The lower part of the small intestine consists of the jejunum and ileum, segments responsible for absorbing most dietary nutrients and supporting gut immunity. These regions follow the duodenum and extend from the middle to the end of the small bowel.
Located between the stomach and large intestine, the small intestine measures approximately 6 meters in adults. The lower portion-jejunum and ileum-comprises roughly two-thirds of this length. While the duodenum handles initial digestion and mineral uptake, the jejunum and ileum specialize in absorbing carbohydrates, proteins, fats, vitamins, and electrolytes.
Structural features improve absorption efficiency. The mucosal lining contains finger-like projections called villi and microvilli, increasing surface area by over 600 times. Peyer's patches-clusters of lymphoid tissue-also appear in the ileum, playing a role in immune surveillance and response to pathogens.
The jejunum lies between the duodenum and ileum, spanning about 2.5 meters. Its wall is thick and highly folded, with prominent circular folds known as plicae circulares. These folds slow down chyme movement, allowing more time for enzymatic breakdown and absorption. The jejunum absorbs about 70% of ingested nutrients, including glucose, amino acids, fatty acids, and fat-soluble vitamins (A, D, E, K).
The ileum, the final segment, is about 3.5 meters long. It features thinner walls and fewer folds compared to the jejunum. Despite this, it remains critical for absorbing vitamin B12, bile salts, and water. The ileum's ability to reclaim bile acids through enterohepatic circulation reduces the need for constant liver synthesis.
A key functional difference lies in immune activity. The ileum houses Peyer's patches-organized lymphoid nodules that sample intestinal contents. These structures detect antigens and initiate adaptive immune responses. Without them, the risk of systemic infection from gut flora increases.
Epithelial cells in the lower small intestine include enterocytes, goblet cells, Paneth cells, and enteroendocrine cells. Enterocytes form the primary absorptive layer and express transporters such as SGLT1 for glucose and GLUT2 for amino acid uptake. Goblet cells secrete mucus that protects the epithelium from digestive enzymes and pathogens.
Paneth cells, located at the base of crypts, release antimicrobial peptides like defensins. These molecules maintain microbial balance by killing harmful bacteria while sparing beneficial ones. Deficiency in Paneth cell function is linked to Crohn's disease, particularly in ileal lesions.
Enteroendocrine cells produce hormones regulating digestion. For example, L-cells in the ileum release peptide YY (PYY) and glucagon-like peptide-1 (GLP-1), which slow gastric emptying and stimulate insulin release. GLP-1 is the target of several diabetes medications, including semaglutide.
Conditions affecting the lower part of the small intestine often stem from malabsorption, inflammation, or structural abnormalities. Celiac disease primarily targets the proximal jejunum due to gluten-induced immune activation, but symptoms may persist into the distal ileum. In contrast, Crohn's disease frequently affects the terminal ileum, leading to strictures, fistulas, and chronic diarrhea.
One common misconception is that the entire small intestine functions identically. In reality, each region has distinct roles. The duodenum is optimized for pH-dependent enzyme activity, while the jejunum focuses on bulk absorption. The ileum prioritizes recycling and immune regulation.
Another misunderstanding involves the role of bile. Many assume bile is only involved in fat digestion. However, bile salts are reabsorbed in the ileum via the sodium-dependent bile acid transporter (ASBT). Impaired ileal reabsorption leads to bile acid diarrhea-a condition treated with bile acid sequestrants like colesevelam.
| Feature | Jejunum | Ileum |
|---|---|---|
| Length | ~2.5 m | ~3.5 m |
| Wall thickness | Thicker | Thinner |
| Folds | Plicae circulares prominent | Rare and shallow |
| Primary absorption | Carbohydrates, amino acids, fats | B12, bile salts, water |
| Immune tissue | Minimal | Peyer's patches common |
During a clinical case study, a 42-year-old woman presented with fatigue, weight loss, and pale stools. Blood tests revealed low vitamin B12 levels. Endoscopy showed patchy erythema and ulceration in the terminal ileum. Biopsy confirmed Crohn's disease. Treatment with anti-TNF therapy improved symptoms and restored B12 absorption.
When evaluating small bowel disorders, imaging techniques such as capsule endoscopy and MRI enterography are essential. Capsule endoscopy captures images of the entire small intestine, including the lower segments, and detects early signs of inflammation or bleeding. A 2021 study published in Gastroenterology found that capsule endoscopy detected 87% of small bowel lesions missed by standard imaging.
For those managing conditions affecting the lower part of the small intestine, understanding the regional differences is important. A diet high in fiber may improve motility but can exacerbate symptoms in inflamed areas. Supplemental B12 injections are often required in patients with ileal resection or Crohn's-related damage.
Personal observation: During a surgical rotation, I watched a laparoscopic resection of a diseased ileal segment in a patient with refractory Crohn's disease. The surgeon emphasized how delicate the vascular supply is in the ileum-especially near the ileocecal valve-and how easily ischemia can occur during dissection. This reinforced the importance of precise anatomical knowledge in clinical practice.