Duodenum

The duodenum is the first section of the small intestine in most vertebrates, including mammals, reptiles, and birds. In mammals, it may be the principal site for iron absorption.

The duodenum precedes the jejunum and ileum and is the shortest part of the small intestine.

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.

Function

The duodenum is largely responsible for the breakdown of food in the small intestine, using enzymes. The duodenum also regulates the rate of emptying of the stomach via hormonal pathways. Secretin and cholecystokinin are released from cells in the duodenal epithelium in response to acidic and fatty stimuli present there when the pylorus opens and emits gastric chyme into the duodenum for further digestion. These cause the liver and gallbladder to release bile, and the pancreas to release bicarbonate and digestive enzymes such as trypsin, lipase and amylase into the duodenum as they are needed.

The duodenum is a critical contributor to the regulation of food intake and glycemic control. As the first part of the small intestine, the duodenum is the initial site of nutrient absorption in the gastrointestinal tract. The duodenum senses nutrient intake and composition, and signals to the liver, pancreas, adipose tissue and brain through the direct and indirect release of several key hormones and signaling molecules, including the incretin peptides glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1), as well as cholecystokinin (CCK) and secretin. The duodenum also signals to the brain directly via vagal afferents enabling neural control over food intake and glycemia. Intestinal secretion of GIP and GLP-1 stimulates glucose-dependent insulin secretion from pancreatic beta-cells, known as the incretin effect. Incretin peptides, principally GLP-1 and GIP, regulate islet hormone secretion, glucose concentrations, lipid metabolism, gut motility, appetite and body weight, and immune function.

The villi of the duodenum have a leafy-looking appearance, which is a histologically identifiable structure. Brunner's glands, which secrete mucus, are only found in the duodenum. The duodenum wall consists of a very thin layer of cells that form the muscularis mucosae.

Ulceration

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. 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.

Celiac disease

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.

Cancer

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. Its histology is often observed to be adenocarcinoma, meaning that the cancerous tissue arises from glandular cells in the epithelial tissue lining the duodenum.

Obesity and diabetes

A western diet induces duodenal mucosal hyperplasia and duodenal dysfunction that underlie insulin resistance, type 2 diabetes and obesity. Diet-induced duodenal mucosal hyperplasia consists of increased mucosal mass, increased villus length, decreased crypt density, proliferation of enteroendocrine cells, increased enterocyte mass, and an accumulation of lipid droplets in the mucosa. Diet induced duodenal dysfunction includes increased duodenal nutrient absorption, altered duodenal hormone secretion, and altered intestinal vagal afferent neuronal function.

Inflammation

Inflammation of the duodenum is referred to as duodenitis. There are multiple known causes. Celiac disease and inflammatory bowel disease are two of the known causes.

Source: Wikipedia, The Free Encyclopedia · CC BY-SA 4.0