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 %color=#ff0000 %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, %color=#ff0000 %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)
- Oligosccharidases (enterocytes)
- Mucus (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
Achalasia
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