The small intestine is a long, pinkish-gray tube about 1 inch wide that coils and loops extensively through the central and lower abdomen. Stretched out, it measures roughly 20 feet in an average adult, though it sits compressed into tight folds that fit neatly inside your belly. From the outside it looks like smooth, glistening coils of soft tubing, but its inner surface tells a completely different story.
Overall Shape and Size
Picture a hollow tube, roughly the diameter of a quarter, that winds back and forth in overlapping loops. The small intestine connects to the stomach at one end and the large intestine at the other, filling much of the space between your ribcage and pelvis. It’s held in place by a fan-shaped sheet of tissue called the mesentery, which anchors the intestinal loops to the back wall of the abdomen. The mesentery is mostly fat, connective tissue, and blood vessels, and it spirals outward from a central point near a major artery. This is why, during surgery or imaging, the small intestine appears bundled and organized rather than tangled randomly.
Three Distinct Sections
The small intestine has three parts, each with a slightly different look.
The duodenum comes first, curving in a C-shape around the head of the pancreas. It’s the shortest section at only about 10 inches long and has thicker walls than the rest. The jejunum follows, spanning roughly 8 feet. It has a deeper red color because of its rich blood supply, and its walls are noticeably thicker and more muscular than the final section. The ileum is the longest stretch at about 10 feet. It’s thinner-walled, paler, and less vascular than the jejunum. If you placed the two side by side, you could tell them apart by color and thickness alone.
The Outer Surface
From the outside, the small intestine looks wet and shiny. This is because most of it is wrapped in a thin, smooth layer called the serosa, a slippery membrane that lets the intestinal loops glide against each other and the abdominal wall without friction. The surface appears pale pink to reddish, with visible blood vessels branching across it from the mesentery. The one exception is the duodenum, which sits pressed against the back of the abdominal cavity and lacks that shiny outer coating on one side.
What the Inside Looks Like
The interior of the small intestine looks nothing like the smooth outside. If you were to slice it open lengthwise, the first thing you’d notice is a series of permanent ridges running across the tube like stacked shelves. These circular folds are most prominent in the jejunum, where they stand out clearly, and they gradually flatten as you move toward the end of the ileum. Unlike the folds in your stomach, these ridges don’t smooth out when the intestine stretches. They’re built into the wall itself.
Look closer and the surface between those folds has a soft, velvety texture. That velvet appearance comes from millions of tiny finger-like projections called villi, each about 1 millimeter tall. They’re so small you can’t make out individual ones with the naked eye, but together they create a plush, carpet-like lining. The villi are what give the small intestine its remarkable ability to absorb nutrients.
The Microscopic Landscape
Under a microscope, the small intestine’s inner surface becomes even more dramatic. Each villus looks like a small, rounded finger covered in a single layer of cells. The surface of those cells is packed with even tinier projections called microvilli, so densely arranged that they resemble the bristles of a brush. Pathologists call this the “brush border.”
This layered design, circular folds plus villi plus microvilli, is an engineering solution for a specific problem: how to pack enormous absorptive surface area into a compact space. The folds triple the inner surface. The villi multiply it by about 6.5 times on top of that. The microvilli add another 13-fold increase. The end result is an internal surface area of roughly 30 square meters, about the size of a studio apartment floor. (You may have heard the old claim that it equals a tennis court. That’s an overestimate; more recent measurements put it closer to 30 square meters.)
The Wall in Cross-Section
If you sliced through the wall of the small intestine, you’d see four distinct layers stacked like rings. The innermost layer is the mucosa, the one with all the villi and folds that does the actual work of absorbing food. Beneath that sits the submucosa, a layer of connective tissue carrying blood vessels and nerves. In the duodenum specifically, this layer is packed with tiny glands that secrete protective mucus to buffer the acidic contents arriving from the stomach.
The third layer is smooth muscle arranged in two sheets: an inner ring that squeezes the tube narrower and an outer sheet running lengthwise that shortens it. Working together, these muscles create the wave-like contractions that push food along. The outermost layer is the serosa, the same glistening membrane visible from the outside.
How It Differs From the Large Intestine
People sometimes confuse the two, but the small and large intestines look quite different. The small intestine is narrower, smoother on the outside, and uniformly tubular. The large intestine is wider (about 2.5 inches across), has a puckered, segmented appearance from bands of muscle that cinch it at intervals, and often has small fatty pouches clinging to its outer surface. The small intestine’s inner lining is velvety with villi; the large intestine’s lining is flat by comparison, since its main job is absorbing water rather than nutrients.
The color difference is subtle but real. The small intestine tends toward pink or reddish-pink, especially in the jejunum, while the large intestine is generally paler and more gray-toned. During procedures like endoscopy, the small intestine’s circular folds are a reliable visual landmark that tells a doctor exactly where the camera is.