Peristalsis is a series of wave-like muscle contractions
that move food through the digestive tract.
It starts in the esophagus
where strong wave-like motions of the smooth muscle
move balls of swallowed food to the stomach.
There, the food is churned into a liquid mixture called chyme
that moves into the small intestine where peristalsis continues.
Stretching out a piece of intestine
will make it easier to see the wave-like motion.
The motion mixes and shifts the chyme back and forth.
This lets the bloodstream absorb nutrients
through the walls of the small intestine.
In the large intestine peristalsis helps water
from undigested food be absorbed into the blood stream.
Then, the remaining waste products
are excreted through the rectum and anus.