The billowing peristaltic contractions of the alimentary canal, or any other tubular structures, result in the contents of such areas being forced onward toward a bodily opening.
Peristaltic movements are initiated by circular smooth muscles contracting behind the chewed food and drinking of liquids to prevent them from moving back into the mouth, which is then followed by a contraction of longitudinal smooth muscles which push the digested food onward.
Intestinal movements are not always the same.
When food is in the small intestine, the muscles that encircle the tube constrict about seven to twelve times a minute, segmenting the tube so it is similar to a series of sausages. These rapid contractions move the food back and forth, churning it, kneading or pressing it, and mixing it with the digestive juices.
Besides these mixing movements, the small intestine also makes propulsive, or peristaltic, movements: waves that move food through the system. In this part of the gastrointestinal tract, peristaltic movements are usually weak and infrequent.
These activities ensure that food will stay in one place long enough to be absorbed. Only when toxic substances enter the small intestine are the propulsive movements strong and quick, in order to expel the poisons as fast as possible.