Posteriorly, the left part of the diaphragm, which separates it from the pleura, lung, and ribs 9–11
- Inferiorly, the left colic flexure
- Medially, the left kidney.
The spleen varies considerably in size, weight, and shape; however, it is usually approximately 12 cm long and 7 cm wide. (A nonmetric memory device exploits odd numbers: the spleen is 1 inch thick, 3 inches wide, 5 inches long, and weighs 7 ounces.)
The diaphragmatic surface of the spleen is convexly curved to fit the concavity of the diaphragm and curved bodies of the adjacent ribs (Fig. 2.58A, B–C). The close relationship of the spleen to the ribs that normally protect it can be a detrimental one in the presence of rib fractures (see the blue box “Rupture of Spleen,”). The anterior and superior borders of the spleen are sharp and often notched, whereas its posterior (medial) end and inferior border are rounded (Fig. 2.58D). Normally, the spleen does not extend inferior to the left costal margin; thus it is seldom palpable through the anterolateral abdominal wall unless it is enlarged. When it is hardened and enlarged to approximately three times its normal size, it moves inferior to the left costal margin, and its superior (notched) border lies inferomedially (see the blue box “Splenectomy and Splenomegaly,”). The notched border is helpful when palpating an enlarged spleen because when the person takes a deep breath, the notches can often be palpated.
The spleen normally contains a large quantity of blood that is expelled periodically into the circulation by the action of the smooth muscle in its capsule and trabeculae. The large size of the splenic artery (or vein) indicates the volume of blood that passes through the spleen’s capillaries and sinuses. The thin fibrous capsule of the spleen is composed of dense, irregular, fibroelastic connective tissue that is thickened at the splenic hilum (Fig. 2.58E). Internally the trabeculae (small fibrous bands), arising from the deep aspect of the capsule, carry blood vessels to and from the parenchyma or splenic pulp, the substance of the spleen.
The spleen contacts the posterior wall of the stomach and is connected to its greater curvature by the gastrosplenic ligament, and to the left kidney by the splenorenal ligament. These ligaments, containing splenic vessels, are attached to the hilum of the spleen on its medial aspect (Fig. 2.58D). The splenic hilum is often in contact with the tail of the pancreas and constitutes the left boundary of the omental bursa.
The arterial supply of the spleen is from the splenic artery, the largest branch of the celiac trunk (Fig. 2.59A). It follows a tortuous course posterior to the omental bursa, anterior to the left kidney, and along the superior border of the pancreas. Between the layers of the splenorenal ligament, the splenic artery divides into five or more branches that enter the hilum. The lack of anastomosis of these arterial vessels within the spleen results in the formation of vascular segments of the spleen: two in 84% of spleens and three in the others, with relatively avascular planes between them, enabling subtotal splenectomy (see the blue box, “Splenectomy and Splenomegaly,”).
Figure 2.59. Spleen, pancreas, duodenum, and biliary ducts.
A.Relationships of spleen, pancreas, and extrahepatic biliary ducts to other retroperitoneal viscera. B.The entry of the bile duct and pancreatic duct into the duodenum through the hepatopancreatic ampulla. C.The interior of the descending part of the duodenum reveals the major and minor duodenal papillae. D.The structure of the acinar (enzyme-producing) tissue is demonstrated. The photomicrograph of the pancreas displays secretory acini and a pancreatic islet.
Venous drainage from the spleen flows via the splenic vein, formed by several tributaries that emerge from the hilum (Figs. 2.59A and 2.60B). It is joined by the IMV and runs posterior to the body and tail of the pancreas throughout most of its course. The splenic vein unites with the SMV posterior to the neck of the pancreas to form the hepatic portal vein.
Figure 2.60. Arterial supply and venous drainage of pancreas.
Because of the close relationship of the pancreas and duodenum, their blood vessels are the same in whole or in part. A.Arteries. Except for the inferior part of the pancreatic head (including uncinate process), the spleen and pancreas receive blood from the celiac artery. B.Venous drainage. C.Celiac arteriogram. Radiopaque dye was selectively injected into the lumen of the celiac artery.
The splenic lymphatic vessels leave the lymph nodes in the splenic hilum and pass along the splenic vessels to the pancreaticosplenic lymph nodes en route to the celiac nodes (Fig. 2.61A). The pancreaticosplenic nodes relate to the posterior surface and superior border of the pancreas.
Figure 2.61. Lymphatic drainage and innervation of pancreas and spleen.
A.The arrows indicate lymph flow to the lymph nodes. B.The nerves of the pancreas are autonomic nerves from the celiac and superior mesenteric plexuses. A dense network of nerve fibers passes from the celiac plexus along the splenic artery to the spleen. Most are postsynaptic sympathetic fibers to smooth muscle of the splenic capsule, trabeculae, and intrasplenic vessels.
The nerves of the spleen, derived from the celiac plexus (Fig. 2.61B), are distributed mainly along branches of the splenic artery, and are vasomotor in function.
The pancreas is an elongated, accessory digestive gland that lies retroperitoneally, overlying and transversely crossing the bodies of the L1 and L2 vertebra (the level of the transpyloric plane) on the posterior abdominal wall (Fig. 2.58C). It lies posterior to the stomach between the duodenum on the right and the spleen on the left (Fig. 2.59A). The transverse mesocolon attaches to its anterior margin (see Fig. 2.39A). The pancreas produces:
- an exocrine secretion (pancreatic juice from the acinar cells) that enters the duodenum through the main and accessory pancreatic ducts.
- endocrine secretions (glucagon and insulin from the pancreatic islets [of Langerhans]) that enter the blood (Fig. 2.59D).
For descriptive purposes, the pancreas is divided into four parts: head, neck, body, and tail.
The head of the pancreas is the expanded part of the gland that is embraced by the C-shaped curve of the duodenum to the right of the superior mesenteric vessels just inferior to the transpyloric plane. It firmly attaches to the medial aspect of the descending and horizontal parts of the duodenum. The uncinate process, a projection from the inferior part of the pancreatic head, extends medially to the left, posterior to the SMA (Fig. 2.60A). The pancreatic head rests posteriorly on the IVC, right renal artery and vein, and left renal vein. On its way to opening into the descending part of the duodenum, the bile duct lies in a groove on the posterosuperior surface of the head or is embedded in its substance (Fig. 2.59A & B; see also Fig. 2.45).
The neck of the pancreas is short (1.5–2 cm) and overlies the superior mesenteric vessels, which form a groove in its posterior aspect (see Fig. 2.44B & C). The anterior surface of the neck, covered with peritoneum, is adjacent to the pylorus of the stomach. The SMV joins the splenic vein posterior to the neck to form the hepatic portal vein (Fig. 2.60).
The body of the pancreas continues from the neck and lies to the left of the superior mesenteric vessels, passing over the aorta and L2 vertebra, continuing just above the transpyloric plane posterior to the omental bursa. The anterior surface of the body of the pancreas is covered with peritoneum and lies in the floor of the omental bursa and forms part of the stomach bed (see Fig. 2.39A & B). The posterior surface of the body is devoid of peritoneum and is in contact with the aorta, SMA, left suprarenal gland, left kidney, and renal vessels (Fig. 2.59A).
The tail of the pancreas lies anterior to the left kidney, where it is closely related to the splenic hilum and the left colic flexure. The tail is relatively mobile and passes between the layers of the splenorenal ligament with the splenic vessels (Fig. 2.58D).
The main pancreatic duct begins in the tail of the pancreas and runs through the parenchyma of the gland to the pancreatic head: here it turns inferiorly and is closely related to the bile duct (Fig. 2.59A & B). The main pancreatic duct and bile duct usually unite to form the short, dilated hepatopancreatic ampulla (of Vater), which opens into the descending part of the duodenum at the summit of the major duodenal papilla (Fig. 2.59B & C). At least 25% of the time, the ducts open into the duodenum separately.
The sphincter of the pancreatic duct (around the terminal part of the pancreatic duct), the sphincter of the bile duct (around the termination of the bile duct), and the hepatopancreatic sphincter (of Oddi)—around the hepatopancreatic ampulla—are smooth muscle sphincters that control the flow of bile and pancreatic juice into the ampulla and prevent reflux of duodenal content into the ampulla.
The accessory pancreatic duct (Fig. 2.59A) opens into the duodenum at the summit of the minor duodenal papilla (Fig. 2.59C). Usually, the accessory duct communicates with the main pancreatic duct. In some cases, the main pancreatic duct is smaller than the accessory pancreatic duct and the two may not be connected. In such cases, the accessory duct carries most of the pancreatic juice.
The arterial supply of the pancreas is derived mainly from the branches of the markedly tortuous splenic artery. Multiple pancreatic arteries form several arcades with pancreatic branches of the gastroduodenal and superior mesenteric arteries (Fig. 2.60A). As many as 10 branches may pass from the splenic artery to the body and tail of the pancreas. The anterior and posterior superior pancreaticoduodenal arteries, branches of the gastroduodenal artery, and the anterior and posterior inferior pancreaticoduodenal arteries, branches of the SMA, form anteriorly and posteriorly placed arcades that supply the head of the pancreas.
Venous drainage from the pancreas occurs via corresponding pancreatic veins, tributaries of the splenic and superior mesenteric parts of the hepatic portal vein; most empty into the splenic vein (Fig. 2.60B).
The pancreatic lymphatic vessels follow the blood vessels (Fig. 2.61A). Most vessels end in the pancreaticosplenic lymph nodes, which lie along the splenic artery. Some vessels end in the pyloric lymph nodes. Efferent vessels from these nodes drain to the superior mesenteric lymph nodes or to the celiac lymph nodes via the hepatic lymph nodes.
The nerves of the pancreas are derived from the vagus and abdominopelvic splanchnic nerves passing through the diaphragm (Fig. 2.61B). The parasympathetic and sympathetic fibers reach the pancreas by passing along the arteries from the celiac plexus and superior mesenteric plexus (see also “Summary of Innervation of Abdominal Viscera,”). In addition to sympathetic fibers that pass to blood vessels, sympathetic and parasympathetic fibers are distributed to pancreatic acinar cells and islets. The parasympathetic fibers are secretomotor, but pancreatic secretion is primarily mediated by secretin and cholecystokinin, hormones formed by the epithelial cells of the duodenum and proximal intestinal mucosa under the stimulus of acid contents from the stomach.