The small intestine is responsible for completing digestion and for absorbing the usable food products into the lymphatic system and bloodstream. The small intestine itself consists of a coiled, narrow tube (1-2 inches in diameter), between 19 and 22 feet (about 6-7 meters) long, in the lower abdomen, below the stomach.
The small intestine extends from the duodenum, where it accepts the chyme (predigested food), to the ileocecal orifice, where it passes semifluid food byproducts to the large intestine. The food is passed through the intestinal tract be wavelike contractions, called peristaltic waves, in the intestinal wall. The food is further digested by bile and other digestive juices deposited into the duodenum from the gallbladder, pancreas, and liver.
The digesting food passes by the millions of villi (projections) on the inside wall of the intestines, which absorb proteins and carbohydrates into their capillaries, and lymphatic nodules, which absorb fats. The villi pass the proteins and carbohydrates to the liver for metabolic processing, and the lymphatic nodules pass the fats through the lymphatic system into the bloodstream. The small intestine is anchored to the spinal column by a vascular membrane called the mesentery.