Pancreas

The pancreas is a vital organ of the digestive and endocrine systems in vertebrates, performing an essential dual role in nutrient digestion and metabolic regulation. In humans it lies within the upper abdomen, positioned behind the stomach and extending towards the spleen. As a mixed gland, it is composed primarily of exocrine tissue with a smaller yet critical endocrine component, both of which contribute to the maintenance of digestive efficiency and blood glucose homeostasis.

General Functions and Physiological Roles

The majority of the pancreas functions as an exocrine gland, constituting approximately ninety-nine per cent of its tissue. This portion secretes pancreatic juice into the duodenum via the pancreatic duct system. The secretion contains bicarbonate ions to neutralise gastric acid entering from the stomach, and a range of digestive enzymes—including lipases, proteases and amylases—which facilitate the breakdown of lipids, proteins and carbohydrates.
The endocrine component comprises about one per cent of the organ and regulates blood sugar levels. Specialised cells of the pancreatic islets secrete hormones such as insulin, glucagon, somatostatin and pancreatic polypeptide, each contributing to metabolic regulation. Dysfunction within this system plays a central role in the development of diabetes mellitus.

Structural Organisation

Anatomical Position and Gross Structure

In adults the pancreas measures roughly 15 centimetres in length and appears salmon-coloured. It is anatomically divided into four principal regions:

• Head: Located within the curvature of the duodenum, it envelopes the superior mesenteric vessels. A small extension, the uncinate process, passes posterior to these vessels.
• Neck: A short segment approximately 2 centimetres wide, lying anterior to the formation of the portal vein and adjacent to the pylorus of the stomach.
• Body: The largest section, extending across the abdomen behind the stomach. It lies over major vessels including the aorta and splenic vein.
• Tail: Narrow and tapering, this portion reaches towards the spleen and sits within the splenorenal ligament.

Two ducts traverse the pancreatic parenchyma: the main pancreatic duct, which merges with the common bile duct at the ampulla of Vater, and the accessory pancreatic duct, which opens separately into the duodenum. The ampulla is controlled by the sphincter of Oddi, while the common bile duct opening is regulated by the sphincter of Boyden.

Blood and Lymphatic Supply

The organ receives a rich arterial supply from branches of the coeliac trunk and superior mesenteric artery. The splenic artery supplies the body and tail via numerous pancreatic branches, whereas the superior and inferior pancreaticoduodenal arteries serve the head. Venous drainage mirrors the arterial pattern, with the body and neck draining into the splenic vein and the head into tributaries of the portal system.
Lymphatic drainage is equally extensive, with vessels accompanying the arteries to reach splenic, pancreaticoduodenal and mesenteric lymph nodes.

Microanatomy and Cellular Composition

At the microscopic level the pancreas consists of two distinct cellular systems:

• Exocrine acini: Clusters of pyramid-shaped cells surrounding small intercalated ducts. These cells contain abundant endoplasmic reticulum and zymogen granules storing inactive digestive enzymes. The duct system progresses from intercalated to intralobular and finally to interlobular ducts, transitioning from simple columnar epithelium to multilayered configurations as the ducts enlarge.
• Endocrine islets (islets of Langerhans): Scattered throughout the parenchyma, each islet contains up to several thousand cells supplied by arterioles and venules. Key cell types include:
  • Alpha cells at the islet periphery, secreting glucagon.
  • Beta cells more centrally located, responsible for insulin secretion.
  • Delta cells, which release somatostatin.
  • Additional enterochromaffin cells dispersed within the islets.

These endocrine cells secrete hormones directly into the bloodstream, enabling rapid systemic effects.

Developmental Variation and Anomalies

The pancreas forms embryologically from two buds arising from opposite sides of the developing duodenum. These rotate and fuse, with their ducts typically merging. Several variations may occur:

• Accessory pancreatic duct formation if the dorsal duct persists.
• Pancreas divisum, where the dorsal and ventral ducts fail to fuse; this is usually clinically silent.
• Annular pancreas, resulting from incomplete rotation of the ventral bud, may encircle the duodenum and is sometimes associated with duodenal atresia.

The size and exact morphology of the adult pancreas can vary widely without impairing function.

Gene and Protein Expression

Approximately half of all human protein-coding genes are expressed within the pancreas, reflecting its diverse functional repertoire. Fewer than one hundred genes show pancreas-specific expression, primarily encoding secreted proteins. Many of these relate to digestive activity—for example chymotrypsinogen and pancreatic lipase—whereas others are associated with hormone regulation in the endocrine compartment.

Clinical Significance

Pancreatitis and Digestive Disorders

Pancreatitis, an inflammatory condition of the pancreas, may be acute or chronic. Common causes include heavy alcohol consumption, gallstones and certain medications. Severe inflammation can impair both exocrine and endocrine functions, leading to digestive difficulties and metabolic disturbances.

Diabetes Mellitus

The discovery of the pancreas’s role in blood sugar control dates to the late nineteenth century, culminating in the identification of insulin in 1921. Damage to beta cells or failure of insulin production underlies type 1 diabetes, while impaired insulin action characterises type 2 diabetes. Ongoing research continues to explore pancreatic islet biology to improve therapeutic strategies.

Pancreatic Cancer

Pancreatic cancer often arises in the exocrine tissue and is strongly associated with chronic pancreatitis, smoking and genetic factors. It typically presents at an advanced stage due to its deep anatomical location and nonspecific symptoms, leading to a poor prognosis.
The pancreas thus constitutes a complex and indispensable organ, integral to digestion, metabolic regulation and endocrine signalling. Its structural intricacy, diverse cellular functions and susceptibility to significant disease underscore its importance within human physiology.