Humans have two intestines: the small intestine and the large intestine. Together, they form one continuous tube that runs from the stomach to the anus, but they differ dramatically in size, structure, and function. Each one is further divided into distinct segments, giving the intestinal tract a total of about nine named sections.
The Small Intestine: Three Segments
The small intestine connects the stomach to the large intestine. Despite its name, it’s the longer of the two, stretching roughly 290 centimeters (about 9.5 feet) in a living person. It earns the label “small” because of its narrower diameter, averaging just 2.5 centimeters across. The small intestine has three segments, each with a slightly different job:
- Duodenum: The first and shortest section, connecting directly to the stomach. This is where digestive juices from the pancreas and liver mix with food to break it down chemically. Most digestion of fats, sugars, and proteins begins here.
- Jejunum: The middle section and the primary site for absorbing sugars, amino acids from proteins, and fatty acids from dietary fats. Specialized transport proteins in the lining pull these nutrients into the bloodstream.
- Ileum: The final section, which absorbs remaining nutrients, certain vitamins, and bile salts before passing leftover material into the large intestine through a one-way valve called the ileocecal valve.
What makes the small intestine so effective at absorption is its inner surface. The lining is covered in tiny finger-like projections called villi, and those villi are themselves covered in even tinier projections called microvilli. These folds increase the absorbing surface roughly 6.5 times for the villi and another 13 times for the microvilli. The result: the small intestine’s internal surface area is approximately 30 square meters, about the size of a studio apartment. Nearly all the calories and nutrients from the food you eat are absorbed here.
The Large Intestine: Six Named Parts
The large intestine is shorter but wider. It averages about 190 centimeters (roughly 6 feet) in length with a diameter of 4.8 centimeters, nearly double the width of the small intestine. It wraps around the edges of your abdominal cavity like a picture frame surrounding the coiled-up small intestine. The large intestine has six recognized parts:
- Cecum: A small pouch about 3 inches long where the small intestine empties into the large intestine. The appendix dangles from its end.
- Ascending colon: Runs upward along the right side of the abdomen, about 8 inches long. This is where most water and electrolyte absorption happens.
- Transverse colon: Crosses the abdomen horizontally from right to left.
- Descending colon: Travels down the left side, compacting waste as it goes.
- Sigmoid colon: An S-shaped curve that connects to the rectum.
- Rectum: The final holding area before stool exits through the anus.
The large intestine doesn’t absorb many calories. Its main jobs are reclaiming water so you don’t become dehydrated, absorbing electrolytes, and hosting trillions of gut bacteria. Those bacteria ferment fiber and other undigested material, producing short-chain fatty acids and gases in the process. When certain sugars like lactose reach the colon undigested (as happens with lactose intolerance), bacteria ferment them into gas and organic acids that draw water into the colon, which is why the result is bloating and loose stools.
How the Two Intestines Compare
Both intestines share the same four-layer wall structure: an inner lining (mucosa), a connective tissue layer beneath it, a muscular layer that contracts to push contents along, and an outer protective coating. But the small intestine’s lining is heavily folded to maximize absorption, while the large intestine’s lining is smoother. The large intestine’s internal surface area is only about 1.9 square meters, a fraction of the small intestine’s 30 square meters. That difference reflects their roles: one is built for extracting nutrients, the other for consolidating waste.
Combined, the entire digestive tract (including the mouth, esophagus, and stomach) has a total internal surface area of roughly 32 square meters. The small intestine accounts for the vast majority of that.
How Much Intestine You Actually Need
Losing portions of the intestine to surgery or disease can lead to a condition called short bowel syndrome, where the body can no longer absorb enough nutrients from food alone. The minimum length needed to maintain nutrition without intravenous feeding depends on which parts remain. If a portion of the colon is still intact, as little as 60 to 90 centimeters of small intestine can be enough. Without any colon, a person typically needs at least 150 centimeters of combined small intestine to absorb adequate nutrition. Below those thresholds, long-term intravenous feeding usually becomes necessary.
The large intestine is more forgiving. People who have their entire colon removed can still absorb nutrients normally because that work is done by the small intestine. The main trade-off is more frequent and looser stools, since the colon’s water-absorbing role is lost.