The pancreas is an accessory digestive gland with exocrine and endocrine functions. It is an elongated pinkish gland, generally between 12 and 15cm in length. It is a largely retroperitoneal structure which is closely related to the stomach, spleen and duodenum. Disorders of the pancreas are frequently encountered in clinical practice.
The pancreas begins to develop at 5 weeks in-utero. It arises as two separate outpouchings of the duodenum, a dorsal and ventral pancreatic bud. As the organs begin to enlarge, the ventral bud begins to rotate to fuse with the dorsal bud to form one pancreas. The ductal system initially begins as two separate outflow systems, each contained in the ventral and dorsal bud. As the ventral bud and the dorsal bud begin to join, the corresponding ducts fuse to form the major pancreatic duct. The pancreatic duct will eventually join the bile duct, which will empty at the ampulla of Vater into the second (descending) part of the duodenum. In the adult pancreas, the uncinate process is derived from the ventral bud and the head, body and tail from the dorsal bud.
The pancreas lies deep within the abdominal cavity and is therefore impalpable. The majority sits in the epigastrium with the tail advancing into the left hypochondrium. The surface anatomy of the pancreas is best described in relation to the transpyloric plane. The neck of the pancreas lies anterior to the L1 and L2 vertebrae along the transpyloric plane. The head is to the right and inferior to this, the body and tail run to the left and superiorly.
Macro-Anatomy (e.g. lobes, surfaces, impressions etc)
The pancreas is anatomically divided into five regions: head, uncinate process, neck, body and tail. The head sits in close relation to the C-shaped curvature of the duodenum. The uncinate process is an extension of the inferior portion of the head, the superior mesenteric artery lies anterior to the uncinate process. The neck overlies the superior mesenteric vessels, which sit in a groove on the posterior aspect of the neck. The neck continues to form the body of the pancreas which is to the left of the superior mesenteric vessels. The tail is closely related to the hilum of the spleen and the left colic flexure and is the most mobile portion of the pancreas. It passes through the layers of the splenorenal ligament.
The ductal system of the pancreas starts with the small intercalated ducts that drain into the intralobular collecting ducts then into the main pancreatic duct to transport pancreatic secretions. The main pancreatic duct traverses the pancreas and joins the common bile duct. The ampulla of Vater is an enlargement where the two ducts join, and open into the duodenum. The sphincter of Oddi controls the secretions into the duodenum by acting as a valve.
A capsule of connective tissue covers the pancreas and partitions the gland into lobules by way of septa. The majority of the pancreas is composed of exocrine tissue, with only 1% of the total volume taken up by the endocrine portion. The endocrine pancreas consists of the Islets of Langerhans arranged in clusters. The exocrine portion is made up of acini which are cells responsible for the secretion of digestive enzymes into the intralobular ducts which eventually join the main pancreatic duct.
The majority of the arterial blood supply is derived from the splenic artery, the largest branch of the coeliac trunk. The splenic artery traverses the posterior surface of the pancreas to supply the tail before reaching the splenic hilum. The inferior and superior pancreaticoduodenal arteries (branches of the superior mesenteric and gastroduodenal arteries respectively) mainly supply the head and body of the pancreas. The body and neck of pancreas drain into the splenic vein, and the head into the superior mesenteric vein and portal vein. This means that the exocrine hormones the pancreas produces arrive directly at the liver.
The lymphatics follow the major arteries supplying the pancreas, with drainage to the coeliac and superior mesenteric lymph nodes.
Both parasympathetic and sympathetic nerves fibres pass alongside the arteries of the coeliac and superior mesenteric plexuses. Parasympathetic innervation is via the posterior vagus nerve and stimulates pancreatic secretions. Autonomic sympathetic nerve supply is derived from the coeliac, superior mesenteric and hepatic plexi.
The pancreas has both exocrine and endocrine functions and is involved in metabolism and glucose homeostasis. The exocrine pancreas secretes enzymes to aid digestion and bicarbonate and water to neutralise the acidity of gastric secretions.
The exocrine pancreas is responsible for the release of:
- Trypsinogen: converted to the active form trypsin in the small bowel, and is a protease.
- Amylase: breaks down starch to maltose.
- Lipase: hydrolyses triglycerides to aid later digestion of fats.
- Bicarbonate ions: secreted from the ductal epithelial cells.
The control of exocrine secretion is mediated by three main hormones. Cholecystokinin, secreted by the duodenum, triggers pancreatic juice secretion. Secretin is released in response to acid being released into the duodenum and this triggers bicarbonate and water release from the ductal pancreatic cells. Gastrin is secreted by the stomach in response to gastric dilatation and causes the secretion of pancreatic enzymes. The exocrine secretions are drained via the pancreatic duct into the duodenum.
The primary function of the endocrine portion of the pancreas is glucose homeostasis. The differing cell types within the Islets of Langerhans are responsible for hormone release. The hormones involved are:
- Glucagon: released by alpha cells, converts glycogen to glucose.
- Insulin: from beta cells, converts glucose to glycogen.
- Somatostatins: from delta cells, reduces glucagons, insulin and growth hormone.
Development: foregut, as dorsal and ventral outpouchings of the duodenal endoderm.
Artery: splenic, inferior and superior pancreaticoduodenal arteries.
Vein: into the portal vein via inferior and superior pancreaticoduodenal arteries.
Lymph: coeliac and superior mesenteric lymph nodes.
Nerve: parasympathetic and sympathetic innervation from the vagus.
The pancreas is a retroperitoneal gland with exocrine and endocrine functions. It has a rich blood supply and is related to the stomach, spleen and duodenum. Pancreatic enzyme secretion is triggered by hormone release in response to food in the stomach. Pancreatitis and pancreatic cancer are commonly encountered in surgical practice and an understanding of the anatomy and physiology is useful to predict the symptoms and complications of both.
Essential Clinical Anatomy. 3rd edition. Moore and Agur.