The first section of the small intestine is called the duodenum, a C-shaped structure that links the stomach to the jejunum. This region plays a pivotal role in the early stages of chemical digestion and nutrient absorption.
Positioned immediately after the stomach, the duodenum receives chyme mixed with bile and pancreatic enzymes. Its mucosal surface coordinates neutralization, digestion, and regulated passage of intestinal contents.
| Region | Length | Main Digestive Role | Key Structural Features |
|---|---|---|---|
| Duodenum | Approximately 25–30 cm | Initial mixing of chyme with bile and pancreatic secretions | Brunner's glands in submucosa, circular folds |
| Jejunum | Approximately 2.5 m | Majority of nutrient absorption | Thick walls, prominent villi |
| Ileum | Approximately 3.5 m | Final digestion and absorption, vitamin B12 uptake | Peyer’s patches, thinner walls |
| Transition Zones | Varies anatomically | Gradual shift in mucosal specialization | Less distinct boundaries in proximal ileum |
Anatomy of the Duodenum
The duodenum is the shortest and most fixed portion of the small intestine. It begins at the pyloric sphincter and forms a nearly complete loop around the head of the pancreas.
Its walls contain submucosal Brunner's glands that secrete an alkaline mucus, protecting the mucosa from acidic gastric chyme. The inner surface features circular folds and villi that increase surface area for early digestive processes.
Blood supply comes from the gastroduodenal and superior mesenteric arteries, while innervation coordinates local secretion and motility through the enteric nervous system.
Chemical Digestion in the Duodenum
Chemical digestion in the duodenum begins when acidic chyme enters the lumen and stimulates the release of secretin and cholecystokinin. These hormones regulate the flow of bile from the liver and gallbladder, and pancreatic enzymes such as amylase, proteases, and lipase.
Bile salts emulsify fats, increasing the surface area for lipase action. Pancreatic bicarbonate neutralizes the chyme, creating an optimal pH for enzymatic activity. Disruption in this process can lead to maldigestion and malabsorption.
Relation to the Jejunum and Ileum
After the duodenum, the jejunum continues digestion and absorbs most carbohydrates, proteins, and fats. The ileum completes absorption, particularly of vitamin B12 and bile salts, before passing residual material to the colon.
Together, these regions form a continuous tube with region-specific mucosal adaptations. The transition from duodenum to jejunum is marked by a reduction in Brunner's glands and an increase in villus height and density.
Common Pathologies of the Duodenum
Conditions such as peptic ulcer disease, duodenal atresia, and tumors can impair digestion early in the small intestine. Ulcers often relate to Helicobacter pylori infection or chronic use of nonsteroidal anti-inflammatory drugs, affecting the duodenal mucosa.
Malrotation or obstruction near the duodenojejunal flexure may present in infants, while adults may experience symptoms from tumors or chronic inflammation. Early diagnosis and targeted treatment help preserve normal intestinal function.
Key Takeaways on Duodenal Function
- The duodenum is the first section of the small intestine.
- It mixes chyme with bile and pancreatic secretions.
- Its mucosal glands protect against gastric acidity.
- Hormonal regulation controls digestive enzyme release.
- Efficient duodenal function supports downstream absorption in jejunum and ileum.
FAQ
Reader questions
What is the medical name for the first section of the small intestine?
It is called the duodenum.
How long is the duodenum in adults?
The duodenum is approximately 25–30 cm long.
Which substances does the duodenum receive from other organs?
The duodenum receives bile from the liver and gallbladder and pancreatic enzymes from the pancreas.
Why is the duodenum important for nutrient absorption?
It initiates digestion and absorption by neutralizing acid and enabling early breakdown of carbohydrates, proteins, and fats.