two types of gastrointestinal contraction

Because the small intestines have to do a lot of absorption, they do not want the chyme zooming through them quickly.  Peristalsis occurs, but weakly, and it doesn't push the food far.  Mixing occurs quite a bit, but a particular kind of mixing called segmentation.  Segmentation would look almost like random squeezing movements and allow for the chyme to get continually broken up... it also can push the chyme backward a bit, enabling more time for absorption.


This is a coordinated movement, caused by coordinated contractions of the muscular layers. Circular muscle contracting behind a bolus of food or behind the chyme, pushes it along.  This is most elegantly done in the esophagus.  After one location of circular smooth muscle contracts, the adjacent location contracts, then the next adjacent location contracts, etc., propelling the food along.  Meanwhile, the circular smooth muscle relaxes right after it contracts.


In the large intestines, the goal of the movement is to gather the chyme materials into a mass, forming the feces, and send this mass toward the anus.  The movements that enable the large intestines to reach their goal are still just mixing and peristalsis.   But now, these movements are slower, and more coordinated along longer stretches of the large intestines.  The peristaltic waves are referred to as "mass movements," since they push the mass within them toward the anus.  These larger movements force accumulation of materials, rather than break up of the mass.  It is also peristalsis that enables the feces to leave the rectum when the anal sphincters are relaxed.

Excellent!  Thankyou!