What are the anatomical, histological, vascular, and functional differences between the jejunum and ileum?

Anatomical and Functional Differences Between Jejunum and Ileum

Anatomical Distinctions

The jejunum and ileum differ fundamentally in their vascular architecture, with the ileum having significantly more arterial arcades (p < 0.0001), more numerous arteriae rectae (p = 0.02), and shorter (p = 0.007) and narrower (p = 0.004) blood vessels compared to the jejunum. 1

Vascular Supply

• Jejunal arteries tend to be larger in diameter (mean 2.2 mm vs. 2.0 mm in ileum) with fewer, longer arterial arcades 1
• Ileal arteries form more complex anastomotic networks with shorter, more numerous straight vessels entering the bowel wall 1
• Despite these architectural differences, there is no significant difference in arterial muscularity (proportion of vessel wall occupied by tunica media) between jejunum and ileum 1

Histological Features

• Recent molecular mapping identifies five distinct metabolic domains across the small intestine rather than the traditional three-segment model, with specific transcriptional patterns defining regional identity 2
• The proximal jejunum shows distinct cellular ecology optimized for rapid nutrient processing 3

Functional Differences in Absorption

The jejunum is the primary site for absorption of most nutrients, water, and electrolytes, with the majority of gastrointestinal secretions reabsorbed in the upper jejunum, while the terminal ileum is uniquely specialized for vitamin B12 and bile salt absorption—functions that cannot be compensated by other intestinal segments. 4

Nutrient-Specific Absorption Patterns

• Glucose and glycine are absorbed significantly faster from jejunum than ileum in all subjects studied 5
• L-alanine shows preferential jejunal absorption in 83% of individuals (5 of 6 subjects) 5
• Oligopeptides (such as glycyl-L-alanine) are absorbed at comparable rates from both jejunum and ileum, making peptide transport particularly important in the ileum 5
• Higher concentrations of free amino acids accumulate in ileal luminal contents during peptide perfusion, indicating active peptide hydrolysis 5

Critical Ileal-Specific Functions

• More than 90% of bile acids are actively reabsorbed exclusively in the distal ileum through specialized sodium-dependent transporters 6
• Vitamin B12 and fat malabsorption occurs when more than 60-100 cm of terminal ileum is resected, as hepatic bile salt synthesis cannot compensate for this loss 7, 4, 6
• Unabsorbed bile salts reaching the colon stimulate secretion and cause characteristic postprandial diarrhea 8, 6

Clinical Implications in Disease States

Short Bowel Syndrome Outcomes

Patients retaining more than 10 cm of terminal ileum with intact colon rarely develop nutritional problems and typically do not require nutritional support, whereas jejunum-colon patients develop diarrhea and steatorrhea due to loss of ileal bile salt and vitamin B12 absorption. 4

• The preserved ileum and colon maintain the "ileal brake" mechanism and allow for colonic fluid reabsorption 4
• Jejunostomy patients show no structural or functional adaptation over time, meaning their nutritional and fluid needs remain constant 7
• In contrast, jejunum-colon patients may show gradual reduction in nutritional requirements due to functional adaptation with slowing of gastric emptying and small bowel transit 7

Fluid Management Considerations

A critical pitfall: drinking water or hypotonic solutions in jejunostomy patients causes paradoxical net sodium efflux from plasma into the bowel lumen, worsening dehydration. 4

• Oral rehydration solutions must contain at least 90 mmol/L sodium to prevent this phenomenon 4
• This reflects the jejunum's role in rapid fluid and electrolyte exchange, requiring isotonic or hypertonic solutions for net absorption 7

Surgical Planning

• Restoration of intestinal continuity and recruitment of distal bowel (especially ileum and colon) should be accomplished as soon as safely possible to improve outcomes 7
• Shifting from jejunostomy anatomy to colon-in-continuity confers significantly improved prognosis 7
• Magnesium deficiency occurs commonly due to chelation with unabsorbed fatty acids and increased renal excretion from secondary hyperaldosteronism 7

Common Pitfalls to Avoid

• Do not overlook the risk of enteric hyperoxaluria in patients with intact colons after ileal resection, as unabsorbed bile acids and fatty acids increase colonic oxalate absorption, leading to kidney stones 6
• Sustained-release and delayed-release medications should be avoided in short bowel syndrome patients, particularly those with proximal jejunal anatomy 7
• Do not assume all small intestinal segments have equivalent absorptive capacity—the terminal ileum's specialized functions for B12 and bile salts cannot be replaced by jejunum 4