thumb|a time-space diagram of a peristaltic wave after a water swallow. high pressure values are red, zero pressure is blue-green. the ridge in the upper part of the picture is the high pressure of the upper esophageal sphincter which only opens for a short time to let water pass. peristalsis is a radially symmetrical contraction and relaxation of muscles that propagates in a wave down a tube, in an anterograde direction. in much of a digestive tract such as the human gastrointestinal tract, smooth muscle tissue contracts in sequence to produce a peristaltic wave, which propels a ball of food (called a bolus while in the esophagus and upper gastrointestinal tract and chyme in the stomach) along the tract. peristaltic movement comprises relaxation of circular smooth muscles, then their contraction behind the chewed material to keep it from moving backward, then longitudinal contraction to push it forward. catastalsis is a related intestinal muscle process.marieb, elaine n. & hoehn, katja "human anatomy & physiology" 8th ed., benjamin cummings/pearson, 2010 earthworms use a similar mechanism to drive their locomotion, and some modern machinery imitates this design. the word comes from new latin and is derived from the greek peristallein, "to wrap around," from peri-, "around" + stallein, "to place". ==human physiology== ===esophagus=== after food is chewed into a bolus, it is swallowed and moved through the esophagus. smooth muscles contract behind the bolus to prevent it from being squeezed back into the mouth. then rhythmic, unidirectional waves of contractions will work to rapidly force the food into the stomach. the migrating motor complex (mmc) helps trigger peristaltic waves. this process works in one direction only and its sole esophageal function is to move food from the mouth into the stomach (the mmc also functions to clear out remaining food in the stomach to the small bowel, and remaining particles in the small bowel into the colon). thumb|right|a simplified image showing peristalsis in the esophagus, two types of peristalsis occur: * first, there is a primary peristaltic wave, which occurs when the bolus enters the esophagus during swallowing. the primary peristaltic wave forces the bolus down the esophagus and into the stomach in a wave lasting about 8â9 seconds. the wave travels down to the stomach even if the bolus of food descends at a greater rate than the wave itself, and will continue even if for some reason the bolus gets stuck further up
Digestive_system