The duodenum is the first section of the small intestine in the digestive system. It receives partially digested food from the stomach and plays an important role in continuing the digestive process. But what comes after the duodenum? Let’s take a closer look at the anatomy and physiology of the small intestine to understand what comes next.
An overview of the small intestine
The small intestine is the longest part of the digestive tract, extending around 6 meters or 20 feet in length. It is divided into three main sections:
- Duodenum
- Jejunum
- Ileum
The duodenum is the first and shortest section, only about 25-30 cm or 10-12 inches long. It connects to the pyloric sphincter of the stomach and receives chyme, the acidic, partially digested food mixture from the stomach. The duodenum plays a key role in neutralizing the acidity of chyme and in initiating further digestion and absorption of nutrients.
After the duodenum comes the jejunum, making up about two-fifths of the small intestine’s length. This highly vascularized section is the primary site of nutrient absorption in the small intestine. The jejunum contains circular folds and finger-like villi on its lining which work to increase the surface area for optimal absorption.
The final and longest segment of the small intestine is the ileum, extending about 3 meters or 10 feet in length. The ileum continues the process of absorption, primarily of vitamin B12, bile salts and any remaining nutrients. The ileum connects to the cecum of the large intestine via the ileocecal valve.
What comes after the duodenum?
So in summary, the small intestinal sections in order are:
- Duodenum
- Jejunum
- Ileum
This means the jejunum comes immediately after the duodenum. The jejunum plays a crucial role in digestion and absorption to continue the processing of chyme after it passes through the duodenum.
Key functions of the jejunum
As the middle and longest segment of the small intestine, the jejunum carries out vital digestive and absorptive functions including:
- Absorption of nutrients – the jejunum absorbs sugars, amino acids, fatty acids, vitamins and minerals from chyme.
- Circular folds – the mucosa of the jejunum contains circular folds which increase surface area for absorption.
- Villi – the jejunum contains small, finger-like villi on its epithelial lining which further increase absorptive area.
- Enzyme secretion – the jejunum secrets peptidases to break down peptides and amylases to break down carbohydrates.
- Water and electrolyte absorption – the jejunum absorbs water and electrolytes like sodium, chloride and potassium.
These functions allow the jejunum to continue digesting chyme and deliver vital nutrients into the bloodstream after chyme passes through the duodenum. The jejunum’s absorptive capacity is much greater than the duodenum due to its increased length and surface area.
Structural adaptations in the jejunum
The jejunum has several key structural adaptations that facilitate its absorptive functions:
- Circular folds – Deep circular folds in the mucosa and submucosa called plicae circulares. These folds increase surface area and slow passage of chyme.
- Villi – Tiny, finger-like villi extending from the epithelial lining. The villi contain a network of capillaries and lacteals for absorption.
- Microvilli – Microscopic protrusions covering the surface of the villi. These further increase surface area for absorption.
- Brush border enzymes – Digestive enzymes like peptidases attached to the brush border membrane of the microvilli.
Together, these structural adaptations give the jejunum an enormous surface area, estimated to be about 30 square meters in adults. This allows for highly efficient absorption of nutrients.
Jejunum vs. ileum – key differences
Although the jejunum and ileum are both parts of the small intestine, there are some key differences between them:
| Feature | Jejunum | Ileum |
|---|---|---|
| Length | 2.5 m | 3.6 m |
| Diameter | Bigger | Smaller |
| Circular folds | Numerous | Few |
| Blood supply | Rich | Less rich |
| Absorption | Majority of nutrients | Mainly vitamins and water |
These differences in structure and function reflect the jejunum’s primary role in absorption compared to the ileum’s role in retrieving any remnants of digestion.
Diseases of the jejunum
Some diseases and disorders involving the jejunum include:
- Celiac disease – autoimmune reaction to gluten that damages jejunal villi.
- Crohn’s disease – inflammatory bowel disease that may affect the jejunum.
- Intussusception – telescoping of the jejunum into itself causing obstruction.
- Volvulus – twisting of the jejunum that can cut off blood flow.
- Tumors – both cancerous and benign tumors may arise in the jejunum.
- Malabsorption – impairment in jejunal absorption of nutrients.
Symptoms of jejunum-related conditions can include diarrhea, abdominal pain, nausea and vomiting, GI bleeding, obstruction, and malnutrition.
Diagnosing jejunum problems
Some tests that may be used to evaluate jejunum structure, function and health include:
- Capsule endoscopy – patient swallows a tiny camera to image the jejunum.
- Double balloon enteroscopy – uses balloons to navigate an endoscope into the jejunum.
- CT enterography – CT scan with contrast to visualize the jejunum.
- MR enterography – MRI scan to examine the jejunum.
- Jejunal aspirate and culture – taking a fluid sample from the jejunum for analysis.
- Jejunal biospy – taking a tissue sample for microscopic examination.
These tests can identify sources of malabsorption, inflammation, infections, tumors and other disorders involving the jejunum.
Conclusion
In summary, the jejunum is the second segment of the small intestine, connecting after the duodenum. With its circular folds, villi and microvilli, the jejunum is specialized for digesting chyme and absorbing nutrients. A variety of conditions can affect the jejunum, causing symptoms like diarrhea, abdominal pain and malabsorption. Advanced imaging and endoscopic techniques allow physicians to directly visualize and assess the jejunum for optimal diagnosis and treatment.
