Peristalsis is a series of contractions that pushes solids or liquids through tubes in certain parts of the body, primarily the digestive system. Peristalsis is basically a wave of contractions, such as those used by the esophagus and small intestines to move food through the system.
This function is also used to push urine from the kidneys to the bladder and to move bile from the gallbladder to the duodenum.
In peristalsis, the upper muscle contracts while the muscle below it relaxes to allow the food or liquid to pass through. This muscle then continues the process by contracting, further pushing the substance through the system and also preventing anything from travelling back up the tube.
When food enters the mouth, it is chewed into what is known as a bolus, which then enters the esophagus as its swallowed. Peristalsis then forces the food down the esophagus and into the stomach. The bolus is then turned into a liquid substance known as chyme as it is processed in the stomach, and then it moves on to the small intestine. The chyme is pushed through the small intestine by peristalsis, but a different type of contraction pushes it through the large intestine.