The pancreas

The pancreas is a long, slender organ, most of which is located posterior to the bottom half of the stomach.Although it is primarily an exocrine gland, secreting a variety of digestive enzymes, the pancreas also has endocrine cells. 

The pancreas is an extended, accessory digestive gland that is found retroperitoneally, crossing the bodies of the L1 and L2 vertebrae on the posterior abdominal wall. The pancreas lies transversely in the upper abdomen between the duodenum on the right and the spleen on the left. It is divided into the head, neck, body, and tail. The head lies on the inferior venacava and the renal vein and is surrounded by the C loop of the duodenum. The tail of the pancreas extends up to the splenic hilum. The pancreas produces an exocrine secretion (pancreatic juice from the acinar cells) which then enters the duodenum through the main and accessory pancreatic ducts and endocrine secretions (glucagon and insulin from the pancreatic islets of Langerhans) that enter the blood.

Development of pancreas in utero is a complex process resulting in a variety of anomalies, many of which are asymptomatic but some of which can produce symptoms. At week 4, the dorsal and ventral buds of the forgut grow and it is from these that the pancreas develops. The ventral bud develops to the right of the duodenum, while the dorsal bud develops to the left. In order for the pancreas to arrive at its final position, the ventral bud must rotate and this occurs at week 6. The dorsal duct drains the body tail and superior portion of the head via the minor papilla. The ventral duct drains the uncinate process and inferior portion of the head into the major papilla. Around week 7, the dorsal duct fuses with the ventral duct and the pancreas is drained through the ventral duct into the major papilla. The distal segment of the dorsal duct regresses, forming an accessory duct of Santorini. The pancreas drains through the ventral duct (of Wirsung) and the major papilla.

The dorsal duct may be partially or completely obliterated and the minor papilla is often not patent. If obliteration of the dorsal duct is incomplete, it will persist as an accessory pancreatic duct (of Santorini). However, there are many variations upon this normal arrangement.

Exocrine pancreas, the portion of the pancreas that makes and secretes digestive enzymes into the duodenum. This includes acinar and duct cells with associated connective tissue, vessels, and nerves. The exocrine components comprise more than 95% of the pancreatic mass.The enzymes secreted by the exocrine gland in the pancreas help break down carbohydrates, fats, and proteins. These enzymes go down the pancreatic duct into the bile duct in an inactive form. When they go into the duodenum, they are activated. The exocrine tissue also secretes a bicarbonate. It neutralizes stomach acid in the duodenum.

Endocrine pancreas, the portions of the pancreas (the islets) that make and secrete insulin, glucagon, somatostatin and pancreatic polypeptide into the blood. Islets comprise 1-2% of the pancreatic mass.

Insulin synthesis and secretion occurs in pancreatic islet cells. Insulin has many sites of action, including the liver, where it inhibits gluconeogenesis, skeletal muscle, where it increases glucose uptake, and adipose tissue, where it increases glucose uptake and inhibits lipolysis. Also notable are the systemic anabolic effects whereby insulin increases amino acid uptake, RNA, DNA, protein synthesis, and cell growth. Insulin is produced exclusively in the beta cells of the pancreatic islets in mammals, and the Brockmann body in some fish.

Glucagon has a major role in maintaining normal concentrations of glucose in blood, and is often described as having the opposite effect of insulin. That is, glucagon has the effect of increasing blood glucose levels. Glucagon is a linear peptide of 29 amino acids. Its primary sequence is almost perfectly conserved among vertebrates, and it is structurally related to the secretin family of peptide hormones. Glucagon is synthesized as proglucagon and proteolytically processed to yield glucagon within alpha cells of the pancreatic islets. 

When blood glucose levels are high, large amounts of glucose are taken up by the liver. Under the influence of insulin, much of this glucose is stored in the form of glycogen. Later, when blood glucose levels begin to fall, glucagon is secreted and acts on hepatocytes to activate the enzymes that depolymerize glycogen and release glucose.

The delta cell accounts for four percent of the islet cells and secretes the peptide hormone somatostatin. Recall that somatostatin is also released by the hypothalamus, stomach and intestines. An inhibiting hormone, pancreatic somatostatin inhibits the release of both glucagon and insulin. The pancreatic polypeptide cell (PP cell) accounts for about one percent of islet cells and secretes the pancreatic polypeptide hormone. It is thought to play a role in appetite, as well as in the regulation of pancreatic exocrine and endocrine secretions. Pancreatic polypeptide released following a meal may reduce further food consumption; however, it is also released in response to fasting.

The pancreas lies near several major vessels and significant landmarks in vascular anatomy: The aorta and inferior vena cava pass posteriorly to the head of the pancreas. The superior mesenteric artery lies behind the neck of the pancreas and anterior to the uncinate process. Posterior to the neck of the pancreas, the splenic and superior mesenteric veins unite to form the hepatic portal vein. As it journeys from its origin at the celiac plexus to the splenic hilum, the splenic artery traverses the superior border of the pancreas.

The pancreas is supplied by the pancreatic branches of the splenic artery. The head is additionally supplied by the superior and inferior pancreaticoduodenal arteries which are branches of the gastroduodenal (from coeliac trunk) and superior mesenteric arteries, respectively. Venous drainage of the head of the pancreas is into the superior mesenteric branches of the hepatic portal vein. The pancreatic veins draining the rest of the pancreas do so via the splenic vein. The pancreas is drained by lymphatic vessels that follow the arterial supply. They empty into the pancreaticosplenal nodes and the pyloric nodes, which in turn drain into the superior mesenteric and coeliac lymph nodes.

Published : Oct 21 2023