Duodenum

In humans, the duodenum is a hollow jointed tube about 25–38 centimetres (10–15 inches) long connecting the stomach to the jejunum, the middle part of the small intestine. It begins with the duodenal bulb, and ends at the duodenojejunal flexure marked by the suspensory muscle of duodenum. The duodenum can be divided into four parts: the first (superior), the second (descending), the third (transverse) and the fourth (ascending) parts.

Overview

The duodenum is the first section of the small intestine in most higher vertebrates, including mammals, reptiles, and birds. In fish, the divisions of the small intestine are not as clear, and the terms anterior intestine or proximal intestine may be used instead of duodenum. In mammals the duodenum may be the principal site for iron absorption.

In humans, the duodenum is a C-shaped hollow jointed tube, 25–38 centimetres (10–15 inches) in length, lying adjacent to the stomach (and connecting it to the small intestine). It is divided anatomically into four sections. The first part lies within the peritoneum but its other parts are retroperitoneal.

Parts

The first or superior part of the duodenum is a continuation from the pylorus to the transpyloric plane. It is superior (above) to the rest of the segments, at the vertebral level of L1. The duodenal bulb, about 2 cm (3⁄4 in) long, is the first part of the duodenum and is slightly dilated. The duodenal bulb is a remnant of the mesoduodenum, a mesentery that suspends the organ from the posterior abdominal wall in fetal life. The first part of the duodenum is mobile, and connected to the liver by the hepatoduodenal ligament of the lesser omentum. The first part of the duodenum ends at the corner, the superior duodenal flexure.

Relations:

• Anterior
  • Gallbladder
  • Quadrate lobe of liver
• Posterior
  • Bile duct
  • Gastroduodenal artery
  • Portal vein
  • Inferior vena cava
  • Head of pancreas
• Superior
  • Neck of gallbladder
  • Hepatoduodenal ligament (lesser omentum)
• Inferior
  • Neck of pancreas
  • Greater omentum
  • Head of pancreas

The second or descending part of the duodenum begins at the superior duodenal flexure. It goes inferior to the lower border of vertebral body L3, before making a sharp turn medially into the inferior duodenal flexure, the end of the descending part.

The pancreatic duct and common bile duct enter the descending duodenum, through the major duodenal papilla. The second part of the duodenum also contains the minor duodenal papilla, the entrance for the accessory pancreatic duct. The junction between the embryological foregut and midgut lies just below the major duodenal papilla.

The third, horizontal or inferior part of the duodenum is 10~12 cm in length. It begins at the inferior duodenal flexure and passes transversely to the left, passing in front of the inferior vena cava, abdominal aorta and the vertebral column. The superior mesenteric artery and vein are anterior to the third part of the duodenum. This part may be compressed between the aorta and SMA causing superior mesenteric artery syndrome.

The fourth or ascending part of the duodenum passes upward, joining with the jejunum at the duodenojejunal flexure. The fourth part of the duodenum is at the vertebral level L3, and may pass directly on top, or slightly to the left, of the aorta.

Blood supply

The first (superior) part of the duodenum, right after the pylorus of the stomach, is not supplied by the arcades. Instead, it is supplied by the supraduodenal artery and posterior superior pancreaticoduodenal artery, along with some branches of the right gastroepiploic artery and the anterior superior pancreaticoduodenal artery. In many people part of the first centimeter of the duodenum is also supplied by branches of the right gastric artery.

The remaining three parts (descending, horizontal, and ascending) of the duodenum are supplied by two arcades (rings) of arteries, one anterior (in front) of the duodenum and pancreas and one posterior to (behind) them. Each arcade is made of two anastomosed (connected) arteries. The superior artery of each arcade comes from the superior pancreaticoduodenal artery, which arises from the celiac artery via the gastroduodenal artery. The inferior artery of each arcade comes from the inferior pancreaticoduodenal artery, a branch of the superior mesenteric artery. The anterior arcade is formed by the anterior superior pancreaticoduodenal artery and the anterior inferior pancreaticoduodenal artery; the posterior arcade is formed by the posterior superior pancreaticoduodenal artery and the posterior inferior pancreaticoduodenal artery.

Vessels from the arcades supply the muscularis externa (muscular layer) before forming a plexus (network of blood vessels) in the submucosa (a layer of connective tissue) called the submucosal plexus. Vessels continue from the submucosal plexus through the muscularis mucosae (another thin muscular layer) before forming another plexus under the epithelium of the villi, the layer where nutrients are absorbed. These vessels entering the duodenum from the arcades are sometimes called vasae rectae or arteriae rectae.

The venous drainage of the duodenum mainly follows the arteries, ultimately draining into the portal system. The venous arcades are usually superficial to the arterial arcades. The anterior superior pancreaticoduodenal vein drains into the right gastroepiploic vein, as do the veins of the lower first part of the duodenum and the pylorus (subpyloric veins). The upper first part of the duodenum is drained by suprapyloric veins, which can drain into the portal vein or the posterior superior pancreaticoduodenal vein, which drains into the portal vein. The inferior veins of the arcades drain into the superior mesenteric, inferior mesenteric, splenic, or first jejunal vein.

Embryologically, the duodenum arises from both the foregut and midgut, constituting the boundary between the two. However, the "midgut" is defined surgically as the parts of the intestine supplied by the superior mesenteric artery. Since the duodenum is supplied both by the celiac artery and the superior mesenteric artery, these two definitions are similar but not exactly the same.

Lymphatic drainage

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 (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.

Histology

Under microscopy, the duodenum has a villous mucosa. This is distinct from the mucosa of the pylorus, which directly joins the duodenum. Like other structures of the gastrointestinal tract, the duodenum has a mucosa, submucosa, muscularis externa, and adventitia. Glands line the duodenum, known as Brunner's glands, which secrete mucus and bicarbonate in order to neutralise stomach acids. These are distinct glands not found in the ileum or jejunum, the other parts of the small intestine.

Variation

The duodenum's close anatomical association with the pancreas creates differences in function based on the position and orientation of the organs. The congenital abnormality, annular pancreas, causes a portion of the pancreas to encircle the duodenum. In an extramural annular pancreas, the pancreatic duct encircles the duodenum which results in gastrointestinal obstruction. An intramural annular pancreas is characterized by pancreatic tissue that is fused with the duodenal wall, causing duodenal ulceration.

Gene and protein expression

About 20,000 protein coding genes are expressed in human cells and 70% of these genes are expressed in the normal duodenum. Some 300 of these genes are more specifically expressed in the duodenum with very few genes expressed only in the duodenum. The corresponding specific proteins are expressed in the duodenal mucosa, and many of these are also expressed in the small intestine, such as alanine aminopeptidase, a digestive enzyme, angiotensin-converting enzyme, involved in controlling blood pressure, and RBP2, a protein involved in the uptake of vitamin A.