What Does the Small Intestine Do?
The small intestine is part of the digestive system, which is also called the gastrointestinal (GI) system. Specifically, it is part of the alimentary canal, which is the tract that runs from the mouth to the anus. The alimentary canal, along with several organs that aid digestion, make up the GI system. Together, the large intestine and small intestine complete the job of digestion as food moves out of the stomach. Here are the details about the small intestine anatomy and what it does.
Small Intestine Anatomy
The small intestine is quite long. The average small intestine length is about 22 feet, making it the longest section of the alimentary canal. It coils and folds to compactly fit in your abdomen. Its diameter is very narrow compared to the large intestine, which is why it gets the name “small.”
Despite its narrowness, the small intestine has an incredible amount of surface area. This is the key to how it does its job. The inner surface of the small intestine is covered with circular folds called mucosal folds. These folds are covered with millions of finger-like projections called villi. The villi, in turn, have microvilli all over them. These folds and projections give the small intestine a massive surface area of about 2,700 square feet. That’s almost the size of a doubles tennis court! Like other parts of the alimentary canal, the small intestine has a muscular wall and cells that secrete mucus.
There are three main small intestine parts or sections:
- Duodenum: Connected to the bottom of the stomach is the C-shaped first part of the small intestine. It meets the stomach via a one-way valve called the pyloric sphincter. The duodenum is nearly 10 inches long, making it the shortest portion of the small intestine. The liver, gallbladder and pancreas connect to ducts that enter the small intestine in the duodenum.
- Jejunum: This is the middle coiled part of the small intestine. Its approximate eight feet of length connects to the final section.
- Ileum: This is the last section of the small intestine. It is slightly longer than the jejunum and is also coiled securely in the abdomen. It joins the large intestine via another one-way valve—the ileocecal valve.
Small Intestine Physiology
The small intestine has several jobs. It finishes digestion by mixing partially digested food with more digestive enzymes and absorbs water and nutrients. Then, it passes the leftover material to the large intestine to absorb the remaining water and form solid waste—or feces.
Each of the three sections has a role to play in the overall small intestine function:
- Duodenum: The duodenum receives stomach contents—or chyme. The chyme is only partially digested at this point. The liver, gallbladder and pancreas empty bile, digestive enzymes, and bicarbonate into the duodenum to further digest the chyme. Bile helps digest fats and fat-soluble vitamins, bicarbonate neutralizes stomach acid, and digestive enzymes further digest fats and carbohydrates.
- Jejunum: The jejunum takes care of 90% of nutrient absorption, mostly in the first half of this section. This includes proteins, carbohydrates, vitamins and minerals.
- Ileum: The ileum absorbs water and nutrients that weren’t absorbed in the jejunum. It also absorbs vitamin B12 and reabsorbs bile salts. Then, it passes the leftovers into the large intestine.
The time it takes for food to pass through the small intestine is about six hours. Contents move through the digestive tract when muscles in the wall contract in wavelike motions called peristalsis. Peristalsis in the small intestine moves anywhere from 1 to 3 gallons of chyme in a day. When it passes chyme to the large intestine, chemical digestion is complete and about 90% of water has been absorbed. The large intestine completes the process and eliminates the waste.