Pancreas

The pancreas is an elongated, accessory digestive gland that lies retroperitoneally, overlying and transversely crossing the bodies of the L₁ and L₂ vertebra (the level of the transpyloric plane) on the posterior abdominal wall. It lies posterior to the stomach between the duodenum on the right and the spleen on the left. The transverse mesocolon attaches to its anterior margin. The pancreas produces:

• an exocrine secretion (pancreatic juice from the acinar cells) that enters the duodenum through the main and accessory pancreatic ducts.

• endocrine secretions (glucagon and insulin from the pancreatic islets [of Langerhans]) that enter the blood.

For descriptive purposes, the pancreas is divided into four parts: head, neck, body, and tail.

The head of the pancreas is the expanded part of the gland that is embraced by the C-shaped curve of the duodenum to the right of the superior mesenteric vessels just inferior to the transpyloric plane. It firmly attaches to the medial aspect of the descending and horizontal parts of the duodenum. The uncinate process, a projection from the inferior part of the pancreatic head, extends medially to the left, posterior to the SMA. The pancreatic head rests posteriorly on the IVC, right renal artery and vein, and left renal vein. On its way to opening into the descending part of the duodenum, the bile duct lies in a groove on the posterosuperior surface of the head or is embedded in its substance.

The neck of the pancreas is short (1.5-2 cm) and overlies the superior mesenteric vessels, which form a groove in its posterior aspect. The anterior surface of the neck, covered with peritoneum, is adjacent to the pylorus of the stomach. The SMV joins the splenic vein posterior to the neck to form the hepatic portal vein.

The body of the pancreas continues from the neck and lies to the left of the superior mesenteric vessels, passing over the aorta and L₂ vertebra, continuing just above the transpyloric plane posterior to the omental bursa. The anterior surface of the body of the pancreas is covered with peritoneum and lies in the floor of the omental bursa and forms part of the stomach bed. The posterior surface of the body is devoid of peritoneum and is in contact with the aorta, SMA, left suprarenal gland, left kidney, and renal vessels.

The tail of the pancreas lies anterior to the left kidney, where it is closely related to the splenic hilum and the left colic flexure. The tail is relatively mobile and passes between the layers of the splenorenal ligament with the splenic vessels.

The main pancreatic duct begins in the tail of the pancreas and runs through the parenchyma of the gland to the pancreatic head: here it turns inferiorly and is closely related to the bile duct. The main pancreatic duct and bile duct usually unite to form the short, dilated hepatopancreatic ampulla (of Vater), which opens into the descending part of the duodenum at the summit of the major duodenal papilla. At least 25% of the time, the ducts open into the duodenum separately.

The sphincter of the pancreatic duct (around the terminal part of the pancreatic duct), the sphincter of the bile duct (around the termination of the bile duct), and the hepatopancreatic sphincter (of Oddi) - around the hepatopancreatic ampulla - are smooth muscle sphincters that control the flow of bile and pancreatic juice into the ampulla and prevent reflux of duodenal content into the ampulla.

The accessory pancreatic duct opens into the duodenum at the summit of the minor duodenal papilla. Usually, the accessory duct communicates with the main pancreatic duct. In some cases, the main pancreatic duct is smaller than the accessory pancreatic duct and the two may not be connected. In such cases, the accessory duct carries most of the pancreatic juice.

The arterial supply of the pancreas is derived mainly from the branches of the markedly tortuous splenic artery. Multiple pancreatic arteries form several arcades with pancreatic branches of the gastroduodenal and superior mesenteric arteries. As many as 10 branches may pass from the splenic artery to the body and tail of the pancreas. The anterior and posterior superior pancreaticoduodenal arteries, branches of the gastroduodenal artery, and the anterior and posterior inferior pancreaticoduodenal arteries, branches of the SMA, form anteriorly and posteriorly placed arcades that supply the head of the pancreas.

Venous drainage from the pancreas occurs via corresponding pancreatic veins, tributaries of the splenic and superior mesenteric parts of the hepatic portal vein; most empty into the splenic vein.

The pancreatic lymphatic vessels follow the blood vessels. Most vessels end in the pancreaticosplenic lymph nodes, which lie along the splenic artery. Some vessels end in the pyloric lymph nodes. Efferent vessels from these nodes drain to the superior mesenteric lymph nodes or to the celiac lymph nodes via the hepatic lymph nodes.

The nerves of the pancreas are derived from the vagus and abdominopelvic splanchnic nerves passing through the diaphragm. The parasympathetic and sympathetic fibers reach the pancreas by passing along the arteries from the celiac plexus and superior mesenteric plexus. In addition to sympathetic fibers that pass to blood vessels, sympathetic and parasympathetic fibers are distributed to pancreatic acinar cells and islets. The parasympathetic fibers are secretomotor, but pancreatic secretion is primarily mediated by secretin and cholecystokinin, hormones formed by the epithelial cells of the duodenum and proximal intestinal mucosa under the stimulus of acid contents from the stomach.