Duodenum

The first part, or superior part, of the duodenum is a continuation from the pylorus to transpyloric plane. It is superior 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.[9] 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.[8]^: 273

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 part, 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.[8]^: 274

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.[8]^: 274

The third part, or horizontal part 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 duodenum.[8]^: 274 This part may be compressed between the aorta and SMA causing superior mesenteric artery syndrome.

The fourth part, 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.[8]^: 274

The duodenum receives arterial blood from two different sources. The transition between these sources is important as it demarcates the foregut from the midgut. Proximal to the 2nd part of the duodenum (approximately at the major duodenal papilla – where the bile duct enters) the arterial supply is from the gastroduodenal artery and its branch the superior pancreaticoduodenal artery. Distal to this point (the midgut) the arterial supply is from the superior mesenteric artery (SMA), and its branch the inferior pancreaticoduodenal artery supplies the 3rd and 4th sections. The superior and inferior pancreaticoduodenal arteries (from the gastroduodenal artery and SMA respectively) form an anastomotic loop between the celiac trunk and the SMA; so there is potential for collateral circulation here.

The venous drainage of the duodenum follows the arteries. Ultimately these veins drain into the portal system, either directly or indirectly through the splenic or superior mesenteric vein and then to portal vein.

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.

Under microscopy, the duodenum has a villous mucosa. This is distinct from the mucosa of the pylorus, which directly joins to 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.[10]^{: 274–275}

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  Dog Duodenum 100X
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  Duodenum with amyloid deposition in lamina propria
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  Section of duodenum of cat. X 60
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  Micrograph showing giardiasis on a duodenal biopsy (H&E stain)
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  Duodenum with brush border (microvillus)

About 20,000 protein coding genes are expressed in human cells and 70% of these genes are expressed in the normal duodenum.[11][12] 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.[13]

Ulcers of the duodenum commonly occur because of infection by the bacteria Helicobacter pylori. These bacteria, through a number of mechanisms, erode the protective mucosa of the duodenum, predisposing it to damage from gastric acids. The first part of the duodenum is the most common location of ulcers since it is where the acidic chyme meets the duodenal mucosa before mixing with the alkaline secretions of the duodenum.[14] Duodenal ulcers may cause recurrent abdominal pain and dyspepsia, and are often investigated using a urea breath test to test for the bacteria, and endoscopy to confirm ulceration and take a biopsy. If managed, these are often managed through antibiotics that aim to eradicate the bacteria, and proton-pump inhibitors and antacids to reduce the gastric acidity.[15]

The British Society of Gastroenterology guidelines specify that a duodenal biopsy is required for the diagnosis of adult celiac disease. The biopsy is ideally performed at a moment when the patient is on a gluten-containing diet.[16]

Duodenal cancer is a cancer in the first section of the small intestine. Cancer of the duodenum is relatively rare compared to stomach cancer and colorectal cancer; malignant tumors in the duodenum constitute only around 0.3% of all the gastrointestinal tract tumors but around half of cancerous tissues that develop in the small intestine.[17] Its histology is often observed to be adenocarcinoma, meaning that the cancerous tissue arises from glandular cells in the epithelial tissue lining the duodenum.[18]

Inflammation of the duodenum is referred to as duodenitis. There are multiple known causes.[19]