Digestion
February 2015
Key Skills
• Identify the major parts of the gastrointestinal tract,
its accessory organs, and their functions.
• Parts of tract: Mouth, esophagus, stomach, small
intestine, large intestine
• Accessory organs: Tongue, teeth, salivary glands,
pancreas, gallbladder, liver
• Explain where and how different nutrients are
broken down and absorbed.
• Describe causes and effects of several common
digestive system ailments, such as stomach ulcers
and diarrhea.
Where does digestion begin?
A) In the mouth
B) In the stomach
C) In the small intestine
D) In the large intestine
E) Somewhere else
Digestion: Physical and Chemical
Breakdown of Food
Physically separating food into pieces
(Examples: Chewing; emulsifying lipids
into smaller droplets with bile)
Chemically breaking down food
macromolecules into pieces
(Examples: Enzymes digesting starch in
the mouth and protein in the stomach.)
An Unconventional Answer
Most textbooks agree that we begin digestion in
our mouths, with chewing…
But it might be fair to argue that, in a sense,
humans have learned how to start digesting food
outside of their bodies! For example, cooking can
unfold proteins and make them more digestible.
Roles of the Digestive System
Ingestion: Taking food
into the body.
Digestion: Breaking
down food
mechanically and
chemically.
Absorption: Getting
nutrients from food
(small intestine).
Excretion
(defectation):
Releasing solid waste.
The Digestive System
Consists of…
GI/digestive tract:
Mouth, pharynx,
esophagus,
stomach, small
intestine, large
intestine, anus
Accesory organs:
Teeth, tongue,
salivary glands,
pancreas, liver,
gallbladder
How Does Chewing Work?
Take a moment to
reflect on how you
chew and swallow
food. What, exactly,
happens when you
chew a piece of
food and swallow
it? Write down as
many details as you
can think of (limit
one paragraph).
Chewing
Chewing involves:
• Grinding and cutting
by the teeth
• Chemical breakdown
by salivary enzymes,
such as amylase,
which breaks starch.
• Shaping of food into
a bolus by the tongue
• Tasting by the
tongue (to detect
poisons and
nutrients)
Salivary Amylase
The enzyme salivary
amylase (a carbohydrase)
is a protein that is shaped
to stick to starch and help
it break apart!
Electrolytes, or
charged particles,
in the saliva help
the amylase
function properly.
(Chloride and
calcium)
The Teeth
The structure of teeth
suits their function.
Hard enamel
makes them
resistant to wear
and tear.
Molars and
premolars near the
back of the mouth
grind.
Incisors and canine
teeth pinch and cut.
Cavity Formation
Cavities form when bacteria
growing on the teeth secrete
acidic waste that slowly
breaks through the enamel
and dentin to reach the pulp.
Sugary and starchy foods are a
big energy source for these
bacteria.
Bacteria at the gumline may also
get into the bloodstream and
increase the risk of heart
disease.
It’s important to floss and brush
regularly to protect your teeth
and heart!
Swallowing
(real-time MRI source: Biomedizinische NMR Forschungs GmbH)
The Esophagus
Like the rest of the digestive tract, the esophagus is lined
with muscles and a protective layer called the mucosa.
These muscles squeeze food
through the esophagus like
toothpaste through a tube.
This movement is called
peristalsis, and is also used
to move food through the
intestines.
The opening between the
stomach and esophagus is
the cardioesophageal
sphincter. Acid reflux, or
heartburn, occurs when
stomach acid flows back up
through this opening, and
can damage the mucosa.
Peristalsis
Emesis (Vomiting)
In vomiting, food and stomach acid are moved
upwards through the esophagus (reverse peristalsis.)
This allows the body to eliminate poisonous
substances. However, it also depletes the body of
water and electrolytes.
People who throw up frequently, such as bulimics who
purge by vomiting, risk damaging their esophageal
mucosa, teeth, and even their heart (which needs
those electrolytes!)
Tooth erosion
from bulimia:
The Stomach
• Stores and churns food,
then gradually releases it
into the small intestine.
• Secretes acids and
enzymes (pepsin and
trypsin) that digest
proteins. Mucosa
protects stomach.
• Absorbs alcohol, some
medications
Digestion in the
Stomach
Protein digestion begins in
the stomach, and
starch/carbohydrate
digestion continues there!
Fats still need to be
separated into droplets
before they can be digested
properly.
Peptic (Stomach) Ulcers
• Holes in the stomach’s mucosa
(lining).
• Primary cause is usually the
bacterium Helicobacter pylori.
• Painkiller overdose and alcohol
use can also contribute. Taking
painkillers without water is not
recommended.
• Factors that increase stomach
acid secretion, such as stress,
might contribute to the
worsening of ulcers…
• …but the usual treatment now is
antibiotics to get rid of H. pylori.
What Happens to the Stomach
Acid and Enzymes?
• When the stomach
releases its acid and
enzymes into the
duodenum… why
doesn’t that mix digest
our intestines?
• The stomach lining is
thick and protected, but
the intestinal lining has
to be thinner to absorb
nutrients…
Safety Mechanisms
Why don’t we digest our own bodies?
1) Stomach enzymes are inactive without
stomach acid. Low pH activates them.
2) Stomach acid is neutralized when it
reaches the small intestine by pancreatic
IInactive pepsinogen
Active pepsin
bicarbonate.
Example: Pepsin, a
protein that breaks
down other proteins, is
made when pepsinogen
is activated by stomach
acid’s low pH.
(gets cut off)
Small Intestine + Accessory Organs
• The small
intestine absorbs
most nutrients.
• Continues
digestion. Bile
from liver, stored
in gallbladder,
breaks fat to
droplets. Enzymes
from pancreas and
intestine lining
break down starch
(amylase), protein
(chymotrypsin),
and lipids
(lipases).
The Folds of the Small Intestine Maximize
Surface Area for Absorption
Fingerlike projections = villi
(singular: villus).
Microscopic fingerlike
projections on individual
cells: microvilli
Absoption in the Small Intestine
Lipids
Amino
acids
Sugars
The Large Intestine
• The large intestine, or colon, reabsorbs water and some
minerals. Bacteria naturally present in the colon help us with
this. Do not worry about learning its regions in this class.
“Colon Cleaning”
Some people claim that the colon
accumulates toxins and that
people can benefit from washing
out this waste with a “colon
cleaning.”
For a healthy person (not a
person with Hirschsprung’s
disease or fecal impaction), are
enemas or colonic irrigation –
attempts to wash the waste out of
the colon – a good idea?
Gut Flora
The normal bacteria of our guts
are beneficial, provided they don’t
escape.
They can produce vitamins we
need, such as vitamin K and some
B vitamins.
They also outcompete harmful
bacteria, such as Clostridium
difficile, that might otherwise
settle in our guts.
Excessive antibiotic treatment or
colon cleaning can lead to
infection by killing normal flora.
Diarrhea
When the large intestine does not reabsorb water and
electrolytes properly, diarrhea can occur.
In a healthy adult, diarrhea is
not usually a medical
emergency. However, severe
diarrhea can remove water
and electrolytes from the
body, leading to dehydration
and even death.
In infants, young children, and
the elderly, both of whom
have more difficulty
controlling water balance,
diarrhea can be fatal.
Worldwide death toll from
diarrhea in infants and small
children is over 1 million
deaths/year. Oral
rehydration therapy could
prevent these deaths!
Causes of Diarrhea
• Many viruses, bacteria, and parasites that spread bacteria are
spread by the fecal-oral route via contaminated drinking water.
• Example of bacterial cause: Cholera, Vibro cholerae.
Responsible for recent outbreak in Haiti following earthquake
that damaged water/sewage infrastructure.
• Example of viral causes: Noroviruses and rotaviruses. Rotavirus
is the major cause of infant diarrhea.
• Example of parasitic agent: Giardia
Vibrio cholerae
Rotaviruses
(MUCH smaller!)
Giardia lamblia