The small intestine has thee main portions: the duodenum, jejunum and ileum. The duodenum is the first part of the small intestine, located just after the stomach in the intestinal tract. The jejunum follows the duodenum, and the ileum is the final portion of the small intestine found just before the large intestine.
The duodenum, or first part of the small intestine, receives digestive system contents from the stomach through a tight band of muscle known as the pyloric sphincter. This part of the small intestine is approximately 10 inches in length, and it is the site where digestive enzymes from the pancreas and gall bladder empty into the small intestine. The stomach contents are very acidic when they enter the duodenum. Bicarbonate from the pancreas is released into the duodenum to neutralize the digestive contents and prevent the acid from damaging the duodenal lining.
Partially digested food is pushed through the duodenum into the jejunum by wavelike muscular contractions known as peristalsis. The jejunum is between 3 and 6 feet in length and is the part of the small intestine where the majority of chemical digestion and absorption of nutrients occurs. When it reaches the end of the duodenum, the partially digested food travels through approximately 6 - 12 feet of ileum, the past part of the small intestine where additional digestion and absorption occur. It is then emptied into the large intestine through the ileocecal sphincter.