Pancreas Anatomy

The pancreas, named after the Greek words pan (all) and kreas (flesh), is a 12-15-cm long J-shaped (like a field 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]} It spans from the C-shaped curve of the duodenum on the right to the splenic hilum on the left, with its head nestled within the duodenal curve and its tail in close proximity to the spleen. The average volume of the pancreas is 70-80 cm³ (with a range of 40-170 cm³) in adults. The pancreatic volume tends to be greater in males than in females. ^[6]

The pancreas serves both endocrine and exocrine functions: ^[7]

• The endocrine component comprises the islets of Langerhans, which are distributed throughout the pancreas and constitute about 2% of its mass. These islets vary in size, with a majority being in the range of 0.05-0.25 mm. The largest ones can range in size from 0.5 to 0.7 mm. They secrete various hormones, including insulin (beta-cells), glucagon (alpha cells), somatostatin (delta cells), and pancreatic polypeptide (PP cells).
• The exocrine component consists of acini that are composed of roughly spherical clusters of pyramidal cells. They produce digestive enzymes, which are transported via the pancreatic ducts into the duodenum to aid in the digestion of proteins, fats, and carbohydrates. 

Pancreas anatomy, in situ. Image shows the parts of the pancreas: uncinate process, head, neck, body, and tail.

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Embryology

The pancreas develops as two 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 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).

Studies highlight functional differences between the pancreatic head and tail due to their distinct embryological origins. The head derives from both dorsal and ventral pancreatic buds, while the body and tail originate solely from the dorsal bud. This developmental distinction influences islet cell function, with islets in the dorsal bud showing superior insulin secretion capacity. ^[8]

Anatomic anatomy

The pancreas is arbitrarily divided into the head, uncinate process, neck, body, and tail. The pancreatic head and neck constitute about 50% and the body and tail constitute the remaining 50% of the pancreatic parenchymal mass.

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. On the cut surface of the pancreas at its neck, the main pancreatic duct lies closer to the superior border and the posterior surface.

The head of the pancreas lies in a duodenal C-loop in front of the inferior vena cava 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; its size varies and it is wedged between the superior mesenteric vessels (vein on right, and artery on left) in front and the aorta behind it.

Pancreas in situ with the superior mesenteric artery (SMA) and superior mesenteric vein (SMV) coursing anterior to the uncinate process of the pancreas.

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Pancreas posterior view, showing pancreas parts: head (blue), uncinate process (yellow), body (green), and tail (red).

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The lower (terminal) part of the common bile duct runs behind (or sometimes through) the upper half of the head of pancreas before it joins the main pancreatic duct of Wirsung to form a common channel (ampulla), which opens at the papilla on the medial wall of the second part of the duodenum.

The neck of the pancreas is about 2 cm wide and lies in front of the superior mesenteric vein (SMV), splenic vein, and portal vein junction. ^[6] The pancreatic neck is the arbitrary junction between the head and body of the pancreas. It is often the most anterior part of the pancreas. ^[6] Portal vein lies behind the neck of the pancreas; no tributaries drain from the posterior surface of the pancreas into the anterior surface of the portal vein; therefore, a tunnel can be easily created behind the neck of the pancreas before its division.

The body and tail of the pancreas run obliquely upward to the left in front of the aorta and left kidney. The body of the pancreas lies anterior to these structures and is covered by peritoneum. ^[9] The narrow tip of the pancreas tail reaches the splenic hilum in the splenorenal (lienorenal) ligament. It is closely associated with splenic vessels as they pass posteriorly. ^[9]

The duodenum (25 cm long) is horseshoe-shaped, with its inferior limb longer than the superior, and has four 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 duodenal border of the head of the pancreas is flattened and slightly concave. It is adherent to the descending part of the duodenum, primarily around the hepatopancreatic ampulla (ampulla of Vater). The lower border of the head is superior to the horizontal part of the duodenum and is continuous with the uncinate process. ^[6]

The transverse mesocolon (with the middle colic vessels in it) is attached to the anterior surface of the lower (inferior) part of the body and pancreas tail; thus, most of the gland is located in the supracolic compartment. The body and tail of the pancreas lie in the lesser sac (omental bursa) behind the stomach.

The main pancreatic duct begins at the tail and traverses through all regions of the pancreas before merging with the common bile duct at the ampulla of Vater. An accessory pancreatic duct (Santorini's duct) may also be present in some individuals. ^[9]

Radiological anatomy

The pancreas is an elongated retroperitoneal organ that lies obliquely across the upper abdomen at the level of the transpyloric plane. ^[9]  The head of the pancreas lies in front of L2 vertebra, the body lies in front of L1 vertebra, and the tail lies at the level of T12 vertebra, near splenic hilum ^[9] ; pancreatic parenchymal calcification/ductal calculi in patients with chronic pancreatitis is seen at these levels on plain radiography of the abdomen.

CT anatomy

The pancreas is best evaluated with a triphasic (arterial, portal venous, and systemic venous phases), contrast-enhanced (after intravenous injection of contrast medium), computed tomography (CT) scan with three-dimensional (3D), 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 one 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 diameter) and proximal body (2-3 mm diameter) of the pancreas on CT scan. See the images below.

Computed tomography (CT) scan showing the pancreas head (*) and the superior mesenteric artery (black arrow) and vein (white arrow).

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CT scan of the uncinate process of the pancreas (*) behind the superior mesenteric vessels (arrow).

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CT scan of the body of the pancreas (*) with the splenic vein (arrow) behind it.

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CT scan of the tail of the pancreas (*) reaching the hilum of the spleen (arrow).

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Endoscopic anatomy

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 nipple-like 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 two pancreatic ducts (main and accessory) often communicate with each other.

Image shows main pancreatic duct (orange) and common bile duct (green) emptying into the 2nd part of the duodenum.

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Endoscopic ultrasonography anatomy

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, 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.

The pancreatic head, including its uncinate process, is best visualized from the descending duodenum (D2/D3), where key structures such as the ampulla of Vater and convergence of the bile and pancreatic ducts are also assessed. The pancreatic body and tail are typically visualized from the stomach. ^[10]

EUS provides detailed imaging of both the main pancreatic duct (duct of Wirsung) and accessory pancreatic duct (duct of Santorini), along with their terminal portions within the head of the pancreas. This is critical for identifying ductal anomalies, strictures, or dilatations. ^[11]

Blood supply

Pancreas derives a rich blood supply from both celiac axis and superior mesenteric artery, with collaterals between the two systems; therefore, when angiography is performed 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. 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 in front of the uncinate process and the third (horizontal) part of the duodenum to enter the small bowel mesentery.

The gastroduodenal artery, 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 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 first, second, and third parts of the duodenum (vasa recta duodeni). Multiple pancreatic branches (including a dorsal pancreatic artery, 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 drain into the portal vein and inferior pancreaticoduodenal veins drain into the 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 third 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 joins the splenic vein (or the junction of the splenic vein and SMV, or even SMV). The portal vein receives the superior pancreaticoduodenal veins, right gastro-omental (gastroepiploic vein, left gastric vein, and right gastric vein; 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.

Lymphatic drainage

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]}

Nerve supply

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]}

Sensory fibers from both sympathetic and parasympathetic pathways contribute to the innervation of the pancreas. These fibers are involved in detecting chemical and mechanical changes within the organ and play a role in pain perception and inflammatory responses. Visceral afferents convey sensory information via the inferior (nodose) ganglion of the vagus nerve (terminating in the solitary tract) and via the celiac plexus and greater thoracic splanchnic nerves (terminating in the dorsal horn of the spinal cord). ^{[6, 12]}

The pancreas also contains an intrinsic neural network composed of intrapancreatic ganglia scattered throughout its parenchyma. These ganglia act as local integration centers for autonomic input and directly regulate pancreatic functions. They receive inputs from vagal preganglionic neurons, sympathetic postganglionic neurons, sensory fibers, and enteropancreatic interneurons. ^[13]

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.

Acinar cells are highly polarized, with basophilic cytoplasm at their base due to abundant rough endoplasmic reticulum and eosinophilic zymogen granules at their apex. These granules store digestive proenzymes such as trypsinogen, pro-lipase, and amylase, which are secreted into the ducts. ^[8]

Under stimulation of secretin and cholecystokinin, 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), pancreatic polypeptide cells (secrete pancreatic polypeptide), and epsilon cells (secrete ghrelin) ^[14] and several other hormone-secreting cells. Islets constitute only about 2% of pancreatic parenchyma.

Natural 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 may arise from the SMA and accessory right hepatic artery (in addition to the normal one from CHA) from the SMA. ^{[1, 2, 3, 4, 5]}

Pathophysiologic variants

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 bypass in the form of 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 (Wirsung) and accessory (Santorini) 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 (of Santorini).

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 cysts and carries a higher risk for biliary malignancy. APBDJ/ APBDU without cystic dilatation of the common bile duct carries a high risk for gallbladder cancer and is an indication for preventive cholecystectomy.

Accessory pancreatic tissue may be present in the stomach, small intestine, Meckel diverticulum, omentum, and hilum of spleen as soft yellow nodules/lobules.

Polycystic disease may involve the pancreas in addition to the more commonly involved organs (i.e., liver and kidneys).

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 forward. Transmitted aortic pulsations can be seen and felt in pancreatic masses (tumors and cysts) as the pancreas lies on the aorta.

Imaging considerations

The pancreas is difficult to visualize on ultrasonography as it lies behind the stomach and within the C loop of the duodenum.

Inflammatory thickening of the anterior layer of the left perirenal (Gerota) fascia is seen on CT scanning in patients with acute pancreatitis.

Using a side-viewing endoscope, 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.

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 CT scans). EUS can also be used to detect early changes in chronic pancreatitis and diagnose pancreas divisum and for guided fine-needle aspiration cytology from pancreaticoduodenal lymph nodes.

High-resolution optical imaging methods such as optical projection tomography and light sheet fluorescence microscopy are being explored in research settings to provide detailed 3D visualization of pancreatic cellular structures. These techniques show promise for studying beta-cell mass distribution and vascularization but are not yet standard clinical tools. ^[15]

Surgical considerations

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.
• Pain of pancreatic origin may radiate to the back because of its retroperitoneal location.
• Pancreatic head cancers manifest with surgical obstructive jaundice and gastric outlet obstruction owing to involvement of the common bile duct and duodenum.
• The Cattell-Braasch maneuver is the 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 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.
• The most common site of fluid collection/pseudocyst formation in acute pancreatitis is the lesser sac.
• 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 for chronic pancreatitis, the incision in the main pancreatic duct 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 the 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 may be performed for chronic pancreatitis with head mass; this procedure involves the removal of part of the pancreas while preserving the duodenum, stomach, and biliary tree. Preserving a thin rim of pancreatic tissue along the duodenal C loop containing the pancreato-duodenal arcade of vessels helps maintain pancreatic and duodenal functions.
• 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 a collateral between the celiac axis and superior mesenteric artery. During pancreaticoduodenectomy, gastroduodenal artery should first be temporarily occluded with a bulldog clamp before its ligation and division; absence of proper hepatic artery pulsations after the gastroduodenal artery occlusion indicates celiac artery stenosis.
• Infiltration of the middle colic vessels in the transverse mesocolon in pancreatic head cancer may necessitate resection of the transverse colon.
• Aberrant and accessory right hepatic arteries arising from the 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 IPDA and veins) and the posterior pancreatic mesentery (containing the posterior SPDA and veins; the anterior SPDA arises from the gastroduodenal artery and lies in front of pancreas).
• Thoracoscopic splanchnicectomy is used for relief from intractable pain in unresectable pancreatic cancer.
• The tip of the pancreatic tail at the splenic hilum is liable to injury during splenectomy.