The Pancreas: Anatomy, Physiology, Biology, and Histology

Anatomy: the structure

The pancreas lies retroperitoneally across the posterior abdominal wall at roughly the L1 to L2 level, behind the stomach. It is described in four parts: the head nestled in the C-shaped curve of the duodenum, with the uncinate process hooking behind the superior mesenteric vessels; the neck; the body crossing the midline; and the tail, the only part that is intraperitoneal, reaching into the splenorenal ligament toward the splenic hilum. This position explains why pancreatic pain often radiates to the back.

The main pancreatic duct, the duct of Wirsung, runs the length of the organ collecting side branches and joins the common bile duct at the ampulla of Vater, where the sphincter of Oddi controls release into the second part of the duodenum. An accessory duct of Santorini often persists and drains separately into the minor papilla. This shared final pathway with the biliary tree is the anatomical reason gallstones can cause pancreatitis.

Blood supply reflects the organ's foregut and midgut boundary. The head is supplied by superior and inferior pancreaticoduodenal arteries, arising from the gastroduodenal and superior mesenteric arteries respectively, forming arcades shared with the duodenum, which is why the two cannot easily be separated surgically. The body and tail are supplied by branches of the splenic artery. Venous drainage is into the portal system, so insulin released from islets reaches the liver at high concentration before entering the general circulation.

Physiology: how it works

The exocrine pancreas secretes about 1.5 litres of alkaline juice daily with two distinct components. Duct cells secrete bicarbonate under the influence of secretin, released when acid chyme enters the duodenum, raising duodenal pH toward the range where pancreatic enzymes work. Acinar cells secrete the enzymes themselves under the influence of cholecystokinin, released in response to fat and protein, and vagal stimulation. Together these cover every macronutrient: amylase for starch, lipase with colipase for triglycerides, and trypsinogen, chymotrypsinogen, procarboxypeptidase, and elastase for proteins, alongside nucleases.

Protective inactivation is critical. Proteases are secreted as inactive zymogens and only activated once they reach the duodenum, where enterokinase from the brush border converts trypsinogen to trypsin, which then activates the remaining zymogens in a cascade. Acinar cells additionally produce a trypsin inhibitor, and secretory granules keep enzymes physically separated from the cytoplasm. Amylase and lipase are exceptions, secreted in active form, which is why they need no such handling.

The endocrine pancreas regulates fuel in both directions. Beta cells sense rising glucose through GLUT2 uptake and glucokinase phosphorylation; the resulting ATP closes ATP-sensitive potassium channels, depolarises the cell, opens voltage-gated calcium channels, and triggers biphasic insulin release. Insulin promotes glucose uptake through GLUT4 in muscle and adipose tissue, glycogen and triglyceride synthesis, and protein anabolism. Alpha cells release glucagon when glucose falls, driving hepatic glycogenolysis and gluconeogenesis. Delta cells release somatostatin, which restrains both, and the islet arrangement allows these cells to regulate one another locally as well as systemically.

Biology: the living cells

Acinar cells are among the most protein-productive cells in the body and look the part: intensely basophilic basal cytoplasm packed with rough endoplasmic reticulum, a prominent Golgi apparatus, and an apex crowded with eosinophilic zymogen granules waiting for the signal to exocytose. Centroacinar cells, the beginning of the duct system, sit within the acinus itself and are unique to the pancreas. Duct cells carry the CFTR chloride channel that drives bicarbonate secretion, which is why cystic fibrosis produces pancreatic insufficiency. The islets contain at least four endocrine populations: beta cells, about 65 to 80 percent and mostly central, producing insulin and C-peptide from proinsulin; alpha cells, about 15 to 20 percent and more peripheral, producing glucagon; delta cells, about 5 percent, producing somatostatin; and PP cells producing pancreatic polypeptide. Islets are far more richly vascularised than the surrounding exocrine tissue, with blood flowing from the core outward, allowing insulin to influence the alpha cells it passes.

Histology: under the microscope

Pancreatic histology is one of the easiest slides to recognise, because the two functional systems look completely different at low power. Dark purple, densely packed serous acini fill most of the field, while pale, rounded, well-vascularised islets of Langerhans stand out among them like clearings. The exocrine tissue is a compound acinar gland divided into lobules by connective tissue septa carrying ducts, vessels, and nerves. Duct calibre increases from intercalated ducts, beginning with centroacinar cells, to intralobular and then interlobular ducts, with epithelium rising from simple cuboidal to columnar. Distinguishing the pancreas from the parotid gland, its closest histological mimic, comes down to two features: the pancreas has islets and centroacinar cells, and it lacks the striated ducts and adipose infiltration typical of the parotid. Specific islet cell types cannot be told apart on routine staining and require immunohistochemistry for insulin, glucagon, or somatostatin.

Clinical correlations

Acute pancreatitis is autodigestion. Gallstones obstructing the ampulla and alcohol account for most cases, and the mechanism is premature intracellular activation of trypsinogen, which triggers the whole zymogen cascade inside the gland. Serum lipase, more specific than amylase, rises within hours. Because the pancreas is retroperitoneal, released enzymes track into the retroperitoneum, and severe cases can produce fat necrosis, hypocalcaemia through calcium soap formation, and systemic inflammatory response.

Chronic pancreatitis destroys acinar tissue and replaces it with fibrosis, producing exocrine insufficiency with steatorrhoea and fat-soluble vitamin deficiency once roughly 90 percent of function is lost. Because islets are relatively resistant, endocrine failure usually appears later. This ordering is diagnostically useful: exocrine symptoms before diabetes suggests parenchymal loss rather than primary islet disease.

Diabetes mellitus separates by mechanism. Type 1 is autoimmune destruction of beta cells, so insulin is absent and treatment must replace it. Type 2 begins with peripheral insulin resistance and relative insufficiency, with beta cell function declining over time. Pancreatic ductal adenocarcinoma, most often in the head, presents late and characteristically with painless obstructive jaundice as the tumour compresses the common bile duct, which is why prognosis remains poor despite the organ being anatomically accessible.