Abdomen and Small Intestine
Even though it is called the "small" intestine, it is about 16 feet (5 m) long. It is the longest part of the gastrointestinal tract. As can be seen in the figure to the right, the small intestine winds and folds upon itself to squeeze into the lower abdomen. It is called the small intestine because of its 1 to 1-1/2 inch (3.5 cm) diameter as compared to the large intestine with a 2-1/2 inch (6 cm) diameter.
The small intestine is made up of three parts: the duodenum, jejunum and ileum. The stomach empties into the duodenum through the pyloric sphincter. Hormonal control regulates the rate of stomach emptying into the duodenum. The chyme, or partially digested food from the stomach, is pushed along the small intestine by muscle contractions called peristaltic waves.
Most of the chemical digestion and breakdown of the food happens in the duodenum. Food is mixed with bile from the gallbladder and digestive juices from the pancreas. The pancreas also releases bicarbonate into the duodenum to neutralize potentially harmful acid from the stomach. Absorption of vitamins, minerals and other nutrients begins in the lower part of the duodenum.
Once the chyme leaves the duodenum, it enters the middle section of the small intestine, the jejunum. The jejunum is responsible for absorption the majority of nutrients from the chyme. The digested food passes into the capillaries and lymphatic vessels in the wall of the intestine. Nutrients such as glucose and amino acids are absorbed into capillaries, while fats are absorbed into the lymph vessels.
The ileum is the last section of the small intestine. It mainly absorbs vitamin B12 and bile salts. Once the nutrients enter the capillaries and lymphatic vessels they are transported to tissue and organs of the body to support cell processes. The ileum connects with the large intestine. Any food that remains undigested and unabsorbed by the small intestine passes into the large intestine.