Jejunum

Overview

The jejunum is the middle segment of the small intestine, positioned between the duodenum and the ileum. It is a major site for nutrient absorption, including carbohydrates, amino acids, and vitamins. Characterized by prominent mucosal folds and rich vascular supply, the jejunum plays a vital role in digestion and absorption during the post-gastric phase of food processing.

Location

The jejunum occupies the upper left quadrant and central part of the abdomen. It:

• Begins at the duodenojejunal flexure, located to the left of the second lumbar vertebra (L2), where the duodenum transitions into the jejunum

• Is suspended by the mesentery, allowing mobility within the abdominal cavity

• Gradually transitions into the ileum without a clear anatomical landmark

The first 40% of the intraperitoneal small intestine after the duodenum is considered the jejunum.

Structure

The jejunum has distinct structural features that differentiate it from the duodenum and ileum:

• Mucosa: Lined with simple columnar epithelium with microvilli; contains numerous plicae circulares (circular folds) for increased surface area

• Villi: Long, finger-like projections that maximize absorptive efficiency

• Submucosa: Contains blood vessels, nerves, and connective tissue, but lacks the Brunner’s glands of the duodenum and Peyer’s patches of the ileum

• Muscularis externa: Two layers of smooth muscle: inner circular and outer longitudinal, enabling peristalsis and segmentation

• Serosa: Covered by visceral peritoneum and suspended by the mesentery, which contains arterial arcades and long vasa recta

Function

The primary functions of the jejunum include:

• Absorption of nutrients: Most of the digested carbohydrates, proteins, and water-soluble vitamins are absorbed here

• Enzymatic digestion: Brush-border enzymes aid in the final breakdown of disaccharides and peptides

• Fluid absorption: Absorbs water and electrolytes as chyme moves through the lumen

Physiological Role(s)

The jejunum plays a central role in gastrointestinal physiology:

• Maximized surface area: Due to extensive villi and plicae circulares, the jejunum has a large surface area optimized for absorption

• Transport mechanisms: Active transport (e.g., sodium-glucose cotransporters) and passive diffusion facilitate nutrient uptake

• pH regulation: The jejunal environment is slightly alkaline (pH 7–8), ideal for enzymatic activity from pancreatic secretions

• Hormonal signaling: Interacts with hormones like cholecystokinin (CCK) and secretin to coordinate digestive processes

Clinical Significance

Several medical conditions and procedures are associated with the jejunum:

• Malabsorption syndromes: Disorders like celiac disease can cause villous atrophy, reducing the jejunum’s ability to absorb nutrients

• Short bowel syndrome: Surgical resection of a large portion of the jejunum can severely impair nutrient and fluid absorption

• Jejunal atresia: A congenital condition where a portion of the jejunum is absent or narrowed, often presenting with bilious vomiting in neonates

• Peptic ulcers: Though rare in the jejunum, ulcers may occur in the setting of Zollinger-Ellison syndrome or ectopic gastric mucosa

• Jejunal diverticulosis: Outpouchings of the jejunal wall can lead to stasis, inflammation, or perforation

• Feeding access: A jejunostomy tube may be placed surgically for enteral nutrition in patients unable to use the upper GI tract

The jejunum can be assessed using imaging (e.g., CT enterography), endoscopy, or biopsy, especially in cases of suspected malabsorption or chronic diarrhea.