Peristalsis: How Does The Food Move?

One of the questions raised by a student on the reflection part of the last worksheet was, "How does the food move upward through the digestive system?"

It took me a moment to realize what he was asking.  He wanted to know how the food goes up in the large intestine, when it seemed that up to that point, all the food was moving down due to gravity.

I then asked, well, what would happen if we ate dinner, then laid down on the floor?  Wouldn't all our food stop moving down?  And what keeps it from coming back up every time we eat?

I shared the story of Cody, my friend Todd's dog. Cody has an unfortunate problem with his esophagus where his food will not stay down.  It is a condition called megaesophagus.  As a dog, he walks around on all fours, and his esophagus is essentially horizontal.  After he eats, his food just comes back out his mouth!  Poor dog!

He wasn't vomiting – his muscles just didn't keep the food going down.  This is the point where we examined a plastic tube cut to the actual length of our digestive system.  It's very long!  Here are the measurements of the average digestive tract:

• Mouth:  11cm, or 4.3 inches
• Esophagus:  25cm, or 9.8 inches
• Stomach:  22cm, or 8.7 inches
• Small intestine:  690cm, or 271.7 inches!
• Large intestine:  152cm, or 59.8 inches
• Rectum:  14cm, or 5.5 inches

To get a feel for how food is moved through our digestive tract, we greased a tennis ball in corn oil to simulate swallowed food, and pushed it into the plastic bag.  The ball barely fit, so it was hard to move it through.  The corn oil simulated mucus and saliva, which helps to lubricate food when we swallow it.  We pushed the ball through the tube, using our hands to squeeze it through.  This is very close to the actual movement that pushes food through the tract:  there are a series of muscles surrounding the digestive tract, and they contract or squeeze in a wave motion, pushing the food along.  This motion of muscles is called peristalsis.

We could have made this model a little more accurate by adding rubber bands wherever there was a border between organs, which is where most of the sphincter muscles are located, which close off the tract between organs.  This muscle is what helps the stomach send food down to the intestines a little at a time, and it's what keeps everything from falling out at the end of the digestive system!