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The duodenum is the first, the shortest, and the widest portion of the small intestine. Its length, which is ten to twelve inches is less variable than that of the jejunum and ileum. It commences at the pyloric sphincter, and ends in the jejunum to the left of the body of the second lumbar vertebra.
Parts and Relations
The duodenum consists of four portions: first ascending, descending, transverse, and terminal ascending portion. The first, or first ascending, portion is nearly horizontal, and is about two inches long. Like the stomach, this portion is comparatively movable, more so than any of the other portions of the duodenum. It passes upward, laterally, and posteriorly from the pyloric end of the stomach to the neck of the gallbladder. Like the stomach, it is completely surrounded by peritoneum, which, in turning to the right, again forms the free border of the gastro-hepatic omentum just above it. Situated superior to it are the omental foramen and the lesser omentum; inferior, is the head of the pancreas; anterior to are the liver and the neck of the gall-bladder, and posterior are the common bile duct, the hepatic artery, the portal vein, and the gastro-duodenal artery. The second, or descending, portion is about three inches in length, and extends from the neck of the gallbladder to the level of the third lumbar vertebra. Its anterior surface is covered with peritoneum-the ascending layer of the transverse mesocolon except where the transverse colon passes anterior to it, and is firmly fixed to the head of the pancreas by connective tissue and peritoneum. anterior to it is the commencement of the transverse colon; posterior are the common bile duct, the medial border of the right kidney, the renal vessels, and the inferior vena cava; on its medial, or left, side are the common bile and the pancreatic ducts, both of which open into it, the superior and inferior pancreaticoduodenal arteries, and the head of the pancreas. After obliquely traversing the wall of the bowel, the pancreatic and the common bile ducts open into the postero-internal aspect of the lower part of this segment of the duodenum, on the summit of a papilla. The third, or transverse, portion, the longest and narrowest part of the duodenum, is about five inches in length; it passes obliquely upward and toward the left side, across the vertebral column, at the third lumbar vertebra, to the left of the second lumbar vertebra, where it terminates. A line drawn from a point three inches to the right of the umbilicus, to a point two inches to the left and above it, will nearly indicate its position. Its anterior surface, except where the superior mesenteric artery and vein cross it, is covered by peritoneum, the descending layer of the transverse mesocolon. anterior to it are the superior mesenteric artery and vein, the jejunum, and the lower layer of the transverse mesocolon; posterior are the crura of the diaphragm, the inferior vena cava, and the abdominal aorta; superior are the superior mesenteric artery and vein, the pancreas, and the inferior pancreaticoduodenal artery; inferior are parts of the mesentery and posterior parietal peritoneum. The fourth, or terminal ascending, portion is the shortest portion, being about one inch in length. It commences immediately to the left of the second lumbar vertebra, passes upward on the surface of the left psoas muscle, and terminates at the point where the mesentery begins, namely, at the origin of the jejunum. It is covered by peritoneum on the anterior surface and partly on the sides, and is firmly fixed to the left crus of the diaphragm and the connective tissue around the celiac artery by a band of fibrous tissue that contains some muscle tissue as well, and is called the suspensory ligament of the duodenum(ligament of Treitz). The junction of the terminal portion of the duodenum with the jejunum constitutes the duodenojejunal flexure, below and to the left of which is the duodenojejunal fossa.
The proximal portion (1st and 2nd sections) of the duodenum arises from the embryonic foregut whose blood supply comes from branches of the celiac artery. The branch of the celiac artery that supplies the proximal portion of the duodenum is the gastroduodenal artery and its branch the superior pancreaticoduodenal artery. The distal portion (3rd and 4th sections) of the duodenum is derived from the midgut whose blood supply is from the the superior mesenteric artery and it’s branches. The distal portion of the duodenum is supplied by the superior mesenteric artery and its branch the inferior pancreaticoduodenal artery. The superior pancreaticoduodenal artery from the gastroduodenal artery and the inferior pancreaticoduodenal artery from the superior mesenteric artery form an anastomotic loop between the celiac trunk and the superior mesenteric artery allowing for collateral circulation.
The venous drainage of the duodenum follows the arteries. The veins draining the duodenum ultimately drain into the portal system, directly or indirectly through the splenic or superior mesenteric vein.
Parasympathetic stimulation from the vagus nerve stimulates peristalsis and secretion. Sympathetic stimulation by nerve fibers from celiac ganglia inhibits peristalsis and promotes vasoconstriction.
The lymphatic vessels follow the arteries in a retrograde fashion. The anterior lymphatic vessels drain into the pancreatoduodenal lymph nodes located along the superior and inferior pancreatoduodenal arteries and then into the pyloric lymph nodes located along the gastroduodenal artery. The posterior lymphatic vessels pass posterior to the head of the pancreas and drain into the superior mesenteric lymph nodes. Efferent lymphatic vessels from the duodenal lymph nodes ultimately pass into the celiac lymph nodes.
The duodenum mixes the gastric contents (chyme) with enzymes from the pancreas (pancreatic juice) and gallbladder (bile) and is largely responsible for the breakdown of food in the small intestine. Bile breaks down or emulsifies fats into smaller particles called micelles. Breakdown into these particles increases the surface area for pancreatic lipase to digest the fats, breaking the triglycerides down into components that can be absorbed through the intestinal wall. Gastric acid from the stomach is also neutralized in the duodenum by pancreatic and biliary secretions as well as mucous secretions from Brunner’s glands, which are found the wall of the duodenum. Significant absorption of electrolytes and nutrients takes place in the duodenum. The duodenum also regulates the rate of emptying of the stomach and triggers hunger signals through the release of hormones. Cells in the duodenal epithelium release hormones secretin and cholecystokinin in response to acids and fats present in the chyme that is released from the stomach when the pylorus opens. These hormones cause the liver and gall bladder to release bile, and the pancreas to release bicarbonate and digestive enzymes (trypsin, lipase, amylase) into the duodenum.
The term duodenum stems from the Latin "duodenum digitorum", meaning twelve fingers or inches, relating to the length of the duodenum.
The proximal portion of the duodenum is derived from the foregut. The distal duodenum is derived from the midgut. The distal duodenum develops from the most cranial part of the midgut. During development the endodermal lining of the duodenum proliferates rapidly and obliterates the lumen recanalization occurs later in development.
Peptic ulcer disease