Peristalsis
Peristalsis ( , ) is a radially symmetrical contraction and relaxation of muscles that propagates in a wave down a tube, in an anterograde direction. Peristalsis is progression of coordinated contraction of involuntary circular muscles, which is preceded by a simultaneous contraction of the longitudinal muscle and relaxation of the circular muscle in the lining of the gut. 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 before being transformed into chyme in the stomach) along the tract. The  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. Earthworms use a similar mechanism to drive their locomotion, and some modern machinery imitate this design. The word comes from New Latin and is derived from the Greek "peristellein", "to wrap around," from "peri"-, "around" + "stellein", "draw in, bring together; set in order". Human physiology. Peristalsis is generally directed caudal, that is, towards the anus. This sense of direction might be attributable to the polarisation of the myenteric plexus. Because of  the reliance of the peristaltic reflex on the myenteric plexus, it is also referred to as the myenteric reflex. Mechanism of the peristaltic reflex. The food bolus causes a stretch of the gut smooth muscle that causes serotonin to be secreted to sensory neurons, which then get activated. These sensory neurons, in turn, activate neurons of the myenteric plexus, which then proceed to split into two cholinergic pathways: a retrograde and an anterograde. Activated neurons of the retrograde pathway release substance P and acetylcholine to contract the smooth muscle behind the bolus. The activated neurons of the anterograde pathway instead  release nitric oxide and vasoactive intestinal polypeptide to relax the smooth muscle caudal to the bolus. This allows the food bolus to effectively be pushed forward along the digestive tract. 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 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). In the esophagus, two types of peristalsis occur: During vomiting, the propulsion of food up the esophagus and out the mouth comes from the contraction of the abdominal muscles; peristalsis does not reverse in the esophagus. Stomach. When a peristaltic wave reaches at the end of the esophagus, the cardiac sphincter (gastroesophageal sphincter) opens allowing the passage of bolus into the stomach. The gastroesophageal sphincter normally  remains closed and does not allow the stomach's food contents to move back. The churning movements of the stomach's thick muscular wall mix the food thoroughly with the acidic gastric juice and are called the chyme. The muscularis layer of the stomach is thickest and maximum peristalsis occurs here. After short intervals, the pyloric sphincter keeps on opening and closing so the chyme is fed into the intestine in installments. Small intestine. Once processed and digested by the stomach, the semifluid chyme is passed through the pyloric sphincter into the small intestine. Once past the stomach, a typical  peristaltic wave lasts only a few seconds, traveling at only a few centimeters per second. Its primary purpose is to mix the chyme in the intestine rather than to move it forward in the intestine. Through this process of mixing and continued digestion and absorption of nutrients, the chyme gradually works its way through the small intestine to the large intestine. In contrast to peristalsis, segmentation contractions result in that churning and mixing without pushing materials further down the digestive tract. Large intestine. Although the large intestine has peristalsis of the type that the small intestine uses, it is not  the primary propulsion. Instead, general contractions called mass action contractions occur one to three times per day in the large intestine, propelling the chyme (now feces) toward the rectum. Mass movements often tend to be triggered by meals, as the presence of chyme in the stomach and duodenum prompts them (gastrocolic reflex). Minimum peristalsis is found in the rectum part of the large intestine as a result of the thinnest muscularis layer. Lymph. The human lymphatic system has no central pump. Instead, lymph circulates through peristalsis in the lymph capillaries, as well as valves in the capillaries, compression during contraction  of adjacent skeletal muscle, and arterial pulsation. Sperm. During ejaculation, the smooth muscle in the walls of the vas deferens contracts reflexively in peristalsis, propelling sperm from the testicles to the urethra. Earthworms. The earthworm is a limbless annelid worm with a hydrostatic skeleton that moves by peristalsis. Its hydrostatic skeleton consists of a fluid-filled body cavity surrounded by an extensible body wall. The worm moves by radially constricting the anterior portion of its body, increasing length via hydrostatic pressure. This constricted region propagates posteriorly along the worm's body. As a result, each segment is extended forward, then  relaxes and re-contacts the substrate, with hair-like setae preventing backward slipping. Various other invertebrates, such as caterpillars and millipedes, also move by peristalsis. Machinery. A peristaltic pump is a positive-displacement pump in which a motor pinches advancing portions of a flexible tube to propel a fluid within the tube. The pump isolates the fluid from the machinery, which is important if the fluid is abrasive or must remain sterile. Robots have been designed that use peristalsis to achieve locomotion, as the earthworm uses it.