The small bowel, also called the small intestine, is the long, coiled tube connecting your stomach to your large intestine. Despite its name, it’s the longest part of your digestive tract, stretching about 22 feet in a living adult. Its primary job is breaking down food and absorbing nutrients into your bloodstream, a role it handles with remarkable efficiency thanks to its unique internal structure.
The Three Segments
The small bowel is divided into three connected sections, each with a slightly different role. The duodenum comes first, a short C-shaped segment that receives partially digested food from your stomach. This is where the most intense chemical digestion happens: your pancreas delivers enzymes that break down carbohydrates, fats, and proteins through small ducts, while your gallbladder squeezes stored bile into the duodenum to help dissolve fats. The duodenum is also a key absorption site for iron and calcium.
Next is the jejunum, the middle section, where the bulk of nutrient absorption takes place. Most sugars, amino acids, and fatty acids cross into your bloodstream here. The final and longest segment is the ileum, which absorbs whatever the jejunum didn’t catch, including vitamin B12 and bile salts that get recycled back to your liver. The ileum connects to the large intestine at a valve that prevents bacteria from flowing backward.
Why the Surface Area Matters
If you spread the small bowel flat, its inner surface would cover roughly 30 square meters, about the size of a small studio apartment. That’s far more than you’d expect from a tube only about 2.5 centimeters wide. The secret is a series of structural folds that dramatically increase the absorbing surface.
The inner wall is lined with millions of tiny, finger-like projections called villi, each about 0.5 to 1.6 millimeters long. Covering each villus are even smaller projections called microvilli. Together, these structures increase the intestinal surface area by hundreds of times compared to a smooth tube. Transport proteins sit on the surface of these villi, pulling nutrients across the intestinal wall and into the blood vessels and lymph channels embedded just beneath.
How Food Moves Through
Waves of muscle contraction called peristalsis push food through the small bowel. These contractions originate in the stomach and ripple through the entire length of the small intestine, mixing food with digestive juices and moving it steadily toward the large intestine.
Between meals, a different pattern takes over: the migrating motor complex. Think of it as a cleaning cycle. These periodic waves of contraction sweep residual, undigested material out of the small bowel and push it downstream. At the same time, digestive secretions from the stomach, pancreas, and liver increase to help flush the tube clean. This housekeeping function also keeps bacterial populations in check, preventing them from building up in the upper sections of the small intestine where they don’t belong.
Bacteria and the Small Bowel
Unlike the large intestine, which teems with trillions of bacteria, the small bowel is relatively sparse. Under normal conditions, the upper small bowel contains fewer than 100,000 bacteria per milliliter. When that number climbs too high, a condition called small intestinal bacterial overgrowth (SIBO) develops. Excess bacteria in the small bowel ferment food prematurely, producing gas and interfering with normal absorption. Common symptoms include bloating, diarrhea, and abdominal pain. Diagnosis typically involves a breath test that measures hydrogen or methane gas produced by the overgrown bacteria.
Common Small Bowel Conditions
Several conditions specifically target the small bowel. Celiac disease is an autoimmune condition triggered by gluten, a protein found in wheat, barley, and rye. In people with celiac disease, eating gluten causes the immune system to attack the villi lining the small intestine, gradually flattening them. This process, called villous atrophy, reduces the absorbing surface and leads to nutrient deficiencies, diarrhea, weight loss, and fatigue. Blood tests can detect specific antibodies, and a biopsy of the small bowel confirms the diagnosis by showing the characteristic tissue damage.
Small bowel obstruction is another common problem. This happens when something physically blocks the passage of food and fluid. The leading cause is adhesions, bands of scar tissue that form after abdominal surgery. Roughly 75% of diagnosed adhesions result from a previous operation. Hernias and inflammatory conditions are other frequent causes. A bowel obstruction typically causes severe cramping, vomiting, and an inability to pass gas or stool, and it often requires hospital treatment.
Crohn’s disease, a type of inflammatory bowel disease, frequently affects the ileum. It causes patches of deep inflammation that can lead to strictures (narrowings), fistulas (abnormal tunnels between organs), and malabsorption of nutrients normally absorbed in the lower small bowel.
How Doctors Examine the Small Bowel
The small bowel has historically been one of the hardest parts of the digestive tract to examine because of its length and its location between the stomach and colon. Standard upper endoscopy only reaches the duodenum, and colonoscopy only reaches the very end of the ileum. Two technologies fill that gap.
Capsule endoscopy involves swallowing a vitamin-sized camera that takes thousands of images as it travels through the entire small bowel. It’s painless and doesn’t require sedation, making it a common first step for investigating unexplained bleeding, chronic diarrhea, or abdominal pain. If something abnormal is found, or if a biopsy or treatment is needed, doctors can use balloon-assisted enteroscopy, a specialized scope that advances through the small bowel in stages, allowing direct visualization, tissue sampling, and even therapeutic procedures deep within the small intestine.