The three parts of the small intestine are the duodenum, the jejunum, and the ileum, in that order. Together, they form a narrow, winding tube about 5 meters (roughly 16 feet) long in a living adult, with each section handling a distinct phase of digestion and nutrient absorption. Food passes through all three segments in about 2.5 to 3 hours.
Duodenum: Where Digestion Kicks Into High Gear
The duodenum is the shortest segment at roughly 25 centimeters (10 inches). It receives partially digested food, called chyme, directly from the stomach. Chyme arriving here is highly acidic, so the duodenum’s first job is to neutralize that acid. Specialized glands in its walls, called Brunner’s glands, secrete alkaline mucus. The duodenum also releases a hormone that triggers the release of bicarbonate, which further lowers the acidity so enzymes can do their work without damaging the intestinal lining.
At the same time, the duodenum signals the liver, gallbladder, and pancreas to deliver their digestive secretions. The liver and gallbladder release bile, which breaks down fats into smaller droplets that enzymes can access. The pancreas sends in its own suite of enzymes: one to break down fats, one for carbohydrates, and one for proteins. This makes the duodenum the intestine’s main chemical mixing chamber. Some nutrient absorption begins here, but the duodenum’s primary role is preparation, turning a thick, acidic paste into something the rest of the intestine can actually absorb.
Jejunum: The Primary Absorption Site
The jejunum makes up the middle stretch of the small intestine, averaging about 190 centimeters (just over 6 feet). This is where the bulk of nutrient absorption happens. Sugars, amino acids, fatty acids, and most vitamins and minerals pass through the jejunum’s walls and into the bloodstream.
What makes the jejunum so efficient at absorption is its internal architecture. Its inner surface is covered in permanent circular folds called plicae circulares, which are taller and more prominent here than in any other segment. These folds don’t flatten out when food fills the tube. On top of those folds sit millions of tiny, finger-like projections called villi, and each villus is covered in even tinier projections called microvilli. This layered design massively increases the surface area available to absorb nutrients. When you account for folds, villi, and microvilli together, the total internal surface area of the small intestine reaches approximately 30 square meters, roughly the size of a studio apartment. The jejunum contributes the lion’s share of that absorptive real estate.
The villi in the jejunum are longer and more finger-shaped compared to the flatter villi in the duodenum, which is one reason nutrient uptake peaks in this segment. If the duodenum is the prep kitchen, the jejunum is the loading dock where nutrients actually enter the body.
Ileum: Final Absorption and Immune Defense
The ileum is the longest of the three segments at about 285 centimeters (roughly 9 feet) and connects to the large intestine. Food spends the most time here before moving on. While the jejunum handles most general nutrient absorption, the ileum has two specialized jobs that no other part of the intestine can perform.
First, the ileum is the only place where vitamin B12 is absorbed. This vitamin is essential for nerve function and red blood cell production, and if the ileum is damaged or surgically removed, B12 deficiency is a predictable consequence. Second, the ileum recycles about 95% of the bile acids the body sends into the small intestine. Rather than losing those bile acids into the colon, the ileum captures them and routes them back to the liver for reuse. When this recycling system breaks down, excess bile acids reach the colon and can cause chronic diarrhea, a condition known as bile acid malabsorption.
The ileum also plays a surprisingly important role in immune defense. Its walls are packed with clusters of immune tissue called Peyer’s patches, which are far more concentrated here than in the jejunum or duodenum. These patches contain immune cells that monitor the intestine’s contents for harmful bacteria and other pathogens. Specialized cells in the lining above each patch sample proteins and microbes from the intestine, transport them inward, and present them to immune cells for evaluation. This is one of the body’s frontline systems for detecting gut infections before they spread. The ileum also contains proportionally more mucus-producing cells than the upper segments, adding a physical barrier against microbial invasion as material approaches the bacteria-dense large intestine.
How the Three Segments Work Together
Each segment transitions smoothly into the next, and the entire tube is only about 2 centimeters wide, roughly the diameter of your index finger. Food moves through via peristalsis, rhythmic waves of muscle contraction that squeeze contents forward. The whole journey from stomach to large intestine takes about 2.5 to 3 hours for half of a meal to pass through.
The division of labor is efficient. The duodenum neutralizes stomach acid and mixes in digestive chemicals. The jejunum absorbs the majority of nutrients through its enormous folded surface. The ileum handles the specialized pickups (B12, bile acids) and runs immune surveillance before passing leftover material into the colon. Damage or disease in any one segment produces distinct problems: duodenal issues often involve acid-related ulcers, jejunal damage leads to broad malnutrition, and ileal disease causes B12 deficiency and bile acid issues.