Duodenum

Structure and Function

Structure

Approximately 30cm long and C shaped. Begins at the pylorus and ends at the jejunum. Four parts:

• The bulb, where the acid and chyme enter from the stomach, is on the transpyloric line. Anterior are the quadrate lobe and the gall bladder. Posterior the portal vein, the bile duct and the gastroduodenal artery – if ulcer bursts, it bursts here as the ulcers are usually in the bulb. Also aorta and IVC (retroperitoneal)
• The second part is where the pancreatic juices and bile enter at the major duodenal papilla. If there is an accessory pancreatic duct present, it opens into the duodenum higher up at the minor duodenal papilla. Next to it is the head of the pancreas, the bile duct and the main pancreatic duct.
• The third part, where nothing happens. It wraps round the bottom of the head of the pancreas. Behind is the aorta and IVC which, when aneurysms occur, can split the wall of the duodenum, giving peritonitis.
• The fourth part, where it joins to the jejunum at the duodenojejunal flexure (bend).

Geared for absorption, so has a very large surface area, via three ways:

• Mucosal and submucosal folds (or plicae) – though these are more common in the jejunum.
• Villi which protrude into the lumen
• Microvilli on the columnar epithelial cells.

Functions

Although most absorption takes place in the jejunum and ileum, iron and calcium are particularly absorbed in the duodenum. Exocrine secretions from the pancreas and gall bladder:

• Trypsinogen
• Amylase
• Bile salts (emulsify fatty foods)
• Lipase

Secretion from the duodenum:

• Enterokinase (enterocytes) (stimulates secretion of TAL)
• Secretin (enterocytes) (secreted in response to acidic chyme, stimulates gall bladder contraction and pancreatic juice)
• Cholecystokinin (enterocytes) (gallbladder contraction and TAL)
• Oligosaccharidases (enterocytes)
• Mucous (and some bicarbonate) from the brunner's glands

Remember – just use your favourite mnemonic – TABLESCOM! See digestion for more.

Arterial and Venous supply

Nervous Supply

The nerves are derived from sympathetic and parasympathetic (vagus) nerves from the celiac and the superior mesentery plexi.

Lymph

The vessels follow the arteries and drain upwards via pancreatoduodnenal nodes to the gastroduodenal nodes, then the celiac nodes. They drain downwards via the pancreatoduodenal nodes to the superior mesentery nodes (near the start of the artery).

Histology

The epithelium contains goblet cells and endocrine cells between enterocytes. Directly underneath that are the ECL (Enterochromaffin-like cells) and paneth cells. At the bottom of the villi, tubular glands and crypts extend down to the muscularis mucosa. The glandular cells tend to be scarce on the the villi and to confederate in the crypts. In the submucosa are the brunner's glands. The lamina propria contains cellular infiltrates.

Key:

• Goblet cells secrete mucus.
• Endocrine secrete enteroglucagon, CCk, secretin, motilin, gastrin.
• ECLs secrete serotonin which regulates gut motility and blood supply.
• Paneth cells secrete granules of antibacterial peptides.
• Brunner's glands secrete alkali mucus which neutralises gastric juice and epidermal growth factor, which promotes mucosal regeneration after injury.
• Lamina propria is made primarily of connective tissue, and contains the lacteal blood capillaries and cellular infiltrate (lymphocytes, mainly t helper ones, eosinophils, mast cells and plasma cells).

Clinical Conditions

Cancer

The duodenum is not a common place for primary cancers. There is more risk involved with untreated coeliac disease.

Coeliac disease

Immunological reaction to gluten-derived gliadin peptides. The concentration of dietary gluten in highest at the beginning of the small intestines, so it mostly affects the duodenum and proximal jejunum. Reaction to gliadin causes lymphocytes to mass, and causes quicker death of enterocytes, accompanied with increased rate of stem cell proliferation. This leads to an oedematous, swollen mucosa, with deep crypts and short villi, that is poor at absorption and digestion and so leads to malabsorption.

Presentation: Diarrhoea and weight loss (due to malabsorption). Inability to absorb fats results in steatorrhoea. Nutrients usually absorbed in the duodenum, such as iron, Vitamin D and calcium are deficient (leading to anaemia, osteoporosis). In severe cases, there will be deficiency in stuff usually absorbed in the jejunum and ileum. There may be abdominal pain and tiredness, and in 10% of cases, neurological defects.

Treatment: Gluten free diet is the only option. Symptoms soon disappear following this.

Duodenal (peptic) ulcers

As the stomach empties its contents, the chyme is squirted against the anterolateral (top outside) wall of the bulb. Very common, 2-3 times more than gastric ulcers. Approximately 10-15% of the Uk will have one at some point. Associated with H. Pylori – 95% of people with ulcers are infected with H. Pylori.

Presentation: Epigastric pain – key feature. Generally if someone points to the epigastric region to demonstrate where the pain is, it will often be a DU. DU pain is classically worse at night, and when hungry (though this last bit is not very reliable). May be some nausea, and occasional vomiting which relieves pain. May not present until after rupture (50% of people who die from rupture didn't ever realise they had DUs).

Investigations: 13C Urea breath test. Serum test. Stool test. In older patients, endoscopy is recommended to check its not cancer.

Treatment: Proton Pump Inhibitor along with 2 antibiotics for 1 week. Stopping smoking will speed up healing.

Perforation: If this occurs there may be massive peritonitis, widespread abdominal pain, and if it erodes the gastroduodenal artery, massive haemorrhage. Sometimes it is followed by gastric outlet obstruction, which will lead to pyloric stenosis. Treatment for perforation is oversewing.

Congenital

• Atresia: Failure to develop duodenum, common in Down's Syndrome.
• Malrotation: Caecum is still at the top of abdomen, crushing the duodenum.
• Stenosis (narrowing): Also common in Downs syndrome.
• Diverticula: Outpouchings in the wall of the duodenum which can perforate, leading to haemorrhage.