Small Intestine

The small intestine is the part of the digestive tract that follows the stomach and is followed by the large intestine.

Formation and Orientation

The small intestine is on average between 22 feet 6 inches and 24 feet 4 inches. It is composed of three distinct structural parts; the duodenum, the jejunum, and the ileum.

The duodenum is the part of the intestine where most chemical breakdown happens; it is also the shortest part of the intestine. It starts at the duodenal bulb and stops at the ligament of Treitz. The jejunum is the mid-gut and begins at the duodenojejunal flexure. This part of the intestine works to absorb fully digested carbohydrates and proteins. Lastly, the ileum works to absorb vitamin B12 and bile salts. The ileum is the final part of the small intestine before the cecum which acts as the junction between the small and large intestines.


The small intestine is the organ where most of the digestive system’s chemical digestion and absorption takes place. Digestive enzymes produced by the pancreas help the small intestines to breakdown nutrients and minerals found in food.

The three main classes of nutrients that are digested in the intestines are protein, fats, and carbohydrates. Proteins are broken into amino acids and peptides. Fats are changed into glycerol and fatty acids. Carbohydrates are transformed into simple sugars (glucose). After the nutrients are broken down, they can undergo diffusion and gain the ability to be absorbed by the small intestine. Most absorption takes place in the jejunum (mid-gut). Other nutrients such as water, fructose and lipids are absorbed through diffusion.

Image Caption: Stomach colon rectum diagram. Credit: Henry W. Schmitt/Wikipedia