The small intestine (also referred to as the small bowel) is the specialized tubular structure between the stomach and the large intestine (also called the colon or large bowel) that absorbs the nutrition from your food. It is approximately 20-25 feet in length and is about as big around as your middle finger. It is divided into three parts: the duodenum, jejunum and ileum.
The beginning portion of the small intestine (the duodenum) begins at the exit of the stomach (pylorus) and curves around the pancreas to end in the region of the left upper part of the abdominal cavity where it joins the jejunum. The duodenum has an important anatomical feature which is the ampulla of Vater. This is the site at which the bile duct and pancreatic duct empty their contents into the small intestine which helps with digestion. The jejunum is the upper part of the small intestine and the ileum the lower part, though there is no clear delineation between the jejunum and ileum.
The lining of the small intestinal mucosa is very highly specialized for maximizing digestion and absorption of nutrients. The lining is highly folded to form microscopic finger-like projections called villi which increase the surface area to help with absorption. The lining also contains specialized groups of cells that produce chemicals which help digestion, provide immune defenses, and hormones that help to control coordination of digestive process of the intestine, gallbladder, and pancreas.
An important anatomic feature of the small intestine is also its highly integrated nervous system which lies within the wall of the intestine (this is called the enteric nervous system) The enteric nervous system plays a very important role in coordinating much of the activities of the small intestine including its muscular activity of propulsion (the moving of intestinal contents).
Function & Control
The small intestine is responsible for absorption of nutrients, salt, and water. On average, approximately nine liters of fluid enters the jejunum each day. The small intestine absorbs approximately seven liters, leaving only 1.5-2 liters to enter the large intestine. Significant abnormalities of the small intestine therefore, are manifested by malabsorption of nutrients, and diarrhea. The absorptive function of the small intestine is effected by an intricate array of cells within its lining that will absorb and secrete salts and nutrients as well as water in order to maintain normal salt and water balance within the body. The absorptive function is so efficient that in a normal adult with a normal diet, over 95% of ingested carbohydrates and proteins are absorbed.
Specific regions are adapted to perform specific functions. For example, the duodenum plays an important role in coordinating how the stomach empties as well as the rate of emptying of bile duct juices into the intestine. The duodenum is also a major site for absorption of iron. The jejunum is a major site for absorption of the vitamin folic acid and the end of the ileum is the most important site for absorption for the vitamin B12, and bile salts.