The ileum is the final and longest segment of the small intestine, following the duodenum and jejunum. It measures approximately 2-4 meters in length and is responsible for the absorption of nutrients and bile salts.[5] The ileum has a thinner wall and fewer folds compared to the jejunum, and its lining contains Peyer’s patches, which are part of the immune system.
Location
The ileum is located in the lower abdomen and pelvic cavity, primarily in the right lower quadrant (RLQ).[3] It begins at the end of the jejunum and terminates at the ileocecal valve, where it connects to the cecum of the large intestine.
Anatomy
The ileum is the final and longest segment of the small intestine, responsible for absorbing nutrients and facilitating the transition of digestive material to the large intestine. Below is a detailed description of its anatomy:
Length and Dimensions
- The ileum is approximately 2-4 meters in length, making up the last portion of the small intestine.
- It has a narrower lumen compared to the jejunum and thinner walls.[7]
Location
- The ileum is located primarily in the right lower quadrant (RLQ) of the abdomen.
- It begins at the jejunoileal junction, where it continues from the jejunum, and ends at the ileocecal valve, which regulates the flow of material into the cecum of the large intestine.
Structure
The wall of the ileum is composed of the same four layers as the rest of the gastrointestinal tract, with unique features that distinguish it:
- Lined with simple columnar epithelium containing goblet cells that secrete mucus.
- Features:
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- Villi: Finger-like projections that increase surface area for nutrient absorption.
- Crypts of Lieberkühn: Glands located at the base of the villi that secrete digestive enzymes and new epithelial cells.
- Peyer’s Patches: Large aggregations of lymphoid tissue unique to the ileum, primarily involved in immune defense against pathogens.[1]
- Microvilli: Microscopic projections on the surface of epithelial cells, forming the brush border to enhance absorption.
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- Contains connective tissue, blood vessels, lymphatics, and nerves.
- Supports the mucosal layer and Peyer’s patches.
Muscularis Externa:
Two layers of smooth muscle:
- Inner circular layer: Facilitates segmentation and mixing of intestinal contents.
- Outer longitudinal layer: Responsible for peristalsis to move material through the ileum.
The ileum is covered by serosa (visceral peritoneum), as it is an intraperitoneal structure.
Vascular Supply
- Arterial Supply:
- Supplied by the ileal branches of the superior mesenteric artery (SMA).
- Venous Drainage:
- Blood drains into the superior mesenteric vein (SMV), which ultimately contributes to the portal vein system.
Lymphatic Drainage
- Lymph from the ileum drains into the mesenteric lymph nodes, eventually flowing into the cisterna chyli and the thoracic duct.
- The lymphatic vessels within the villi, known as lacteals, are responsible for the absorption and transport of dietary fats.
Nervous Supply
- Sympathetic Innervation:
Supplied by the superior mesenteric plexus, which inhibits motility and secretion.
- Parasympathetic Innervation:
Provided by the vagus nerve, stimulating motility and secretion.
Mesenteric Attachment
- The ileum is suspended by the mesentery, a fold of peritoneum that attaches it to the posterior abdominal wall.[7]
- The mesentery contains blood vessels, lymphatics, and nerves, allowing communication between the ileum and the systemic circulation.
Anatomical Relationships
Superiorly:
Connected to the jejunum.
Inferiorly:
Leads into the ileocecal valve and the cecum of the large intestine.
Anteriorly:
Covered by the greater omentum and may be in contact with the anterior abdominal wall.
Posteriorly:
Lies on the posterior abdominal wall and pelvic cavity structures.
Functions
The ileum, the final segment of the small intestine, plays a vital role in digestion, nutrient absorption, and immune function.[5] Below is a detailed explanation of its functions:
Absorption of Nutrients
The ileum is the primary site for absorbing specific nutrients that have not been fully absorbed by the duodenum and jejunum:
Vitamin B12:
- The ileum is the only site in the gastrointestinal tract where vitamin B12 is absorbed.
- This absorption requires intrinsic factor, a glycoprotein secreted by the stomach.
Bile Salts:
The ileum actively reabsorbs bile salts, which are then recycled through the enterohepatic circulation to the liver for reuse in fat digestion.
Fat-Soluble Vitamins (A, D, E, and K):
These vitamins are absorbed alongside bile salts.
Other Nutrients:
Absorbs residual carbohydrates, proteins, and lipids, although most of these are absorbed earlier in the small intestine.[3]
Transition and Transport of Digestive Material
- The ileum ensures the smooth transition of digested material from the small intestine to the large intestine.
- Ileocecal Valve Regulation:
- The ileum controls the flow of chyme into the large intestine through the ileocecal valve, preventing backflow of colonic contents.
Contribution to Immune Defense
The ileum contains a significant amount of lymphoid tissue, making it a critical component of the immune system:
- Peyer’s Patches:
- Large lymphoid aggregates in the ileum monitor intestinal bacteria and pathogens.
- These patches initiate immune responses when harmful microbes are detected.
- Gut-Associated Lymphoid Tissue (GALT):
- Part of the mucosal immune system, it provides a barrier to prevent pathogens from entering the bloodstream while promoting tolerance to beneficial gut flora.
Maintenance of Gut Microbiota
The ileum provides an environment for specific gut bacteria that assist in:
- Fermentation of undigested carbohydrates.
- Synthesis of certain vitamins (e.g., vitamin K).
Fat Absorption and Transport
- Specialized lymphatic vessels in the ileum, called lacteals, absorb dietary fats in the form of chylomicrons.
- These lacteals transport the fats to the lymphatic system for eventual distribution into the bloodstream.
Regulation of Intestinal Motility
The ileum contributes to peristalsis, mixing, and segmentation:
- Segmental Contractions:
Mix intestinal contents to maximize nutrient absorption.
- Peristaltic Movements:
Propel chyme toward the ileocecal valve, ensuring efficient transit to the large intestine.
Water and Electrolyte Absorption
- The ileum absorbs water and electrolytes (e.g., sodium, chloride, and potassium) to maintain fluid and electrolyte balance in the body.[2]
- This process complements the absorption occurring in the jejunum and precedes the final absorption in the colon.
Role in Enterohepatic Circulation
- By reabsorbing bile salts, the ileum plays a central role in the enterohepatic circulation:
- Bile salts absorbed in the ileum are returned to the liver via the portal vein for reuse in the digestion of fats.
Preparation of Material for the Large Intestine
- The ileum processes the remaining chyme to a form suitable for fermentation and further water absorption in the large intestine.
- It plays a critical role in ensuring that only digestible and nutrient-rich material is absorbed before waste enters the colon.
Clinical Significance
The ileum is critical to nutrient absorption and immune function, and its dysfunction can lead to several clinical conditions:
- Vitamin B12 Deficiency:
- Diseases affecting the ileum, such as Crohn’s disease or surgical resection, can impair vitamin B12 absorption, leading to megaloblastic anemia.
- Bile Salt Malabsorption:
- Failure to reabsorb bile salts in the ileum (e.g., after ileal resection) can result in diarrhea and fat malabsorption, known as bile acid diarrhea.
- Crohn’s Disease:
- The ileum is a common site for inflammation in Crohn’s disease, causing symptoms like abdominal pain, diarrhea, and malabsorption.
- Small Bowel Obstruction:
- Adhesions, hernias, or tumors involving the ileum can lead to obstruction, presenting with severe pain and vomiting.
- Lymphatic Disorders:
- The ileum’s Peyer’s patches and lymphatic tissue can be involved in infections or certain cancers like lymphoma.
