The ileum is the final and longest section of the small intestine, acting as the digestive system’s last checkpoint before material moves into the large intestine. This approximately 11.5-foot-long segment connects the jejunum to the cecum, the beginning of the large intestine, via the ileocecal valve. The primary role of the ileum is to complete the process of nutrient absorption initiated by the preceding segments. While the duodenum and jejunum handle the bulk of digestion and nutrient uptake, the ileum is specialized for reclaiming specific substances and ensuring minimal loss of water and electrolytes. Its architecture, featuring numerous folds and projections, provides a vast surface area for these final absorptive functions.
General Absorption of Water and Electrolytes
The ileum acts as a clean-up crew, ensuring that any remaining nutrients are salvaged before the intestinal contents leave the small intestine. While the upper small intestine absorbs the vast majority of carbohydrates, fats, and proteins, the ileum absorbs the final 5% to 10% of these digested materials. This downstream absorption maximizes the nutritional yield from every meal.
A major function of the ileum is the reabsorption of water and electrolytes, a highly efficient process. About 7 to 9 liters of fluid enter the small intestine daily, consisting of dietary intake and secretions from organs like the stomach, pancreas, and liver. The ileum works alongside the jejunum to absorb approximately 80% of this fluid, preventing dehydration.
The absorption of water is a passive process that relies entirely on the active transport of solutes, especially sodium ions. Sodium ions are absorbed across the ileal wall through various mechanisms, including cotransport with substances like glucose and amino acids, and through ion exchange. The movement of these solutes creates an osmotic gradient that draws water out of the intestinal lumen and into the bloodstream. The ileum also absorbs chloride and exchanges sodium ions for hydrogen ions, which helps regulate the acidity of the intestinal contents.
Specialized Uptake of Vitamin B12
The ileum has an exclusive role in the absorption of Vitamin B12, also known as cobalamin. This water-soluble vitamin is essential for nerve function and the production of red blood cells. Its absorption requires a multi-step process: B12 must first be released from food proteins in the stomach and then bound to a specialized protein called Intrinsic Factor (IF).
Intrinsic Factor is a glycoprotein produced by the parietal cells in the stomach lining. The resulting B12-IF complex travels through the small intestine until it reaches the terminal ileum. Here, the enterocytes—the absorptive cells lining the ileum—possess specific receptors that recognize and bind only the B12-IF complex.
Once bound to the ileal receptor, the complex is internalized into the cell through receptor-mediated endocytosis. Inside the enterocyte, the B12 is released from the Intrinsic Factor. The freed vitamin then binds to transcobalamin II, a transport protein, which carries it into the bloodstream for distribution to the liver and other tissues.
The Recycling of Bile Salts
The ileum performs a specialized conservation task by reabsorbing bile salts. Bile salts are produced by the liver and released into the small intestine to emulsify dietary fats, making them accessible to digestive enzymes. To prevent the constant need for new synthesis, the body recycles nearly all the bile salts through a mechanism called enterohepatic circulation.
The terminal ileum is the main site for this active reabsorption, reclaiming approximately 95% of the bile salts that reach it. This reabsorption is mediated by specific transporters on the ileal cell surface, such as the apical sodium-dependent bile acid transporter (ASBT). After transport across the ileal wall, the bile salts enter the portal vein.
The portal vein carries the bile salts directly back to the liver, where they are efficiently extracted from the blood and re-secreted into the bile. This recycling process is efficient, allowing the body’s small pool of bile salts (about 3 to 5 grams) to be reused multiple times during a single meal. Only a small fraction (about 0.5 grams per day) is lost in the feces, which the liver compensates for through new synthesis.
The Role in Immune Surveillance
Beyond absorption, the ileum plays a significant non-digestive role as a central hub for immune surveillance within the digestive tract. The intestinal wall is constantly exposed to microorganisms and potential pathogens, necessitating a robust defense system. This defense is concentrated in the ileum in structures known as Peyer’s patches.
Peyer’s patches are large, organized clusters of lymphoid tissue embedded in the ileal wall. They are a major component of the Gut-Associated Lymphoid Tissue (GALT), which represents a large portion of the body’s immune cells. These patches function as immune sensors, monitoring the contents of the gut lumen for threats.
Specialized epithelial cells within the patches, called microfold (M) cells, sample antigens and bacteria from the intestinal contents. They transport these materials to the underlying immune cells, such as T cells and B cells, to initiate a precise immune response. This monitoring system prevents harmful bacteria from crossing the intestinal barrier and causing systemic infection.