The shortest stretch of your small intestine quietly talks to your brain
The duodenum is only 25 to 38 centimetres long, a C-shaped tube linking stomach to small intestine. Yet it senses what you have eaten and signals the liver, pancreas, fat tissue and brain, releasing hormones such as GLP-1 that help set blood sugar, appetite and body weight.
Anatomists divide it into four parts that wrap around the head of the pancreas. It begins with the duodenal bulb, about 2 cm long and a leftover of a fetal membrane that once hung the gut from the back wall of the abdomen. The second, descending part receives the bile duct and main pancreatic duct through a small opening called the major duodenal papilla. The third part runs horizontally for 10 to 12 cm, crossing in front of the aorta and the spine, and the fourth climbs to meet the jejunum. Embryologically, it straddles the border between foregut and midgut, so it draws blood from two major arterial systems.
Its chemistry is finely tuned. When acidic, fatty food arrives from the stomach, cells in its lining release secretin and cholecystokinin, prompting the gallbladder to squeeze out bile and the pancreas to send bicarbonate plus enzymes such as trypsin, lipase and amylase. These signals also control how fast the stomach empties. Brunner's glands, found nowhere else in the small intestine, pour out mucus and bicarbonate to neutralise stomach acid. In mammals the duodenum may also be the main site of iron absorption.
The gut hormones GIP and GLP-1, released here and further along the intestine, trigger insulin release from the pancreas only when glucose is present, a response called the incretin effect. They also influence gut movement, appetite and body weight, and the duodenum reaches the brain directly through the vagus nerve. About 70 per cent of the roughly 20,000 protein-coding genes in human cells are active in the normal duodenum, including around 300 that are especially enriched there.
Because acidic stomach contents first hit its opening section, that is where most duodenal ulcers form, usually after Helicobacter pylori bacteria erode the protective lining. Such ulcers are typically diagnosed with a urea breath test and endoscopy. British guidelines require a duodenal biopsy to diagnose coeliac disease in adults. Duodenal cancer is rare, making up only about 0.3 per cent of digestive tract tumours, though it accounts for roughly half of small-intestine cancers.
Source: Duodenum
