Duodenum, Pancreas and Abdominal Aorta LO1
1. Describe the basic structure of the duodenum and pancreas.
The duodenum and pancreas share a close proximity to one another, a common developmental pathway, and complementary functions.
The duodenum is the proximal part of the small intestine, which receives chyme from the stomach and secretions from the pancreas and liver/gallbladder. Owing to its relationships to the stomach, pancreas, and liver/gallbladder, the duodenum is the most anatomically constant part of the small intestine. The pancreas and the proximal third of the duodenum are derivatives of the embryonic foregut, whereas the distal two-thirds of the duodenum is a midgut derivative.
The duodenum is comprised of four parts, described by their relative positions. They include the:
- Superior (1st) part – horizontal, lies at L1 level ~ transpyloric plane;
- Descending (2nd) part – vertical, descends from L1 to L3 (anterior to the right kidney), contain the major & minor duodenal papillae;
- Inferior (3rd) part – horizontal , crosses L3 vertebra, is crossed by the SMA & SMV; and
- Ascending (4th) part – vertical, ascends from L3 to L2.
The proximal three parts of the duodenum form a ‘C’-shape, which cradles the head of the pancreas. It is this region which shares a common blood supply (described below).
The superior (1st) part is a small portion of the duodenum (about 5 cm), and receives chyme from the pylorus. Its first half is intraperitoneal, and the remainder is secondarily retroperitoneal (however, some sources consider the entire 1st part intraperitoneal). The superior is unique from the remainder of the duodenum in that it is smooth-walled in appearance, whereas the other parts of the duodenum have circular folds, permanent infoldings of the mucosa to increase surface area and slow the movement of materials.
The descending (2nd) part receives secretions of the pancreas and liver/gallbladder to mix with chyme from the stomach. The major duodenal papilla (MDP), an eminence of mucosa, transmits the hepatopancreatic ampulla, the distal dilation of the typical junction of the pancreatic and (common) bile ducts. The major duodenal papilla is the landmark differentiating the foregut (proximal to the MDP) from the midgut (distal to the MDP). When an accessory pancreatic duct is present, this duct typically drains into the descending part at the minor duodenal papilla, proximal (superior) to the MDP. The descending part is secondarily retroperitoneal.
The inferior/horizontal (3rd) part of the duodenum is transmitted between the abdominal aorta and the SMA, which in the presence of an aneurysm, makes the inferior part of the duodenum susceptible to superior mesenteric artery syndrome. The inferior part is secondarily retroperitoneal.
The ascending (4th) part is the smallest (about 2.5 cm) of the duodenum, and is contiguous with the jejunum at the duodenojejunal flexure. This flexure is sometimes supported by the suspensory ligament of (Treitz) the duodenum. The ascending part is secondarily retroperitoneal. Beyond the duodenojejunal flexure, the jejunum is intraperitoneal.
The pancreas is an important endocrine and exocrine (e.g. accessory digestive) gland. The endocrine portions of the pancreas (pancreatic islets) regulate blood glucose, and secrete into capillaries. The exocrine portions of the pancreas are acinar, and secrete into ducts coalescing into a pancreatic duct, and sometimes an accessory pancreatic duct.
The pancreas is derived from the embryonic foregut, and is topographically divided into four parts.
- Head - cradled by the duodenum,
- Uncinate process - located posterior to the SMA and SMV,
- Neck - posterior to the pylorus of stomach, anterior to the SMA & SMV (adjacent to the origin of the hepatic portal v.),
- Body - posterior to the stomach, anterior left renal vessels & splenic a.
- Tail - anterior to left kidney, oriented towards the spleen, intraperitoneal.
The (main) pancreatic duct (of Wirsung) runs the length of the pancreas (from tail-to-head), and terminates in the lumen of the descending duodenum. Most typically, it joins with the (common) bile duct to form the hepatopancreatic ampulla (of Vater), which is transmitted through the major duodenal papilla. The accessory pancreatic duct (of Santorini), when present, drains the superior portion of the head of the pancreas, and is transmitted into the descending duodenum through the minor duodenal papilla. As both pancreatic ducts and the (common) bile duct are transmitted through the head of the pancreas, pressure exerted upon the head (from edema, neoplasm, etc.) may interfere with exocrine pancreatic function as well as the drainage of bile.