Ileum

The ileum is the final and longest segment of the small intestine, responsible for the absorption of vital nutrients, electrolytes, and bile salts. It plays a critical role in maintaining digestive efficiency and overall nutrient balance. Understanding its structure, location, and relationships with surrounding organs is essential in clinical anatomy, surgery, and gastrointestinal physiology.

This article explores the anatomy, gross features, and functional characteristics of the ileum, emphasizing its structural adaptations that facilitate absorption and its clinical significance in gastrointestinal health and disease.

Anatomy of the Ileum

Location and Extent

The ileum forms the distal portion of the small intestine and lies between the jejunum and the large intestine. It begins at the jejunoileal junction and ends at the ileocecal junction, where it opens into the cecum of the large intestine through the ileocecal valve. The ileum occupies the lower right quadrant of the abdomen and the pelvic region, extending in loops that are suspended by the mesentery.

Length and Dimensions

The ileum measures approximately 2 to 4 meters in length, constituting about three-fifths of the total length of the small intestine. It has a smaller diameter than the jejunum, averaging about 2.5 cm, and its wall is thinner and less vascular. These structural differences reflect its primary function in absorption rather than vigorous digestion.

Relations and Boundaries

The ileum is situated within the infracolic compartment of the abdominal cavity. Its loops lie freely in the pelvis and lower abdomen, making contact with several surrounding structures.

• Anterior relations: Anterior abdominal wall and greater omentum.
• Posterior relations: Right ureter, psoas major muscle, and iliac vessels.
• Superior relations: Jejunum and coils of small intestine.
• Inferior relations: Pelvic organs, such as the urinary bladder and uterus (in females).

Peritoneal Covering and Mesentery

The ileum is completely invested by the peritoneum and suspended by the mesentery of the small intestine, which attaches it to the posterior abdominal wall. This mesentery provides mobility and carries the vascular, lymphatic, and neural structures essential for its function.

• Attachment: The mesentery extends obliquely from the left side of the second lumbar vertebra to the right sacroiliac joint.
• Root of the mesentery: Approximately 15 cm long, containing the superior mesenteric vessels, lymphatics, and nerves between its two peritoneal layers.
• Contents: Arteries, veins, lymph nodes, lacteals, adipose tissue, and autonomic nerve fibers supporting intestinal activity and absorption.

Gross Features

External Characteristics

The external surface of the ileum differs from that of the jejunum in several key features. It appears paler, thinner, and less vascular due to a reduced blood supply. The walls are smooth with fewer mucosal folds, and the diameter gradually decreases toward the ileocecal junction.

• Diameter and wall thickness: The ileum has a narrower lumen and thinner wall compared to the jejunum.
• Color and texture: Paler pink due to reduced vascularity.
• Peyer’s patches: Prominent lymphoid aggregates on the antimesenteric border, visible as oval or circular raised areas, contributing to intestinal immunity.

Internal Features

The internal lining of the ileum is highly specialized for absorption, featuring numerous folds and microscopic projections that increase surface area.

• Plicae circulares: Permanent circular folds of mucosa and submucosa, smaller and sparser in the ileum than in the jejunum.
• Villi and microvilli: Finger-like projections that greatly amplify the absorptive surface. Villi are shorter and more numerous in the ileum than in proximal segments.
• Lumen: The cavity of the ileum is relatively narrow and smooth near the ileocecal junction, where folds become less pronounced.

Comparison Between Jejunum and Ileum

The ileum and jejunum are continuous but differ in structural and functional adaptations. These differences are clinically relevant in imaging, endoscopy, and surgical procedures.

Feature	Jejunum	Ileum
Location	Upper left abdomen	Lower right abdomen and pelvis
Length	About 2.5 meters	About 3.5 meters
Diameter	Wider lumen	Narrower lumen
Vascularity	Highly vascular with red appearance	Less vascular with pale appearance
Plicae circulares	Large, numerous, and closely spaced	Small, sparse, and disappear distally
Peyer’s patches	Few and small	Numerous and large
Function	Mainly digestion and absorption of carbohydrates and proteins	Absorption of bile salts, vitamin B12, and remaining nutrients

These gross and microscopic adaptations make the ileum uniquely suited for its role in the terminal stages of digestion and the absorption of essential micronutrients before intestinal contents pass into the large intestine.

Microscopic Structure

Histological Layers of the Ileum

The wall of the ileum is composed of four distinct histological layers, each contributing to its absorptive and protective functions. These layers are continuous throughout the small intestine but show unique features in the ileum, particularly related to lymphoid tissue accumulation and mucosal structure.

• Mucosa: The innermost layer composed of epithelium, lamina propria, and muscularis mucosae. The surface epithelium consists of simple columnar cells with microvilli forming the brush border that enhances absorption. The lamina propria contains blood capillaries, lymphatic lacteals, and numerous immune cells.
• Submucosa: Lies beneath the mucosa and contains connective tissue, blood vessels, lymphatics, and the Meissner’s plexus. It supports mucosal folds and houses lymphoid aggregates forming Peyer’s patches in the distal ileum.
• Muscularis externa: Consists of an inner circular and an outer longitudinal layer of smooth muscle, coordinated by the myenteric (Auerbach’s) plexus to produce peristaltic movements for the propulsion of intestinal contents.
• Serosa: The outermost layer of visceral peritoneum that forms part of the mesentery. It secretes serous fluid to reduce friction between intestinal loops.

Specialized Cells and Structures

The mucosal lining of the ileum contains several specialized epithelial cells, each performing a specific function related to digestion, absorption, and immune defense.

• Enterocytes: Columnar absorptive cells with densely packed microvilli that form the brush border, increasing the surface area for nutrient uptake and housing digestive enzymes such as disaccharidases and peptidases.
• Goblet cells: Interspersed among enterocytes, they secrete mucus that lubricates the lumen and protects the epithelium from mechanical and chemical injury. Their number increases progressively toward the distal ileum.
• Paneth cells: Located at the base of intestinal crypts, they secrete antimicrobial peptides such as lysozyme and defensins, providing innate immune defense.
• Enteroendocrine cells: Scattered cells that release hormones like secretin, cholecystokinin (CCK), and serotonin, which regulate digestive secretions and motility.
• M cells (microfold cells): Found over Peyer’s patches, these specialized epithelial cells transport antigens from the lumen to underlying immune cells, facilitating immune surveillance.

Lymphoid Tissue

The ileum contains abundant gut-associated lymphoid tissue (GALT), which plays a vital role in mucosal immunity and defense against pathogens.

• Peyer’s patches: Aggregates of lymphoid follicles found predominantly on the antimesenteric border of the ileum. Each patch consists of multiple lymphatic nodules rich in B and T lymphocytes, acting as immune sensors of intestinal contents.
• Lamina propria lymphatics: Numerous small lymphatic vessels called lacteals absorb dietary lipids and transport them into the lymphatic circulation.
• Immune function: GALT in the ileum forms part of the mucosal immune system, recognizing antigens and initiating immune responses to protect the intestinal mucosa from infection.

Blood Supply and Lymphatic Drainage

Arterial Supply

The ileum receives its blood supply primarily from branches of the superior mesenteric artery (SMA), a major branch of the abdominal aorta. These vessels travel within the mesentery to reach the intestinal wall.

• Ileal branches: Multiple small branches from the SMA form arterial arcades within the mesentery.
• Vasa recta: Straight terminal branches arising from the arcades that penetrate the intestinal wall to supply the mucosa and muscular layers.
• Vascular arrangement: The ileum has more complex arterial arcades with smaller vasa recta compared to the jejunum, reflecting its reduced vascular demand.

Venous Drainage

Venous return from the ileum parallels its arterial supply. The veins drain into the superior mesenteric vein (SMV), which subsequently joins the splenic vein to form the portal vein, directing nutrient-rich blood to the liver for processing.

• Ileal veins: Accompany arterial branches and converge into the SMV.
• Portal circulation: Ensures absorbed nutrients, including glucose and amino acids, are delivered to the liver for metabolism and detoxification.

Lymphatic Drainage

The lymphatic system of the ileum is essential for lipid absorption and immune defense. Lymphatic vessels begin as blind-ended lacteals within the intestinal villi and drain into larger channels within the mesentery.

• Lacteals: Specialized lymphatic capillaries in villi that absorb emulsified dietary fats in the form of chylomicrons.
• Mesenteric lymph nodes: Collect lymph from the intestinal wall before it drains into the superior mesenteric lymph nodes.
• Final drainage pathway: Lymph ultimately drains into the cisterna chyli and then enters the thoracic duct, returning to the venous circulation.

The rich vascular and lymphatic networks of the ileum ensure efficient absorption of nutrients and lipids while supporting immune surveillance and maintaining mucosal homeostasis.

Nerve Supply

Sympathetic Innervation

The sympathetic innervation of the ileum originates from the lower thoracic segments of the spinal cord, primarily through the T9 to T12 levels. Preganglionic fibers travel via the thoracic splanchnic nerves to synapse in the superior mesenteric ganglion. From there, postganglionic fibers reach the ileum through perivascular plexuses accompanying branches of the superior mesenteric artery.

• Effect on motility: Sympathetic stimulation decreases peristaltic activity by inhibiting smooth muscle contractions in the intestinal wall.
• Effect on secretions: Reduces intestinal gland secretion and blood flow, conserving energy during stress responses.
• Vasomotor control: Sympathetic fibers regulate the caliber of intestinal vessels, thereby influencing local blood flow and absorption rates.

Parasympathetic Innervation

Parasympathetic innervation is supplied by the vagus nerve (cranial nerve X), which carries preganglionic fibers to the enteric plexuses located within the wall of the ileum. These fibers synapse with postganglionic neurons in the myenteric and submucosal plexuses, enabling fine regulation of intestinal activity.

• Effect on motility: Enhances smooth muscle contractions, promoting peristalsis and segmentation for efficient mixing and propulsion of intestinal contents.
• Effect on secretion: Stimulates digestive enzyme and mucus secretion, facilitating absorption and lubrication of luminal contents.
• Reflex coordination: Works in coordination with the enteric nervous system to mediate reflexes such as the gastroileal reflex, which increases ileal motility after gastric emptying.

Enteric Nervous System

The enteric nervous system (ENS), often referred to as the “brain of the gut,” operates semi-independently of central control and integrates signals from both sympathetic and parasympathetic fibers. It consists of two main plexuses that coordinate intestinal motility and secretion.

• Auerbach’s (myenteric) plexus: Located between the circular and longitudinal muscle layers, it regulates the strength and rhythm of peristaltic contractions.
• Meissner’s (submucosal) plexus: Found in the submucosa, it controls local secretion, absorption, and blood flow within the mucosa.
• Intrinsic reflexes: The ENS can independently manage local reflexes such as peristaltic and secretory responses without requiring input from the central nervous system.

Overall, the neural supply of the ileum ensures coordinated motor activity, secretion, and absorption, enabling smooth transit of intestinal contents and maintenance of digestive efficiency.

Physiology and Functions

Digestive Functions

The ileum contributes significantly to the final stages of digestion, continuing the breakdown of nutrients initiated in the duodenum and jejunum. Digestive enzymes from the brush border and pancreatic secretions act synergistically to complete the hydrolysis of macronutrients.

• Enzymatic activity: Enzymes such as maltase, sucrase, and peptidase on the microvilli surface complete carbohydrate and protein digestion.
• Bile salt absorption: The terminal ileum actively reabsorbs bile salts, which are then recycled to the liver via the enterohepatic circulation, maintaining efficient fat digestion.
• Vitamin B12 absorption: Occurs exclusively in the terminal ileum through receptor-mediated uptake of the vitamin B12–intrinsic factor complex.

Absorptive Functions

The ileum is specialized for nutrient absorption through its extensive surface area created by villi and microvilli. Both active and passive transport mechanisms are utilized to absorb a wide range of nutrients.

• Carbohydrates and proteins: Absorbed as monosaccharides and amino acids through specific transporters located on enterocyte membranes.
• Lipids: Absorbed as fatty acids and monoglycerides after emulsification by bile salts. They are reassembled into triglycerides and packaged into chylomicrons for lymphatic transport via lacteals.
• Electrolytes and water: Sodium absorption occurs via active transport, while water follows osmotically to maintain fluid balance. The ileum reabsorbs a significant portion of the remaining intestinal water content.

Immune Function

Beyond digestion and absorption, the ileum serves as a vital immune barrier that prevents pathogen entry while maintaining tolerance to commensal gut flora.

• Gut-associated lymphoid tissue (GALT): Peyer’s patches and isolated lymphoid follicles monitor intestinal antigens and initiate immune responses against harmful microorganisms.
• Secretory IgA production: Mucosal plasma cells produce IgA antibodies that neutralize pathogens and toxins within the intestinal lumen.
• Barrier integrity: Tight junctions between epithelial cells prevent bacterial translocation and maintain intestinal homeostasis.

Thus, the ileum functions as a multifunctional organ, performing not only essential roles in nutrient absorption and digestion but also acting as a key component of the body’s immune defense system. Its structural and physiological adaptations ensure the completion of nutrient uptake and the protection of the intestinal environment before contents pass into the large intestine.

Development and Embryology

Origin from the Midgut

The ileum develops from the distal part of the embryonic midgut, which gives rise to the majority of the small intestine and a portion of the large intestine. During early development, the midgut elongates rapidly, forming a U-shaped loop that projects into the umbilical cord. This loop consists of a cranial limb, which forms the distal duodenum and jejunum, and a caudal limb, which develops into the ileum, cecum, ascending colon, and proximal part of the transverse colon.

By the sixth week of gestation, the midgut loop communicates with the yolk sac through the vitelline duct, which later regresses. The ileum occupies the caudal segment of this loop, establishing its continuity with both the jejunum and the future large intestine.

Rotation and Fixation

During the seventh to tenth weeks of development, the midgut undergoes a physiological herniation due to rapid elongation and limited abdominal cavity space. The loop rotates counterclockwise around the axis of the superior mesenteric artery by a total of 270 degrees.

• As rotation proceeds, the ileum and jejunum return to the abdominal cavity first, occupying the central and lower portions of the abdomen.
• The distal ileum then assumes its final position in the lower right quadrant, connecting to the cecum at the ileocecal junction.
• The mesentery of the ileum attaches obliquely from the left upper abdomen to the right iliac fossa, stabilizing the loops while allowing mobility.

This rotation and fixation process ensures the proper anatomical arrangement of the small and large intestines within the abdominal cavity, establishing normal digestive tract orientation.

Formation of Meckel’s Diverticulum

Meckel’s diverticulum represents a developmental remnant of the vitelline (omphalomesenteric) duct, which normally obliterates between the fifth and seventh weeks of gestation. Failure of this process results in a small pouch projecting from the antimesenteric border of the ileum, typically located about 60 cm proximal to the ileocecal valve.

• Structure: The diverticulum contains all layers of the intestinal wall and may include ectopic gastric or pancreatic tissue.
• Clinical relevance: Although often asymptomatic, it can cause complications such as bleeding, inflammation (diverticulitis), or obstruction due to intussusception.

Developmental Anomalies

Several congenital anomalies may arise from aberrations in the development, rotation, or fixation of the ileum and its associated structures.

• Ileal atresia and stenosis: Result from vascular accidents during development, leading to partial or complete obstruction of the lumen.
• Malrotation: Incomplete or abnormal rotation of the midgut loop, predisposing to volvulus and intestinal obstruction.
• Persistent vitelline duct: Can form an ileo-umbilical fistula, sinus, or cyst if communication with the umbilicus remains.

Understanding embryological development is vital for recognizing these anomalies, which often present in neonates and infants with intestinal obstruction or abdominal pain.

Clinical Anatomy and Disorders

Congenital Abnormalities

Several congenital conditions involve the ileum and may manifest early in life with gastrointestinal symptoms.

• Meckel’s diverticulum: The most common congenital anomaly of the small intestine, resulting from incomplete obliteration of the vitelline duct.
• Ileal atresia or stenosis: Congenital absence or narrowing of the ileal lumen leading to neonatal intestinal obstruction, bilious vomiting, and abdominal distension.
• Malrotation: Can cause abnormal positioning of the ileum and cecum, increasing the risk of volvulus (twisting of the intestine).

Inflammatory and Infectious Diseases

The ileum is frequently affected by various inflammatory and infectious conditions due to its high lymphoid content and exposure to intestinal flora.

• Crohn’s disease: A chronic inflammatory bowel disease that commonly involves the terminal ileum, leading to transmural inflammation, ulceration, and fibrosis. Clinical features include abdominal pain, diarrhea, weight loss, and malabsorption.
• Typhoid ulcers: Caused by Salmonella typhi infection, these ulcers develop over Peyer’s patches and may lead to intestinal bleeding or perforation.
• Tuberculosis of the ileum: Often secondary to pulmonary infection, characterized by thickening of the terminal ileum and ileocecal region, producing obstruction and pain.

Neoplastic Conditions

Neoplasms of the ileum are less common compared to other regions of the gastrointestinal tract but may present with obstruction or bleeding.

• Carcinoid tumors: The most frequent small intestinal neoplasm, arising from enterochromaffin cells of the mucosa, typically in the terminal ileum. They secrete serotonin, causing flushing, diarrhea, and bronchospasm (carcinoid syndrome).
• Adenocarcinoma: A malignant epithelial tumor that may develop in long-standing inflammatory conditions like Crohn’s disease.
• Lymphoma: The ileum, rich in lymphoid tissue, is a common site for primary intestinal lymphoma.

Vascular Disorders

Vascular compromise of the ileum can result in ischemia or infarction, often presenting as acute abdominal pain.

• Ischemic enteritis: Occurs due to reduced mesenteric blood flow, leading to mucosal ulceration and necrosis.
• Mesenteric artery thrombosis: Causes acute intestinal ischemia requiring urgent surgical intervention to prevent gangrene.

Surgical Importance

The ileum holds significant surgical importance due to its involvement in various procedures and disease conditions.

• Ileostomy: A surgical opening created in the ileum to divert intestinal contents to the abdominal wall, often performed after colectomy.
• Ileocecal valve significance: Prevents backflow of colonic contents into the small intestine, reducing the risk of bacterial overgrowth.
• Consequences of ileal resection: Removal of the terminal ileum leads to malabsorption of bile salts and vitamin B12, resulting in bile salt diarrhea and megaloblastic anemia.

Recognizing the clinical and surgical relevance of the ileum is crucial for diagnosing gastrointestinal disorders, planning surgical interventions, and managing postoperative complications effectively.

Imaging and Diagnostic Techniques

Imaging studies and diagnostic procedures play a vital role in evaluating ileal structure and function, detecting inflammatory, infectious, neoplastic, and congenital conditions. Due to the ileum’s deep location and complex loops, specialized imaging modalities are used for accurate visualization.

• Barium follow-through: A conventional radiographic technique in which the patient ingests barium sulfate contrast. Sequential X-rays track the passage of contrast through the small intestine, allowing assessment of mucosal patterns, strictures, ulcers, and motility. The terminal ileum is often examined closely for signs of Crohn’s disease or tuberculosis.
• CT enterography: A cross-sectional imaging method using oral and intravenous contrast to visualize bowel wall thickness, enhancement patterns, and surrounding mesenteric structures. It is highly sensitive for detecting inflammation, obstruction, fistulas, or neoplastic lesions.
• MRI enterography: Similar to CT enterography but uses magnetic resonance imaging to evaluate soft tissue contrast without ionizing radiation. It is particularly useful in monitoring Crohn’s disease activity and assessing response to treatment.
• Ultrasonography: High-frequency ultrasound can identify thickened bowel loops, hypervascularity, or abscesses. It is a non-invasive and radiation-free tool useful in children and pregnant patients.
• Endoscopy and capsule endoscopy: Direct visualization of the ileal mucosa using an endoscope or a swallowable capsule camera. Capsule endoscopy provides high-resolution images of small bowel mucosa and is valuable for diagnosing bleeding, ulcers, and early inflammatory lesions.
• Histopathological examination: Tissue biopsies obtained during endoscopy allow microscopic analysis to confirm diagnoses such as Crohn’s disease, tuberculosis, or neoplastic changes.

These diagnostic modalities complement each other, providing comprehensive assessment of both structural and functional aspects of the ileum for precise diagnosis and management planning.

Summary of Key Differences Between Jejunum and Ileum

Comparative Table of Anatomical and Functional Features

The jejunum and ileum, though continuous parts of the small intestine, exhibit several anatomical and physiological distinctions. These differences aid in identifying their respective segments during surgery, radiology, or endoscopy.

Feature	Jejunum	Ileum
Position	Upper left quadrant of the abdomen	Lower right quadrant and pelvic region
Length	About 2.5 meters (proximal two-fifths of the small intestine)	About 3.5 meters (distal three-fifths of the small intestine)
Wall thickness	Thick and muscular	Thinner and less muscular
Lumen diameter	Wider	Narrower
Vascularity	Rich blood supply, appears red	Poor blood supply, appears pale
Plicae circulares	Large, numerous, and closely packed	Smaller, fewer, and sparse toward the distal end
Villi	Long and broad	Short and slender
Peyer’s patches	Few and small	Numerous and large, mainly on the antimesenteric border
Fat in mesentery	Less fat, translucent mesentery	More fat, opaque mesentery
Vasa recta	Long and fewer	Short and numerous
Main function	Absorption of carbohydrates and proteins	Absorption of bile salts, vitamin B12, and electrolytes

The comparative differences between the jejunum and ileum reflect their specialization for different phases of digestion and absorption. The jejunum emphasizes nutrient breakdown and rapid absorption, while the ileum specializes in the selective absorption of vitamins, bile salts, and the final stages of nutrient recovery before intestinal contents pass into the large intestine.

References

1. Gray H, Standring S. Gray’s Anatomy: The Anatomical Basis of Clinical Practice. 42nd ed. London: Elsevier; 2021.
2. Drake RL, Vogl AW, Mitchell AWM, Tibbitts RM, Richardson PE. Gray’s Student Anatomy. 4th ed. Philadelphia: Elsevier; 2022.
3. Junquiera LC, Carneiro J, Kelley RO. Basic Histology: Text and Atlas. 15th ed. New York: McGraw Hill; 2023.
4. Guyton AC, Hall JE. Textbook of Medical Physiology. 15th ed. Philadelphia: Elsevier; 2021.
5. Moore KL, Dalley AF, Agur AMR. Clinically Oriented Anatomy. 8th ed. Philadelphia: Wolters Kluwer; 2018.
6. Sadler TW. Langman’s Medical Embryology. 15th ed. Philadelphia: Wolters Kluwer; 2023.
7. Kumar V, Abbas AK, Aster JC. Robbins and Cotran Pathologic Basis of Disease. 10th ed. Philadelphia: Elsevier; 2021.
8. Raju GS, Gerson L, Das A, Lewis B. Capsule endoscopy in small-bowel evaluation: a review. Gastrointest Endosc. 2015;82(4):620–628.
9. Bailey H, Love RJM, O’Connell PR. Bailey & Love’s Short Practice of Surgery. 28th ed. Boca Raton: CRC Press; 2023.
10. World Health Organization. Global Report on Digestive Diseases. Geneva: WHO Press; 2022.