Peristalsis

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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 the rhythmic contraction of smooth muscles to propel contents through the digestive tract.

In much of the gastrointestinal tract, smooth muscles contract in sequence to produce a peristaltic wave which forces a ball of food (called bolus while in the esophagus and chyme below the stomach) along the gastrointestinal tract. Peristaltic movement is initiated by circular smooth muscles contracting behind the chewed material to prevent backflow, followed by a contraction of longitudinal smooth muscles which pushes the digested food forward.

1 In the esophagus
2 In the small intestine
3 Etymology
4 External link

[edit] In the esophagus

After food is chewed into a bolus, it is swallowed to move it into the esophagus. Smooth muscles will contract behind the bolus to prevent it from being squeezed back onto the mouth, then rhythmic, unidirectional waves of contractions will work to rapidly force the food into the stomach. This process works in one direction only and its sole purpose is to move food from the mouth into the stomach.

In the esophagus, two types of peristalsis occur.

First, there is a primary peristaltic wave; once 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 8-9 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 the esophagus.

In the event that the bolus gets stuck or moves slower than the primary peristaltic wave (as can happen when it is poorly lubricated), stretch receptors in the esophageal lining are stimulated and a local reflex response causes a secondary peristaltic wave around the bolus, forcing it further down the esophagus, and these secondary waves will continue indefinitely until the bolus enters the stomach.

[edit] In the small intestine

Once processed and digested by the stomach, the milky chyme is squeezed through the pyloric valve into the small intestine. Once past the stomach a typical peristaltic wave will only last for 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.

As opposed to the more continuous peristalsis of the small intestines, fecal contents are propelled into the large intestine by periodic mass movements. These mass movements occur one to three times per day in the large intestines and colon, and help propel the contents from the large intestine through the colon to the rectum.

[edit] Etymology

The word is derived from New Latin and comes from the Greek peristaltikos, peristaltic, from peristellein, "to wrap around," and stellein, "to place."