The pancreas is a large, asymmetric gland lying in the central retroperitoneum, consisting of the head, neck, body, and tail. The pancreatic head lies at just right of the L2 vertebral body and extends in an oblique course to the left over the spine, cephalad, and then slightly posterior until the tail terminates near the splenic hilum, at the level of T10. The neck is often defined as that portion of the gland overlying the superior mesenteric artery and vein (SMA and SMV), and separating the head to the right from the body to the left. The dividing line between the body and tail hardly exists and is not surgically relevant. The uncinate process of the pancreas is embryologically separate from the rest of the pancreas and in adults extends from the inferior lateral head of the gland, extending slightly posterior to the SMV and terminating at the SMA (Fig. 1.1).

The arterial supply to the pancreas is abundant and comes via multiple named and unnamed vessels from both the celiac axis and the SMA, a fact largely responsible for the ability of the pancreaticoduodenectomy resection specimen to bleed abundantly until the last fibers of tissue are divided. The head is richly supplied by anastomosing branches of the pancreaticoduodenal arteries, while the body and tail are predominantly supplied by branches of the splenic artery and jejunal branches. The collateral flow often present between the SMA and the celiac axis, chiefly through the gastroduodenal artery (GDA), becomes very important at pancreaticoduodenectomy, during which the GDA is typically divided. In all cases, flow in the hepatic artery is confirmed during clamping of the GDA to detect cases in which hepatic artery flow is significantly dependent on SMA-celiac axis collaterals. In such cases, arterial bypass, preservation of the GDA, or division of a median arcuate ligament may be necessary, depending on the clinical scenario. In all cases, one must be aware of aberrant hepatic arterial anatomy (vida infra), which is common (>25% of cases) and is more commonly replaced than accessory (Fig. 1.2).

The venous drainage of the pancreas is predominantly portal, excepting small unnamed retroperitoneal veins that may drain posteriorly into the lumbar system and
may in cases of portal hypertension become clinically relevant. The predominantly portal drainage of the pancreas accounts for the preponderance of liver metastases compared with lung metastases in cases of advanced pancreatic cancer. The pancreatic head and uncinate process drain via pancreaticoduodenal veins that run with the pancreaticoduodenal arteries and drain into the SMV and portal vein, while the body and tail drain via the splenic vein. During pancreaticoduodenectomy several prominent named veins must be ligated and divided at their confluence with the SMV in order to dissect the neck of the pancreas from the SMV. These include the gastroepiploic vein caudal and to the left, and the vein of Belcher, cephalad and to
the right. After division of the neck of the pancreas, a first jejunal vein is sometimes ligated and divided at its confluence with the SMV during division of the uncinate process (Fig. 1.3).

The vascular anatomy of the pancreas and surrounding structures is crucial to the surgical care of pancreaticobiliary patients, and in fact, often determines resectability of malignant masses. Typically, patients without metastatic disease are grouped into three categories of resectability, depending on the vascular involvement by the tumors: Resectable, borderline, and unresectable. Borderline patients were first defined by the MD Anderson group and may have either encasement of a short segment of the hepatic artery (but no extension to the celiac axis) that is amenable to resection and reconstruction, or tumor abutment (viz, <180 degrees) of the SMA, or short-segment occlusion of the SMV or portal vein that is amenable to resection and reconstruction. Tumor encasement (viz, ≥180 degrees) of the SMA by the tumor typically constitutes a locally advanced, unresectable tumor.

Pancreatic lymphatic vessels travel from the acini and follow the arteries to drain into peripancreatic lymph nodes. The head and neck of the pancreas drain widely into pancreaticoduodenal nodes, superior mesenteric nodes, hepatic artery nodes, pre-aortic, and celiac axis nodes. The body and tail drain predominantly into pancreaticosplenic nodes with a minority of channels draining into pre-aortic nodes. The pancreatic islets have no lymphatics.

The exocrine pancreas is richly innervated with both sympathetic and parasympathetic fibers (the endocrine pancreas is innervated almost exclusively by the parasympathetic system). Sensory fibers from the pancreas travel through the celiac plexus, at which point they are available for ablation, typically via ethanol splanchnicectomy in cases of severe, chronic pain from pancreatitis or locally advanced malignancy.

The extrahepatic biliary tree, as well as associated arteries, is aberrant in one way or another at least as often as they are typical. Although discussion of all the variations of the hepatic, cystic, and common bile ducts, as well as the cystic and hepatic arteries, is beyond the scope of this chapter, aberrancies of the right and left hepatic arteries are worthy of further mention. The most reliable measurement of the frequency and type of hepatic artery aberrancies likely comes from autopsy studies in the 1950s by Michels, who described in a series of 200 autopsies, that 26% of bodies had aberrant right, and 27% aberrant left, hepatic arteries. On both sides, replaced was more common than accessory arteries (60% replaced on the right and 70% on the left). The practicing pancreaticobiliary surgeon must be familiar with standard as well as aberrant anatomy of the extrahepatic biliary tree and associated arteries (Fig. 1.2).