The three parts of the small intestine are the duodenum, the jejunum, and the ileum, in that order from stomach to large intestine. Together, they form a coiled tube roughly 20 feet long that fills most of your abdominal cavity. Each segment has a distinct length and specialized role in breaking down food and absorbing nutrients.
Duodenum: The First Segment
The duodenum is the shortest section at roughly 10 inches long. It connects directly to the stomach through a ring of muscle called the pyloric sphincter, which controls how quickly partially digested food enters the small intestine. Despite its small size, the duodenum is arguably the busiest segment because it’s where chemical digestion kicks into high gear.
As food leaves the stomach, it’s highly acidic. The duodenum receives digestive juice from the pancreas and bile from the liver, both of which neutralize that acid and begin breaking down proteins, carbohydrates, and fats. The duodenum also produces its own digestive enzymes, so by the time food leaves this short segment, it’s been transformed into a soupy mixture that the rest of the small intestine can absorb efficiently.
Jejunum: The Middle Segment
The jejunum picks up where the duodenum leaves off, separated by a sharp bend in the intestine called the duodenojejunal flexure. A thin band of tissue known as the ligament of Treitz anchors this bend to the diaphragm, giving it a fixed position in the upper abdomen. This landmark matters in medicine because bleeding that occurs above it is classified as an upper gastrointestinal bleed, while bleeding below it is considered lower.
At about 8 feet long, the jejunum is the primary site where your body absorbs most nutrients: sugars, amino acids from protein, and fatty acids. Its inner walls are lined with millions of tiny finger-like projections called villi, which dramatically increase the surface area available for absorption. Each villus is further covered in even smaller projections called microvilli. Villi enlarge the intestinal surface by about 6.5 times, and microvilli multiply it another 13 times on top of that. This design turns a relatively narrow tube into an enormous absorptive surface packed into a compact space.
Ileum: The Final Segment
The ileum is the longest section at roughly 10 feet, making up about half the total length of the small intestine. It ends at the ileocecal valve, a one-way gateway into the large intestine (specifically the cecum). This valve prevents the contents of the large intestine, which are full of bacteria, from flowing backward.
The ileum absorbs whatever nutrients the jejunum didn’t catch, particularly vitamin B12 and bile salts. Bile salts are recycled here and sent back to the liver to be reused in digestion, a loop your body runs several times per meal. The ileum also plays a unique role in immune defense. Its walls contain clusters of immune tissue called Peyer’s patches, small lymphoid organs that act as a first line of defense against harmful microbes entering through the gut. Specialized cells on the surface of these patches sample bacteria and other particles directly from the intestinal contents, then alert the immune system to mount a response if needed.
How the Three Parts Work Together
The small intestine operates like an assembly line. The duodenum handles the chemical breakdown, mixing food with bile and pancreatic enzymes until fats are emulsified and proteins are split into smaller pieces. The jejunum then absorbs the bulk of those nutrients into the bloodstream through its densely folded walls. The ileum finishes the job, recovering the last useful molecules and recycling bile for the next round of digestion, while simultaneously monitoring for pathogens.
Muscular contractions along the entire length of the small intestine keep things moving forward. These rhythmic squeezes, called peristalsis, also mix the food thoroughly so it stays in contact with the intestinal walls long enough for absorption. Transit through the entire small intestine typically takes three to five hours, though this varies depending on the composition of your meal. Fatty meals slow things down, while liquid meals pass through more quickly.
Why Segment Location Matters
Certain digestive conditions target specific segments of the small intestine. Celiac disease, an autoimmune reaction to gluten, primarily damages the lining of the duodenum and upper jejunum, which is why nutrient deficiencies are common even in mild cases. Crohn’s disease most frequently affects the ileum, where chronic inflammation can narrow the passage and interfere with bile salt absorption, leading to diarrhea and poor absorption of fat-soluble vitamins.
Knowing which segment is involved helps explain the pattern of symptoms. Damage to the jejunum tends to cause widespread nutrient deficiencies because that’s where most absorption happens. Damage to the ileum is more likely to cause B12 deficiency and problems with fat digestion, since those are functions concentrated at the end of the line. If a significant length of either segment is surgically removed, the remaining intestine can partially adapt over time, but the specific nutrient gaps depend heavily on which section was lost.