The small intestine, which is about 20 feet long in the average adult, breaks down starches, protein, and carbohydrates. Dr. Patel, what else can you tell us about the small intestine?
Well, Dr. Mansfield, food empties out of the stomach through a ringlike bundle of muscles, called the pyloric sphincter, into the small intestine. This first part of the small intestine, where it connects to the stomach, is called the duodenum.
The middle part of the small intestine is called the jejunum. In the jejunum, a layer of muscle contracts and relaxes to move the food through the small intestine. This movement, which resembles an ocean wave moving through the small intestine, is called peristalsis.
As the food travels through the jejunum, it mixes with digestive juices secreted by the pancreas, liver, and small intestine. The walls of the small intestine absorb the digested nutrients and fluid into the bloodstream, and the blood delivers the nutrients to the rest of the body.
The third and last section of the small intestine connects to the large intestine, or colon. It's called the ileum. The ileum mainly absorbs vitamin B12, bile salts, and other nutrients not absorbed by the jejunum. Whatever is not absorbed into the bloodstream in the small intestine passes through the ileocecal valve as waste products into the colon.