The small intestine is a part of the human body's digestive system, coming after the stomach and before the large intestine in the sequence of food digestion. At 5 meters long, it is also the longest organ in the digestive system, though it is significantly narrower than the large intestine.
The primary functions of the small intestine are to break down food by means of chemicals and to absorb some of the food's nutrition through the intestine walls. Food enters the small intestine from the stomach and travels through the intestine by means of peristalsis. The first section of the small intestine, called the duodenum, receives digestive juices from the pancreas that aid in the breakdown of fats, proteins and carbohydrates. These digestive juices also help to neutralize acid that accompanies the food from the stomach. Bile from the gall bladder enters the duodenum to further aid in the breakdown of fatty substances.
The walls of the small intestine contain small protruding components called microvilli, and these small structures help to increase the wall's absorption capabilities so that large amounts of nutrients may pass through the intestine walls into the blood stream. The remaining waste passes into the large intestine for later expulsion.