The pancreas, named for the Greek words pan (all) and kreas (flesh), is a 12-15–cm long J-shaped (like a hockey stick), soft, lobulated, retroperitoneal organ. It lies transversely, although a bit obliquely, on the posterior abdominal wall behind the stomach, across the lumbar (L1-2) spine (see the image below). [1, 2, 3, 4, 5]
The pancreas develops as 2 buds (outpouchings) of endoderm from the primitive duodenum at the junction of the foregut and the midgut. A small ventral bud (pouch) forms the lower (inferior) part of the head and the uncinate process of pancreas, whereas a large dorsal bud (pouch) forms the upper (superior) part of the head as well as the body and tail of the pancreas. The ventral bud rotates behind the duodenum dorsally from right to left and fuses with the dorsal bud, and the duct of the distal part (body and tail) of the dorsal bud unites with the duct of the ventral bud to form the main pancreatic duct (of Wirsung). Because the common bile duct (CBD) also arises from the ventral bud, it forms a common channel with the main pancreatic duct. The remaining proximal part (head) of the duct of the dorsal bud remains as the accessory pancreatic duct (of Santorini).
The duodenum (25 cm long) is horseshoe shaped (with its inferior limb being longer than the superior) and has 4 parts: (1) superior (5 cm) at the level of L1; (2) descending, or C loop (7.5 cm), at L1-L3; (3) horizontal, or transverse (10 cm), at L3; and (4) ascending (2.5 cm), leading to the duodenojejunal flexure (junction). [1, 2, 3, 4, 5]
The pancreas is prismoid in shape and appears triangular in cut section with superior, inferior, and anterior borders as well as anterosuperior, anteroinferior, and posterior surfaces.
The head of the pancreas lies in the duodenal C loop in front of the inferior vena cava (IVC) and the left renal vein (see the following images). The uncinate process is an extension of the lower (inferior) half of the head toward the left; it is of varying size and is wedged between the superior mesenteric vessels (vein on right, and artery on left) in front and the aorta behind it.
The lower (terminal) part of the CBD runs behind (or sometimes through) the upper half of the head of pancreas before it joins the main pancreatic duct (MPD) of Wirsung to form a common channel (ampulla).
The neck of the pancreas lies in front of the superior mesenteric vein, splenic vein and portal vein junction. The body and tail of the pancreas run obliquely upward to the left in front of the aorta and left kidney. The pancreatic neck is the arbitrary junction between the head and body of the pancreas. Portal vein lies behind the neck of the pancreas. The narrow tip of the tail of the pancreas reaches the splenic hilum in the splenorenal (lienorenal) ligament.
The pancreatic head constitutes about 50% and the body and tail the remaining 50% of the pancreatic parenchymal mass.
The transverse mesocolon (with the middle colic vessels in it) is attached to the anterior surface of the lower (inferior) part of body and tail—most of the gland is thus located in the supracolic compartment. The body and tail of the pancreas lie in the lesser sac (omental bursa) behind the stomach.
The pancreas is best evaluated with a triphasic (arterial, portal venous, and systemic venous phases), contrast-enhanced (after intravenous injection of contrast medium), computed tomographic (CT) scan with 3-dimensional (3-D), triplanar (axial, coronal, and sagittal planes) reconstruction. Because the pancreas lies obliquely, all parts of the pancreas are not at the same transverse level and are not seen in 1 section (cut) of the CT scan—the pancreatic head is lower (at the level of L2) than its body (L1) and tail (T12). The normal pancreatic duct may be just seen in the head (3-4 mm) and proximal body (2-3 mm) of the pancreas on CT scan. See the images below.
The main pancreatic duct (of Wirsung) runs from the tail through the body to the head of the pancreas where it descends into the lower (inferior) part of the head. There, it joins the duct of the uncinate process coming from left and then the lower part of the common bile duct to form a common channel (called the hepatopancreatic ampulla, when dilated), which runs through the medial duodenal wall and opens on the dome of the major duodenal papilla (a nipplelike projection on the medial wall of the middle segment of the second part [C loop] of the duodenum). Both the ampulla and papilla are eponymously related to Vater.
A smooth muscle sphincter (of Oddi) is present around the common channel of the pancreatic duct and the common bile duct; this prevents reflux of duodenal juices into the pancreatic duct (and the common bile duct). Another individual smooth muscle sphincter is present around the terminal part of the main pancreatic duct before it joins the common bile duct; this prevents reflux of bile into the pancreatic duct (a similar sphincter present around the lower part of the common bile duct prevents reflux of pancreatic juices into the common bile duct).
An accessory pancreatic duct drains the upper (superior) part of the head of the pancreas and opens in the duodenum at the minor duodenal papilla 2 cm anterosuperior to the major papilla (see the following image). The 2 pancreatic ducts (main and accessory) often communicate with each other.
Endoscopic ultrasonography (EUS) is the latest technical tool to evaluate the pancreas. An ultrasonographic probe is mounted at the tip of an upper gastrointestinal endoscope (UGIE), which is passed into the second part (C loop) of the duodenum. The pancreatic head, distal (terminal) parts of the pancreatic ducts (main and accessory), lower (intrapancreatic) part of the common bile duct, and pancreaticoduodenal lymph nodes are very well visualized on EUS.
Pancreas derives a rich blood supply from both celiac axis and superior mesenteric artery; that is why when angiography is done for bleeding as a complication of acute pancreatitis, chronic pancreatitis or pancreatoduodenectomy both celiac axis and superior mesenteric artery should be evaluated.
The celiac trunk (axis) comes off from the anterior surface of the aorta at the level of T12–L1. It has a short length of about 1 cm and trifurcates into the common hepatic artery (CHA), splenic artery, and left gastric artery (LGA). The CHA runs toward the right on the superior border of the proximal body of the pancreas, and the splenic artery runs toward the left on the superior border of the distal body and tail of the pancreas. [1, 2, 3, 4, 5]
The superior mesenteric artery (SMA) comes off from the anterior surface of the aorta just below the origin of the celiac trunk at the level of L1 behind the neck of the pancreas. Then, it descends down in front of the uncinate process and the 3rd (horizontal) part of the duodenum to enter the small bowel mesentery.
The gastroduodenal artery (GDA), a branch of the CHA, runs down behind the first part of the duodenum in front of the neck of the pancreas and divides into the right gastro-omental (gastroepiploic) artery (RGEA) and superior pancreaticoduodenal artery (SPDA), which further bifurcates into anterior and posterior branches. The inferior pancreaticoduodenal artery (IPDA) arises from the SMA and also bifurcates into anterior and posterior branches.
The anterior and posterior branches of the SPDA and IPDA join each other and form anterior and posterior pancreaticoduodenal arcades in the anterior and posterior pancreaticoduodenal grooves supplying small branches to the pancreatic head and uncinate process of the pancreas as well as the 1st, 2nd, and 3rd parts of the duodenum (vasa recta duodeni). Multiple pancreatic branches (including a great pancreatic artery or arteria magna pancreatica) of the splenic artery supply the pancreatic body and tail. Multiple, small pancreatic branches of a dorsal pancreatic artery from the splenic artery and an inferior pancreatic artery from the superior mesenteric artery supply the body and tail of pancreas.
The arterial supply of the pancreas forms an important collateral circulation between the celiac axis and superior mesenteric artery.
Veins accompany the SPDA and IPDA. Superior pancreaticoduodenal veins (SPDVs) drain into the portal vein and inferior pancreaticoduodenal veins (IPDVs) drain into the superior mesenteric vein (SMV). A few small, fragile uncinate veins drain directly into the SMV. Some veins from the head of the pancreas drain into the gastrocolic trunk. Numerous small, fragile veins drain directly from the pancreatic body and tail into the splenic vein.
The SMV lies to the right of the SMA in front of the uncinate process and the 3rd part of the duodenum. The splenic vein arises in the splenic hilum behind the tail of the pancreas and runs from left to right on the posterior surface of the pancreatic body. Union of the horizontal splenic vein and the vertical SMV forms the portal vein behind the neck of the pancreas.
The inferior mesenteric vein (IMV) joins the splenic vein (or the junction of the splenic vein and SMV, or even SMV). The portal vein receives the SPDVs, right gastro-omental (gastroepiploic vein, left gastric vein (LGV), and right gastric vein (RGV); then, it runs up (superiorly) behind the first part of the duodenum in the hepatoduodenal ligament behind (posterior to) the common bile duct on the right and proper hepatic artery on the left.
The portal venous system (splenic vein, SMV, and portal vein) has no valves.
The head of the pancreas drains into pancreaticoduodenal lymph nodes and lymph nodes in the hepatoduodenal ligament, as well as prepyloric and postpyloric lymph nodes. The pancreatic body and tail drain into mesocolic lymph nodes (around the middle colic artery) and lymph nodes along the hepatic and splenic arteries. Final drainage occurs into celiac, superior mesenteric, and para-aortic and aortocaval lymph nodes. [1, 2, 3, 4, 5]
The pancreas receives parasympathetic nerve fibers from the posterior vagal trunk via its celiac branch. Sympathetic supply comes from T6-T10 via the thoracic splanchnic nerves and the celiac plexus. [1, 2, 3, 4, 5]
The pancreas is a composite gland containing both exocrine and endocrine components. Acini, formed of zymogenic cells around a central lumen, are arranged in lobules. Each lobule has its own ductule, and many ductules join to form intralobular ducts, which then form interlobular ducts that drain into branches of the main pancreatic duct.
Under stimulation of secretin and cholecystokinin (CCK), the zymogenic cells secrete a variety of enzymes — trypsin (digests proteins), lipase (digests fats), amylase (digests carbohydrates), and many others. Ductular cells produce bicarbonate, which makes the pancreatic fluid (juice) alkaline.
Scattered throughout the gland are pancreatic islets (clusters) (of Langerhans) containing beta cells (about 75% of islets; these secrete insulin), alpha cells (about 20% of islets; these secrete glucagon), delta cells (these secrete somatostatin), and several other hormone-secreting cells. Islets constitute only about 2% of the pancreatic parenchyma.
Natural and Pathophysiologic Variants
The main pancreatic duct and common bile duct may not unite to form a common channel and open separately at the major duodenal papilla. In addition, an aberrant (normal vessel is not present) right hepatic artery (RHA) may arise from the superior mesenteric artery (SMA) and accessory RHA (in addition to the normal one from common hepatic artery [CHA]) from the SMA. [1, 2, 3, 4, 5]
An annular pancreas is caused by failure of rotation of the ventral bud of the pancreas. A ring of pancreas is present around and obstructs the second part (C loop) of the duodenum. Neonates with this pancreatic variant present with vomiting; abdominal x-rays show a double-bubble (gastric and duodenal) appearance. Treatment includes dudodeno-jejunostomy and not division of the pancreatic ring because it may result in pancreatic juice leak and fistula.
Pancreas divisum is due to failure of the main and accessory pancreatic ducts to fuse. In addition to the upper (superior) half of the head of pancreas (which it normally also drains), the accessory pancreatic duct (of Santorini) also drains the body and tail of pancreas. This drainage may not be adequate (because of the smaller size of the accessory duct) and may cause functional obstruction, resulting in recurrent attacks of acute pancreatitis. The main pancreatic duct (of Wirsung) drains only the lower (inferior) half of the head and uncinate process and does not communicate with the accessory duct.
A long (> 15 mm common channel of pancreatic duct and common bile duct is described as anomalous pancreatio biliary ductal junction/ union (APBDJ/ APBDU) - it is associated with choledochal cyst and carries a higher risk of biliary malignancy.
Accessory pancreatic tissue may be present in the stomach, small intestine, Meckel diverticulum, omentum, and hilum of spleen as soft yellow nodules/lobules.
Periampullary cancers include those of the lower common bile duct, ampulla, pancreas head, and duodenum (including papilla) within 1-2 cm of the ampulla. Pain of pancreatic origin in acute pancreatitis, chronic pancreatitis, and pancreatic cancer is felt in the epigastrium and bores into the back; it is aggravated when lying down and may be relieved by sitting and bending forwards. Transmitted aortic pulsations can be seen and felt in pancreatic masses (tumors and cysts) as the pancreas lies on the aorta.
Using a side-viewing endoscope (SVE), the pancreatic duct (and the common bile duct) can be cannulated through the papilla and radiographs obtained after injecting contrast, which is called endoscopic retrograde cholangiopancreatography (ERCP).
Endoscopic ultrasonography (EUS) can detect small stones in the lower part of the common bile duct (not detected on regular ultrasonogram) and small tumors in the head of the pancreas (not detected on computed tomography [CT] scan). EUS can also be used to diagnose pancreas divisum and for guided fine-needle aspiration cytology (FNAC) from pancreaticoduodenal lymph nodes.
Inflammatory thickening of the anterior layer of the left perirenal (Gerota) fascia is seen on CT scanning in acute pancreatitis.
The following should be kept in mind when considering surgical intervention in pancreatic disorders:
Trauma to lumbar spine, especially at the level of L2 vertebra, may result in pancreatic neck injury
Retroperitoneal inflammation of acute pancreatitis spreads easily into perinephric and paracolic spaces
Surgical obstructive jaundice occurs early in periampullary cancers because of common bile duct obstruction
Pancreatic head cancers manifest with surgical obstructive jaundice and gastric outlet obstruction because of involvement of the common bile duct and duodenum
The Cattell-Braasch maneuver is a downward (inferior) mobilization of the hepatic flexure of colon and right transverse colon to expose the head of pancreas
Kocherization is the mobilization of the second part (C loop) of the duodenum anterior and to the left to mobilize the head of the pancreas; the inferior vena cava (IVC) and left renal vein are encountered posteriorly
Opening the lesser sac by division of the gastrocolic ligament exposes the body and tail of the pancreas
Tail of the pancreas may get injured during splenectomy
Cystogastrostomy for pancreatic pseudocysts is feasible because of the proximity of the posterior wall of the stomach to the anterior wall of a pancreatic pseudocyst, which lies in the lesser sac
During lateral pancreatico-jejunostomy (LPJ) for chronic pancreatitis, the incision in the main pancreatic duct (MPD) should stop a few mm short of the pancreatico-duodenal groove to avoid injury to the anterior pancreaticoduodenal arcade of vessels
Pancreaticoduodenectomy is required for resection of pancreatic head and periampullary cancers because of the shared blood supply between the head of pancreas and second part (C loop) of the duodenum
Duodenum-preserving pancreatic head resection (DPPHR) may be performed for chronic pancreatitis with head mass; a thin rim of pancreatic tissue along the duodenal C loop containing the pancreato-duodenal arcade of vessels
Infiltration of the superior mesenteric vessels (vein more often than artery) and the portal vein in pancreatic cancer (head and uncinate process) makes them unresectable
Pancreato-duodenal arcade acts as collateral between celiac axis and superior mesenteric artery. During pancreaticoduodenectomy, gastroduodenal artery should first be temporarily occluded with a bull dog clamp before its ligation and division; absence of proper hepatic artery pulsations after GDA occlusion indicates celiac artery stenosis
Infiltration of the middle colic vessels in the transverse mesocolon in the pancreatic head cancer may necessitate resection of the transverse colon
Aberrant and accessory right hepatic arteries arising from the superior mesenteric artery (SMA) may be at risk during pancreaticoduodenectomy
The pancreatic duct lies nearer to the upper border and the posterior surface of the pancreatic neck stump after pancreaticoduodenectomy.
The margins of a pancreaticoduodenectomy specimen include the inferior uncinate mesentery (containing the inferior pancreaticoduodenal artery and veins) and the posterior pancreatic mesentery (containing the posterior superior pancreaticoduodenal artery and veins; anterior superior pancreaticoduodenal artery arises from the gastroduodenal artery and lies in front of pancreas).
Thoracoscopic splanchnicectomy is used for relief of intractable pain in unresectable pancreatic cancer
The tip of the pancreatic tail at the splenic hilum is liable to injury during splenectomy.