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The Digestive System
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Dr. Timmel
What is the digestive
system?
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The digestive system is a one-way tube the begins
at the mouth and ends at the anus. It is also
known as the alimentary canal.
The purpose of the digestive system is to break
down large organic molecules (Carbohydrates,
Proteins, and Lipids) into smaller ones that can
diffuse into the circulatory system.
The digestive system also absorbs vitamins,
minerals and water (inorganic molecules). These
do not need to be broken down into smaller
molecules.
Interactive diagram of human
digestive system
Lets eat!
To understand what happens in the
digestive system we will investigate the path
a roast beef sandwich with mayo and
lettuce takes when we eat it.
 Roast beef= Protein; Bread= Carbs;
Mayo = Lipid; Lettuce = roughage.
(Vitamins, minerals and water are also present
in the sandwich)
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Enter the mouth
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As you bite into the sandwich, your teeth help to
shred and mash up the food. This is a type of
physical or mechanical digestion.
As you chew, your salivary glands secrete saliva
which helps moisten your food. Saliva also contains
Salivary Amylase, an enzyme which begins to break
down carbohydrates. This is a type of chemical
digestion.
Your tongue helps to position the food between
your teeth for chewing, and then helps move the
food to the back of your mouth for swallowing.
Once your food leaves your mouth and enters your
throat it is known as bolus.
Down the gullet
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Once the bolus leaves the mouth it goes through
your throat and into your esophagus.
No digestion occurs here, the esophagus is just a
tube which connects your mouth to your stomach.
Prior to entering the stomach it passes through the
gastro-esophogeal spincter. This muscular ring
helps prevent the contents of your stomach from
moving back into the esophagus.
Sometimes the acidic contents of your stomach
does get into the esophagus. This is what causes
the painful condition known as “heart burn”.
Going down the “wrong
pipe.”
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As you swallow, a flap of tissue called the
epiglottis covers the top of the trachea to
prevent food from entering the lungs.
Sometimes food or drinks can get past the
trachea which causes a fit of coughing to
help get it out of there and back into the
esophagus.
It is impossible to breathe and swallow at
the same time.
Get in my stomach!
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The environment of the stomach is very
acidic. This is because the enzymes
present which begin the digestion of
proteins are only active at a pH of about 2.
Enzymes which break down proteins are
called proteases.
The acidic environment also helps to kill the
bacteria which are present in/on all food.
Stomach continued.
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The churning of your stomach is a type of
physical digestion. All the food you eat
becomes completely liquefied in the
stomach.
This semi-digested, acidic liquid is now
referred to as chyme.
Chyme is then squirted through the pyloric
spincther into the small intestine.
The small intestine.
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It is here in the small intestine where
most of the business of digestion takes
place.
The first 10 centimeters of the small
intestine is called the duodenum.
The chyme is very acidic when it first
enters the S.I., so it needs to be
neutralized.
The Pancreas and the
Liver
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These are two very important organs
in the digestive system.
Though food never enters into either
the pancreas or the liver, their
secretions are vital to digestion.
First we will look at the pancreas
The pancreas
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The pancreas neutralizes stomach acid by
secreting bicarbonate into the duodenum.
The pancreas also secretes three types of
digestive enzymes: Proteases, Amylases
and Lipases.
These secretions travel from the pancreas
to the duodenum through the common bile
duct.
The liver
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The liver secretes a substance called bile.
Bile helps in the physical digestion of lipids
by emulsifying them. This means it breaks
a large glob of fat into smaller ones for
easier digestion.
Bile is stored in the gall bladder and
secreted into the duodenum through the
common bile duct.
Duodenum summary.
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In this first section of the small
intestine:
– The chyme is neutralized.
– Carbohydrates complete their chemical
digestion.
– Proteins complete their chemical
digestion
– Lipids begin and complete their chemical
digestion.
Absorption in the S.I.
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As your nutrients pass out of the duodenum into
the rest of the S.I., they begin to be absorbed into
the blood stream.
Absorption is carried out by special finger-like
extensions in the walls of the S.I. called villi.
The villi’s shape greatly increases the surface area
of the S.I., allowing it to absorb nutrients at a much
faster rate.
By the time your food reaches the end of the S.I.,
all that remains are roughage, water and bacteria.
Drawing of villi
The Appendix
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Near the point where the small intestine and
large intestine meet, there is a small
fingerlike projection called the appendix.
In modern humans, the appendix serves no
purpose. It is believed that in our
evolutionary past the appendix did aid in
digestion.
Organs, such as the appendix are called
vestigial organs.
The Large intestine
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Here in the large intestine most everything
useful except water and some vitamins has
been absorbed.
The main job of the large intestine is to
reabsorb water.
When the Large intestine removes too much
water, constipation is the result. If not
enough water is reabsorbed, diarrhea is the
result. Both conditions can actually kill!
Bacteria and the large
intestine.
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The large intestine is also home to
E.coli bacteria.
These bacteria are actually beneficial
to use here as they synthesize vitamin
K.
The rectum
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After water has been removed, your
food, now called feces, is temporarily
stored in the rectum.
At this point all that is left is roughage,
bacteria and a small amount of water.
Feces are then eliminated from the
body through the anus.
Recap.
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Here is the pathway your food takes
through your body.
MouthEsophagusStomach
Small IntestineLarge Intestine 
Rectum Anus.
Chemical digestion
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Chemical digestion for carbohydrates
begins in the mouth and ends in the
Small Intestine.
Chemical digestion of proteins begins
in the stomach and ends in the small
intestine.
Chemical digestion of lipids begins and
ends in the Small intestine.
Physical (mechanical)
digestion.
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In the mouth, all food in physically
digested by the action of your teeth,
tongue and jaws.
In the stomach, all food is physically
digested by churning.
In the duodenum lipids are physically
digested by bile. (Emulsification).