Bodily Defenses Image Map

Over the years scientists have worked to unlock the once mysterious process of how humans digest food. Some of the approaches can even be used by you to discover what happens in the digestive tract!

How do we learn about the different parts of the digestive system?
When we dissect a dead animal, we can see the different parts along the tract that begins in the mouth and ends in the anus. See pictures here. The cells can be seen by making microscope slides.

How could we see where digestion begins?
How would you find out if saliva has any digestion function?  A quick way to find out is to compare a piece of steak (which has a lot of protein) and a cracker (which has a lot of starches).  Place the meat in your mouth and do not chew it for about 2 minutes. What happens?   You should discover that the steak keeps its shape. Next, place the cracker in your mouth without chewing it for about two minutes. You should notice that the cracker becomes soft in your mouth and you may even taste something sweet. This is because the cracker is composed of carbohydrates, which saliva is able to digest at least partly. The carbohydrates in the cracker begin to be broken down into individual glucose molecules by an enzyme in your saliva. This enzyme only breaks down carbohydrates, not proteins. Since the steak was mostly composed of proteins, it did not start breaking down like the cracker.

How do we know what goes on in our stomach?
The stomach contains acids that turn most of your food into a liquid. Have you ever been sick and felt a burn in the back of your throat after vomiting? If you have, then you have experienced the power of the acids in your stomach. Stomach secretions also contain strong enzymes.  The digestive functions of the stomach were discovered during an experiment performed by Doctor Beaumont.  In his experiment, Doctor Beaumont tested the digestive power of stomach fluids through a "fistula" (see the graphic on the right) that connected the stomach to the patient's skin.  Doctor Beaumont tied food to a string, poked it through the fistula, and pulled it back out at varying times to see what had happened to the food.  He observed that the food had changed, varying with the amount of time spent in the stomach and the nature of the food itself. One of the major things that was noticed by Doctor Beaumont is that proteins were especially degraded by the acids of the stomach. See Story Time for more.


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