The primary function of the small intestine is digestion and nutrient absorption. Approximately 90 percent of food digestion and absorption occurs in the small intestine, while the rest takes place in the large intestine.
The small intestine is made up of several parts called the duodenum, jejunum and ileum. The duodenum is the first part; here, food is squirted from the stomach into the small intestine. The pancreas secretes digestive juices into the duodenum, which breaks down most of the materials in the small intestine. The gallbladder releases bile into the duodenum as well and is crucial for breaking down fats.
The food is transferred down the intestine via peristalsis, which consists of rhythmic contractions that cause food to be squeezed down the digestive tract. Small microscopic fibers coat the walls of the small intestine, called microvilli. These protrusions increase the surface area of the small intestine. Each microvillus contains a capillary, thereby promoting the absorption of nutrients into the blood.
The small intestine is also important in the functioning of the immune system. Probiotic flora growing inside the small intestine contribute to the host's immune system. In humans the small intestine is around 19 feet long. Material that cannot be digested by the small intestine passes down to the large intestine.