How the Small Intestine Works
The small intestine, or small bowel, is a hollow tube about 20 feet long that runs from the stomach to the beginning of the large intestine. The small intestine breaks down food from the stomach and absorbs much of the nutrients from the food.
The small intestine has three parts:
The duodenum is the first part of the small intestine. The main role of the duodenum is to complete the first phase of digestion. In this section of the intestine, food from the stomach is mixed with enzymes from the pancreas and bile from the gallbladder. The enzymes and bile help break down food.
The jejunum is the second part of the small intestine. After food is broken down in the duodenum, it moves to the jejunum, where the inside walls absorb the food's nutrients. The inside walls of the jejunum have many circular folds, which make its surface area large enough to absorb all of the nutrients that the body needs.
The ileum is the third part of the small intestine. It absorbs bile acids, which are returned to the liver to be made into more bile, then stored in the gallbladder for future use in the duodenum. The ileum also absorbs vitamin B12, which the body uses to make nerve cells and red blood cells.
After food is processed in the small intestine, it passes into the large intestine, also called the large bowel or colon. The large intestine, which is about 5 feet long, extracts most of the water from this food and distributes it to the body; the remaining material passes through the colon and out of the body as feces.
Learn more about the intestine.