III.
How Does Your Digestive System Work?
Changing food
for the body to use
Your digestive system
changes food into a form that your body cells can use. Complex
food is transformed into simple nutritive substances that can be
absorbed into the blood stream.
Food enters your digestive system when you put food in your
mouth. The food is moved from one part of the digestive system
to another. Along the way, it is broken down and changed. This
changing of food into a form your body cells can use is called
digestion. Chemicals called enzymes cause most of
the changes of digestion.
The Mouth and
Esophagus
Think about biting into an apple. Biting is the first step in
breaking down food for your body. The mouth and teeth are the
first step in breaking food down. When you chew food, your teeth
grind and cut it into smaller pieces. The more you chew, the
smaller the pieces become.
As
you chew, your tongue helps mix the food with saliva, the
liquid in your mouth. Salivary glands near your mouth
make saliva. Tiny tubes carry the saliva to your mouth. Saliva
makes your food wet and easy to swallow.
An
enzyme in saliva begins to break apart some of the material that
makes up food. For example, bread is made of starches. Simple
sugars are joined together to make up starches.
starch
Simple
sugar
Saliva
begins to break the starches into the sugars that they are made
of. When you chew such foods as bread and crackers, you might
notice a sweet taste. This taste tells you that the enzyme in
saliva is at work.
After you
chew your food, your tongue moves it to the back of your mouth.
You swallow, and the food enters a tube called the esophagus.
Muscles in the esophagus push the food along toward the stomach.
The tongue helps push the food in the back of the mouth, and the
muscles in the esophagus move the food down the tube.
The Stomach and Small Intestine
The
stomach is a sac shaped like a “j” that receives food. In
the stomach, food is mixed with acids. The muscles in the
stomach move, which breaks down the food. The stomach is
protected from the acid by a lining. Cells in the lining of the
stomach make juices. The wall of the stomach is made of muscles
that squeeze and mix food with the juices. Enzymes in these
juices break down more of the food, which stays in the stomach
for 2 to 4 hours. The food is a thick liquid by the time it
leaves the stomach.
The
stomach squeezes the food into the small intestine, which
is the final place for digestion. This organ is a long tube
whose muscles push food along in a squeezing motion called
peristalsis. This motion is much the same as squeezing a tube of
toothpaste. All of this movement causes the noise when we say
our stomach is” growling”. Most digestion takes place in the
small intestine. Juices made in the lining of its walls mix with
the food. Nearby organs like the liver and the
pancreas also make juices. Tiny tubes carry these juices to
the small intestine. Different kinds of enzymes in all the
juices break down the food into forms that the body can use.
The
next job of the small intestine is to send these nutrients to
the rest of the body. Blood vessels fill the wall of the small
intestine. The nutrients move through the thin walls of the
blood vessels into the blood, which carries them everywhere in
the body.
Food that is
not digested
Not
everything in food can be broken down into the parts that make
it up. Waste products and food that are not absorbed in the
small intestine pass into the large intestine.
The material that enters the large intestine is mostly liquid.
The large intestine removes much of the water in this liquid.
The water passes through the thin lining of the large intestine
into blood vessels. The undigested food is almost solid by the
time it reaches the end of the large intestine. The large
intestine stores this undigested food until it leaves the body.
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