In the healthy adult, the pancreas is a soft, retroperitoneal glandular organ, lying transversely and oblique and draped over the vertebral column at the level of L1–L2 vertebrae. The bulk or volume of the pancreas varies and increases during the first 2–3 decades of life but progressively atrophies with aging. The pancreas is divided into five parts: the head, neck, body, tail, and uncinate process. The neck, head, and uncinate process are encompassed by the C-loop of the duodenum, to the anatomic right of the midline, and are in intimate relationship with the superior mesenteric vessels medially. The body extends laterally to the anatomic left, posterior to the stomach, with the tail terminating in the splenic hilum.
The organ is surrounded by a thin capsule that is loosely attached to its surface. Most of the anterior surface of the pancreas is covered with peritoneum, except where it is crossed by the root of the transverse mesocolon, as well as where there is direct contact with the first part of the duodenum and the splenic hilum. The head of the pancreas is the thickest part of the gland. Anteriorly, it is related to the origin of the transverse mesocolon. Posteriorly, the head is related to the inferior vena cava (IVC), the right gonadal vein near its entrance into the vena cava, and the right crus of the diaphragm. The common bile duct runs either on the posterior surface of the pancreatic head or is embedded within the parenchyma of the gland.
The transitional zone between the head and the body of the pancreas is termed the neck. It is defined by its anatomic location anterior to the formation of the portal vein (usually by the confluence of the superior mesenteric and splenic veins).It is approximately 2 cm wide and usually the most anteriorly located portion of the pancreas. Anteriorly the neck is covered by peritoneum and is related to the pylorus superiorly. Its posterior aspect is grooved by the superior mesenteric vein (SMV) and the portal vein (PV).
The anterior body of the pancreas is covered by the peritoneal layer that constitutes part of the posterior wall of the lesser sac. Toward the inferior border of the pancreas, the peritoneal layer is reflected anteroinferiorly to form the superior leaf of the transverse mesocolon. The posterior surface of the body lies on the fusion fascia of Toldt in the retroperitoneum, the so-called bloodless plane of Treves. The posterior body is related to the abdominal aorta and the origin of the superior mesenteric artery (SMA), the left crus of the diaphragm, the left renal vein, the left kidney, and the left adrenal gland, from right to left.
The pancreas has important relationships to major blood vessels, of relevance to surgery of the pancreas. The splenic vein runs along the posterior surface of the gland in a groove of variable depth, sometimes almost entirely embedded within the pancreatic parenchyma. The celiac trunk and its branches emanate along the superior border of the body, with the common hepatic artery running to the right and the splenic artery to the left. The inferior border of the pancreas is crossed posteriorly by the inferior mesenteric vein (IMV), typically at its confluence with the splenic vein, and it serves as a useful landmark for identification of the former vessel on cross-sectional imaging.
The tail of the pancreas is the relatively mobile, left-most part of the pancreas that is confined between the layers of the splenorenal ligament together with the splenic artery and the origin of the splenic vein. It is 1.5–3.5 cm long in adults and may extend variably to the hilum of the spleen in 50% of cases and may extend posterior to vessels in the hilum. This makes the tail of the pancreas vulnerable to injury during splenectomy and needs to be visualized prior to ligating the splenic vessels. The uncinate process can be considered as a distinct part of the pancreas due to its different embryologic origin and its location extending posterior to the superior mesenteric vessels.
It extends in the plane between the superior mesenteric vessels anteriorly and the aorta posteriorly. Superiorly, it relates to the left renal vein. It lies immediately superior to the third part of the duodenum, such that tumors arising in the uncinate process can compress the former leading to duodenal obstruction. The main pancreatic duct of Wirsung begins at the tail of the pancreas and runs through the body roughly midway between the superior and inferior border. It receives multiple small ductules throughout its course that drain the pancreatic parenchyma, thus increasing progressively in diameter from 1 mm in the tail to 3 mm in the head. It deviates inferiorly and posteriorly in the head as it courses toward the main ampulla. The pancreatic duct and bile duct are usually separated by the transampullary septum before joining in a “Y” configuration within the duodenal wall.
The terminal part of the two ducts is surrounded by a complex circular arrangement of smooth muscle fibers known as the sphincter of Oddi. The sphincter of Oddi is anatomically distinct from the muscular layers of the duodenum, and it has a dual function: (a) to regulate flow of biliary and pancreatic secretions into the duodenal lumen and (b) to impede reflux of intestinal content into the pancreatobiliary ductal system. The accessory duct of Santorini runs superior and parallel to the duct of Wirsung. It drains part of the head of the pancreas into the minor duodenal papilla, roughly 1–2 cm proximal to the ampulla of Vater. The pattern of fusion of the main and accessory ducts is variable and can be entirely separate (pancreas divisum).