Pancreas

Pancreas

The pancreas is an elongated mixed gland lying across the posterior abdominal wall behind the stomach. About 98–99% of it is exocrine and secretes about 1–1.5 L of alkaline, enzyme-rich juice a day. The remaining 1–2% is the endocrine islets.

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Overview

Length / mass≈12–15 cm / ≈80–100 g

Exocrine : endocrine≈98–99% : 1–2%

Pancreatic juice≈1–1.5 L/day, pH ≈8

Islets≈1 million

Vertebral levelL1–L2 (transpyloric plane)

The pancreas is about 12–15 cm long and weighs about 80–100 g. It lies at the level of L1–L2, across the transpyloric plane, in the "stomach bed" behind the lesser sac. Except for its tail, it is secondarily retroperitoneal. It runs from the C-loop of the duodenum on the right to the splenic hilum on the left.

The head sits within the duodenal C-loop. The uncinate process hooks from the lower head behind the superior mesenteric vessels. The short neck lies in front of the point where the superior mesenteric and splenic veins unite to form the hepatic portal vein. The body crosses the aorta and L2, with the splenic vein running along its posterior surface and the tortuous splenic artery along its upper border. The tail runs in the splenorenal ligament to the splenic hilum.

The exocrine pancreas is a compound acinar gland. Serous acini secrete digestive enzymes, mostly as inactive zymogens. A branching duct system, from centroacinar cells and intercalated ducts to the main and accessory pancreatic ducts, adds a large volume of bicarbonate-rich fluid that neutralises gastric acid. Secretin drives the fluid and bicarbonate output, and CCK and vagal acetylcholine drive enzyme output.

The endocrine pancreas consists of about a million islets of Langerhans, most numerous in the tail. They contain β cells (insulin), α cells (glucagon), δ cells (somatostatin), PP cells and ε cells (ghrelin), and are described with the endocrine system.

Location: Epigastrium and left hypochondrium, behind the stomach and lesser sac, across L1–L2. The head lies in the duodenal C-loop and the tail at the splenic hilum.

Parts & subdivisions

Head and uncinate process
Inside the C-loop of the duodenum. The bile duct runs in a groove on its posterior surface. The uncinate process passes behind the SMV (and often the SMA).
Neck
Short segment in front of the portal vein formation (SMV + splenic vein).
Body
Crosses the aorta, SMA origin, left crus, left kidney and adrenal. Forms part of the stomach bed.
Tail
Intraperitoneal in the splenorenal ligament. Reaches the splenic hilum; at risk in splenectomy. Has the highest islet density.
Ducts
Main (Wirsung) and accessory (Santorini) pancreatic ducts.
Acini
Enzyme-secreting exocrine units.
Islets of Langerhans
Endocrine micro-organs (insulin, glucagon, somatostatin); about 1–2% of pancreatic mass.

Anatomy

Innervation: Parasympathetic: vagus (secretomotor to acini and islets). Sympathetic: greater and lesser splanchnic nerves via the coeliac and superior mesenteric plexuses (vasomotor). Visceral pain afferents (about T6–T10) give epigastric pain radiating to the back. Coeliac plexus block can relieve pancreatic cancer pain.
Arterial supply: Head: anterior and posterior superior pancreaticoduodenal arteries (from the gastroduodenal artery, coeliac territory) anastomose with the inferior pancreaticoduodenal artery (SMA), forming arcades. Neck, body and tail: branches of the splenic artery, including the dorsal pancreatic artery, the great pancreatic artery (arteria pancreatica magna) and caudal pancreatic arteries.
Venous drainage: Pancreaticoduodenal veins → SMV and portal vein; pancreatic veins → splenic vein. All drain to the hepatic portal vein.
Lymphatic drainage: Pancreaticosplenic, pancreaticoduodenal and pyloric nodes → coeliac and superior mesenteric nodes → para-aortic nodes.
Relations: Anterior: stomach (across the lesser sac), transverse mesocolon, and jejunal loops below. Posterior: IVC, right renal vessels and bile duct (head); SMV, SMA and left renal vein (neck and uncinate; the left renal vein lies between the aorta and SMA); aorta, left crus, left kidney and suprarenal (body); splenic vein along the back of the body.
Histology: Lobules of serous acini with basal basophilia and apical zymogen granules. Centroacinar cells inside the acini are a unique feature. Pale islets are scattered among the acini.
Development: Dorsal and ventral pancreatic buds form from foregut endoderm in week 5. The ventral bud rotates with the duodenum behind the gut to fuse with the dorsal bud in week 7. The ventral bud forms the uncinate process and lower head; the dorsal bud forms the rest of the head, the neck, body and tail. The main duct is made from the distal dorsal duct and the whole ventral duct; the proximal dorsal duct persists as the accessory duct.

Function

Exocrine: secretes about 1–1.5 L/day of pancreatic juice, containing proteases, amylase, lipases, nucleases and bicarbonate.
Neutralises acid chyme in the duodenum, giving the pH that pancreatic enzymes need.
Endocrine: glucose homeostasis through insulin and glucagon (islets).

Clinical significance

Acute pancreatitis

Mostly caused by gallstones and alcohol. Premature activation of trypsinogen leads to autodigestion. Patients have severe epigastric pain radiating to the back, often eased by leaning forward, and lipase or amylase more than 3× the upper limit of normal. Haemorrhagic forms produce Grey Turner and Cullen signs. Complications are necrosis, pseudocyst in the lesser sac, ARDS, hypocalcaemia and hyperglycaemia.

Pancreatic ductal adenocarcinoma

About two-thirds arise in the head. They present with painless obstructive jaundice, a palpable gallbladder (Courvoisier), weight loss, new-onset diabetes and migratory thrombophlebitis (Trousseau sign). CA 19-9 is raised. Only a minority are resectable (Whipple pancreaticoduodenectomy), and 5-year survival is about 10%.

Developmental anomalies

Pancreas divisum (failure of duct fusion, in about 5–10% of people) drains most of the gland through the minor papilla and may cause recurrent pancreatitis. Annular pancreas is a ring of ventral-bud tissue around the 2nd part of the duodenum that can obstruct it.

Mnemonics

Causes of acute pancreatitis: I GET SMASHED

Idiopathic, Gallstones, Ethanol, Trauma, Steroids, Mumps, Autoimmune, Scorpion venom, Hypercalcaemia/Hypertriglyceridaemia, ERCP, Drugs.

Compiled from standard anatomy and physiology references — see the Digestive System sources. For education only; not medical advice.