What are the key differences between the ileum and jejunum in the small intestine?

Anatomical and Functional Differences Between Jejunum and Ileum

The jejunum and ileum differ primarily in their absorptive specialization, vascular architecture, wall thickness, and motility patterns, with the jejunum optimized for rapid nutrient absorption and the ileum specialized for selective absorption of vitamin B12 and bile salts.

Macroscopic Anatomical Differences

Luminal Diameter and Wall Structure

• The jejunum has a larger luminal diameter (mean 24.5 mm) compared to the ileum (proximal ileum 19.5 mm, distal ileum 18.9 mm, terminal ileum 18.7 mm) 1
• The bowel wall thickness remains relatively constant throughout both segments at approximately 1.5 ± 0.5 mm 1
• Jejunal mucosa displays more prominent circular folds (plicae circulares/valvulae conniventes) with 4.6 folds per 2.5 cm, while the terminal ileum has only 1.5 folds per 2.5 cm 1
• Fold thickness decreases progressively from jejunum (2.1 mm in duodenum) to terminal ileum (1.8 mm) 1

Vascular Architecture

• The jejunum has fewer arterial arcades (typically 1-2) with longer, wider arteriae rectae (straight arteries), while the ileum has more numerous arterial arcades (3-5) with shorter, narrower, and more numerous arteriae rectae 2
• Jejunal parent arteries tend to be larger (mean diameter 2.2 mm) compared to ileal arteries (mean diameter 2.0 mm), though this difference approaches but does not reach statistical significance 2
• Despite these architectural differences, there is no significant difference in arterial muscularity between jejunal and ileal vessels 2

Physiological and Functional Differences

Absorptive Specialization

• The jejunum is the primary site for absorption of most nutrients, water, and electrolytes, with the majority of gastrointestinal secretions (approximately 4 liters daily) reabsorbed in the upper jejunum 3
• The terminal ileum (distal 60-100 cm) is uniquely specialized for vitamin B12 and bile salt absorption—functions that cannot be compensated by other intestinal segments 3
• Jejunal mucosa is "leaky" with rapid bidirectional sodium fluxes, requiring luminal sodium concentrations of approximately 100 mmol/L to prevent net sodium efflux from plasma into the lumen 3

Motility Patterns

• Migrating motor complexes (MMC) occur with similar frequency in jejunum and ileum, but jejunal MMC cycles are significantly shorter at night 4
• Ileal motility is less intense and less propagative than jejunal motility, with slower propagation velocity, shorter propagation distance, and fewer migrating clustered contractions during both fasting (phases II and III) and postprandially 4
• The ileum exhibits unique prolonged propagated contractions not typically seen in the jejunum, representing a distinct propulsive pattern 4
• The ileum provides a "braking mechanism" that slows transit and enhances nutrient absorption—this function is lost in patients with jejunostomy, resulting in rapid transit and massive fluid losses 3

Clinical Implications in Short Bowel Syndrome

Jejunum-Predominant Resection

• Patients retaining more than 10 cm of terminal ileum with intact colon (jejunum-ileum configuration) rarely develop nutritional problems and typically do not require nutritional support 3
• The preserved ileum and colon maintain the braking mechanism and allow for colonic fluid reabsorption 3

Ileum-Predominant Resection

• Jejunum-colon patients (jejunoileal resection with jejunocolic anastomosis) develop diarrhea/steatorrhea and progressive malnutrition due to loss of ileal bile salt and vitamin B12 absorption 3
• Vitamin B12 and fat malabsorption occurs when more than 60-100 cm of terminal ileum is resected, as increased hepatic bile salt synthesis cannot compensate for lost ileal absorptive surface 3
• Unabsorbed bile salts entering the colon contribute to colonic secretory diarrhea 3

Jejunostomy Patients

• Patients with jejunostomy (jejunoileal resection with colectomy and stoma formation) face immediate severe dehydration from massive stomal water and sodium losses, particularly after oral intake 3
• If less than 100 cm of jejunum remains proximal to a jejunostomy, patients may lose more fluid through the stoma than they consume orally 3
• Jejunostomy effluent contains approximately 100 mmol/L sodium but relatively little potassium (approximately 15 mmol/L) 5

Common Clinical Pitfalls

Fluid Management Errors

• Drinking water or hypotonic solutions (sodium <90 mmol/L) in jejunostomy patients causes paradoxical net sodium efflux from plasma into bowel lumen, worsening dehydration rather than improving it 3
• Oral rehydration solutions must contain at least 90 mmol/L sodium to prevent this phenomenon 3

Tumor Distribution Patterns

• In mouse models of intestinal cancer, tumors occur primarily in duodenum and jejunum with fewer in the ileum and minimal involvement of cecum and colon, reflecting regional differences in susceptibility to neoplastic transformation 3