The pancreas is both an exocrine and an endocrine gland. Most of its substance is involved in producing pancreatic juice, which is conveyed by a duct system into the descending duodenum. In addition, microscopic clumps of endocrine tissue, the pancreatic islets (islets of Langerhans), are dispersed throughout the gland.
The pancreas is a long and narrow, lobulated organ, deeply located on the posterior abdominal wall. It lies approximately on the transpyloric plane (L1 vertebral level) and slopes slightly upwards from right to left. Its extremities lie in the right and left paravertebral gutters, while the intermediate portion is thrust forwards in the midline by the prominence of the vertebral column and aorta.
The gland is divided into four parts, from right to left: the head, neck, body and tail (Fig. 4.53). The head is the broadest part and is surrounded by the loop of the duodenum. Projecting to the left from its lower portion is the uncinate process. The neck is indented posteriorly by the portal and superior mesenteric veins. The body, of fairly uniform width, extends to the left and is continuous with the tail, which tapers as it approaches the hilum of the spleen. The tail is contained within the splenorenal ligament but all other parts are retroperitoneal. The pancreas develops from two embry- onic structures that fuse. This process may lead to constriction of the duodenum (annular pancreas), causing obstruction of the duodenum.
In cross-section, the pancreas is triangular. It has well-defined superior and inferior borders and a blunt anterior border, to which is attached the transverse mesocolon. The inferior half of the gland is related to the infracolic compartment of the peritoneal cavity, while the upper half lies in the floor of the omental bursa and contributes to the stomach bed.
Surrounded by the loop of the duodenum, the head of the pancreas lies anterior to the inferior vena cava, the right renal vessels and the terminal portion of the bile duct, which is sometimes embedded within the substance of this part of the gland (Fig. 4.54). A tumour in the head of the pancreas can obstruct the bile duct, leading to jaundice. The anterior surface of the head is related to the first part of the duodenum, the transverse colon and the gastroduodenal artery (Fig. 4.53). The uncinate process lies immediately superior to the horizontal duodenum. The superior mesenteric vessels cross the anterior surface of the uncinate process and separate it from the neck of the gland, which lies in front of the superior mesenteric vein and the commencement of the portal vein. The anterior surface of the neck is separated from the pylorus by the omental bursa.
The body of the pancreas crosses the aorta and usually covers the origins of its ventral branches, the coeliac trunk, the common hepatic and splenic branches of which are related to the superior border of the gland (Fig. 4.55), and the superior mesenteric artery, which emerges at the lower border (Fig. 4.53). Further to the left, the gland covers the left renal vessels and suprarenal gland, and the hilum of the left kidney. Immediately posterior to the body, the splenic vein receives a major tributary, the inferior mesenteric vein (Figs 4.54 & 4.55). The inferior border of the body is related to the duodenojejunal flexure, coils of jejunum and the left colic flexure. Overlying the anterior surface of the body are the transverse mesocolon, the stomach and omental bursa, and part of the lesser omentum.
The tail of the gland lies within the splenorenal ligament and is accompanied posteriorly by the splenic vessels (Fig. 4.55). It lies anterior to the visceral surface and hilum of the spleen and posterior to the stomach and omental bursa.
The main pancreatic duct arises in the tail of the gland and traverses the body and neck to reach the head where it curves downwards and to the right, to reach the medial wall of the descending duodenum (Fig. 4.54). The duct receives numerous tributaries and gradually increases in calibre from left to right. It pierces the intestinal wall at the greater duodenal papilla (Fig. 4.51).
The main pancreatic duct and the bile duct usually enter the duodenal wall together and unite to form a common chamber, the hepatopancreatic ampulla (of Vater), in which pancreatic juice and bile may mix before entering the duodenal lumen. There is usually a second and smaller duct, the accessory pancreatic duct, which opens into the duodenum about 2 cm above the main duct, at the minor duodenal papilla. Reflux of bile into the pancreatic duct, due for example to impaction of a gallstone in the hepatopancreatic ampulla, can lead to pancreatitis.
This is derived from branches of the coeliac and superior mesenteric arteries. The head and uncinate process receive superior pancreaticoduodenal branches from the gastroduodenal artery (a branch of the hepatic artery; Fig. 4.53) and inferior pancreaticoduodenal branches from the superior mesenteric artery (Fig. 4.52). The remainder of the gland is supplied by branches from the splenic artery (Fig. 4.55). The venous drainage of the pancreas passes into the portal system. Superior and inferior pancreaticoduodenal veins from the head of the gland pass, respectively, into the portal vein and the superior mesenteric vein. Veins from the remainder of the gland terminate in the splenic vein.