The Digestive System
Passageway of food as it enters the
digestive system
• Mouth, pharynx, esophagus, stomach, small
intestine, large intestine, rectum, anal canal
• Organs involved in digestion that food never pass
through: liver, gall bladder, pancreas
There are two types of digestion
Mechanical Digestion
• Large pieces of food
become smaller pieces,
getting it ready for chemical
digestion
• Begins with the teeth
chewing food in the mouth
• Churning and mixing of food
in stomach
Chemical Digestion
• Digestive enzymes break
down macromolecules into
small organic molecules
that can be absorbed by
cells
Some Digestive Enzymes
• Carbohydrates are first broken down in the mouth
chemically by the enzyme in saliva called amylase
• Proteins are first broken down in the stomach by the
enzyme, pepsin
• Lipase first begins the break down of fats in the
small intestine
Most of the digestion in the mouth is
physical digestion, but some of it is
chemical with the enzyme, amylase
Mouth
• Digestion begins here!!
• Sensory receptors on tongue, taste buds, are activated
by the presence of food which causes a nerve impulse
to travel to the brain via the cranial nerve
• Teeth are involved in mechanical digestion, allowing us
to chew food to pieces small enough for swallowing
• Three pairs of salivary gland in the mouth secrete
saliva which contains an enzyme called, amylase,
which begins the process of chemically digesting
starches, which are carbohydrates
The Three pairs of salivary glands in the mouth secrete
saliva which moisten food and breaks down carbohydrates
with its amylase enzyme
The Pharynx (throat)
• The region that receives air from the nasal cavity and
food from the mouth
• Swallowing, which occurs in the pharynx is a reflex
action performed automatically, without conscious
thought
• When you swallow, a flap of tissue called the epiglottis,
closes over the trachea so that food does not go down
the airway and cause a person to choke
• For this reason, we do not breathe when we swallow
The green pathway represents what happens when you
swallow- red is when you are not swallowing-the
epiglottis closes over the trachea when you swallow
Esophagus
• A muscular tube that passes from the pharynx to the stomach
that opens to receive the bolus (chewed up food)
• Waves of muscular contraction called peristalsis pushes food
along the digestive tract
• Peristalsis begins in the esophagus and continues in the
stomach and intestines
• Sphincters are muscles that encircle tubes that act as valves.
There is a sphincter at the meeting of the esophagus and
stomach
• Relaxation of the sphincter allows the bolus to enters, while
contraction prevents the acidic content from the stomach
from backing up into the esophagus
Peristalsis of the esophagus wall passes
food along to the stomach
Did you know?
• Heartburn feels like a burning pain rising up in
the throat. It occurs when some of the contents
in the stomach escape into the esophagus
• When vomiting occurs, a contraction of the
abdominal muscles and diaphragm propels food
upward through the esophagus
• Did you know that if you stand on your head, you
can eat and the food will still go down (or in this
case- up ) your esophagus and enter your
stomach? That is how powerful peristalsis action
is!
The Stomach
• A thick-muscular walled, J-shaped organ that lies on
the left side of the liver
• Length of stomach remains about 10 in regardless
of amount of food, but it can expand in diameter,
holding up to a 1 to 1.5 liters of food!
• The stomach receives food from the esophagus,
stores it, mixes it with its digestive juices, and
breaks down proteins into smaller pieces
Stomach
• The term gastric always refers to the stomach, so what
does a gastroenterologist treat? Diseases of the
digestive organs.
• The gastric glands produce gastric juice which contains
pepsinogen, HCI, and mucus
• Chief cells secrete pepsinogen, which becomes the
enzyme pepsin (breaks down protein)when exposed to
hydrochloric acid
• HCI is released by parietal cells- the HCI causes the
stomach to have a pH of about 2 and is beneficial
because it kills most of the bacteria present in food and
helps break down food . Stomach cells are replaced
through mitosis every 2-3 days because of the harsh
acidic enviroment
More on the Stomach
• Alcohol is absorbed in the stomach, but food and
other substances are not. They are absorbed later
in the small intestine
• Normally, the stomach empties in about 2-4 hours
• When food leaves the stomach, it is a thick, soupy
liquid called chyme.
• Chyme enters the small intestine by way of the
pyloric sphincter, which acts as a valve between the
stomach and the small intestine
Hydrochloric Acid (HCl)
• Kills bacteria and other living cells in food
• Causes pepsinogen to be converted to its
active form pepsin
• Denatures proteins which makes it easier for
enzymes to act on them
• Causes the stomach to have a low pH which is
optimum for the pepsin enzyme
Hydrochloric Acid
• HCl simply breaks down the connective tissue of meat
and activates pepsin
• The wall of the stomach is protected by a thick mucous
layer which secretes mucus
• If HCl penetrates the mucous layer, the wall can begin
to break down and an ulcer results
• An ulcer is an open sore in the wall caused by the
gradual destruction of tissue. Most are due to a
bacterial infection (Helicobacter pylori) that impairs
the ability of mucous cells to secrete mucus
• Stress and spicy foods don’t cause ulcers, but
they can aggravate the condition
Helicobacter pylori (H. Pylori)cause
ulcers
You Be The
Doctor!
How do you think a
doctor might treat a
patient with an ulcer
caused by H. Pylori
bacteria?
1. Reduce stomach
acid
2. Kill the bacteria
with antibiotics
3. Protect the
stomach lining
The Small Intestine
• The small intestine is about 20 feet in length
• Ducts from the liver and pancreas join to form one duct
that enters the duodenum. The small intestine receives
bile from liver and pancreatic juices from the pancreas
• Bile, a yellowish green, thick mucus produced by the
liver emulsifies (loosens up) fat, causing fat droplets to
disperse in water, then lipase can easily break down
fats
• The small intestine has a slightly basic pH because the
pancreatic juice contains sodium bicarbonate
(NaHCO3) which neutralizes chyme
Villi
1.
2.
3.
4.
5.
The wall of the small intestine
contain tiny, finger-like projections
called villi, which give the wall a
soft velvety appearance
Each villus has an outer layer of
columnar epithelial cells, and each
of these cells has thousands of
microscopic extensions called
microvilli
The microvilli greatly increase the
surface area of the villus for the
absorption of nutrients
In each villus there are blood
capillaries and lymph vessels
After nutrients are absorbed, they
are carried to all the cells of the
body by the bloodstream
The Small Intestine
• The pancreas secretes digestive juices with
enzymes into the small intestine to complete
the digestion of food
• Bile is secreted into the small intestine to
loosen up fat (emulsify)
• The small intestine is where the absorption of
most nutrients takes place
• The surface area of the small intestine is
approximately the size of a tennis court!
Imagine that.
Pancreas
• The pancreas is a gland that lies deep in the
abdominal cavity, resting just beneath the
stomach
• Most pancreatic cells produce pancreatic juice,
which contains sodium bicarbonate and a mix of
digestive enzymes that are secreted in the small
intestine to finish the break down of nutrients:
• Pancreatic amylase completes the digestion of
starch
• Trypsin completes the digestion of proteins
• Lipase completes the digestion of fats
Notice the pancreas and liver both have ducts emptying into the small intestine; the
pancreas secretes digestive juices into the small intestine to finish the break down o
carbohydrates, fats, and protein and bile loosens up fat
The Liver
• Produces bile
• It detoxifies blood by removing and metabolizing poisonous
substances
• Stores sugar, iron (Fe) and the fat soluble vitamins, A, D, K, E
• Helps regulate blood cholesterol, converting some to bile salts
• It maintains the blood glucose levels by working with the pancreas
• Removes bilirubin, a breakdown product of hemoglobin from the
blood and excretes it in bile
• Produces urea after breaking down amino acids
• Makes clotting factors to stop excess bleeding
The Liver
The largest organ in the
body. It is a complex
organ that you cannot
live without!
Drinking too much
alcohol and a eating a
diet heavy in fat can
damage liver tissue over
time.
Question for you to
research: Do you think
you can you live
without a gall bladder?
Gall bladder
• The gall bladder is attached to the surface of the
liver
• It stores excess bile produced by the liver and
when needed, it releases bile into the duodenum
• The passage of the stones may block the common
bile duct and cause jaundice, a yellowish tint of
the whites of the eyes and skin. Jaundice is
caused by abnormal amounts of bilirubin in the
blood. At this point, the gall bladder must be
removed
Gall Bladder
The Large Intestine
• The large intestine includes the cecum, the
colon, the rectum, and the anus
• It is larger in diameter than the small
intestine, but shorter in length. It is about 5
feet long
• Water, salts, and some vitamins are absorbed
here.
• It also stores indigestible materials until it is
eliminated at the anus
Large Intestine
• Inside the large intestine are bacteria. Over 99%
of the intestinal bacteria are anaerobes (bacteria
that die in the presence of oxygen)
• The bacteria are helpful in that they break down
indigestible material, and produce some
vitamins that are body uses (they also produce
gas, or as you know it as- farts)
So the next time you think all bacteria are
bad…think again. Some are actually beneficial to
us…
Problems with the Large Intestine
• Two common everyday complaints associated
with the large intestine are diarrhea and
constipation
• The major causes of diarrhea are infection of the
lower intestinal tract. In the case of infection,
such as food poisoning, the intestinal wall
becomes irritated, and peristalsis increases.
Water is not absorbed, and diarrhea results
• The major causes of constipation, dry and hard
feces, is a lack of water or fiber in the diet
Gastric Bypass
Surgery
This weight loss surgery is not for
everyone! Here’s what’s involved:
The surgeon creates a small
pouch at the top of your stomach
and adds a bypass around a
segment of the stomach and the
small intestine. The surgeon
staples the stomach at the top,
sealing it off from the rest of the
stomach.
The resulting pouch is about the
size of a walnut and can hold only
and ounce of food,
Then the surgeon cuts part of the
small intestine and sews part of it
to the pouch. Food enters the
small intestine directly, limiting
your ability to absorb calories
Cirrhosis of the Liver
• Chronic disease of the liver where the liver becomes
fatty, and then the liver tissue is replaced by fibrous
scar tissue
• This disease is often seen in alcoholics, due to
malnutrition and to the excessive amounts of alcohol
(a toxin) the liver is forced to break down
• The liver has amazing regeneration powers and can
recover if the rate of regeneration exceeds the rate of
damage
• During liver failure, however, there may not be enough
time for it to heal itself, and liver transplant might be
only option
Hepatitis
• Inflammation of the liver caused by a virus; three
common types:
• Hepatitis A- usually acquired from sewagecontaminated drinking water; common in developing
countries
• Hepatitis B- usually spread through sexual contact, but
also blood transfusions (prior to 1991) and infected
needles; can be spread from mother to child
• Hepatitis C- usually acquired by coming in direct
contact with infected blood; likely to lead to chronic
hepatitis, liver cancer and death
Tips: How to Take Care of Your Liver
• Wash your hands thoroughly after using the washroom (A)
• Eat a sensible diet with plenty of fruits and vegetables
(washed and clean) (A)
• If traveling to undeveloped countries- drink only bottled
water and consider a hepatitis A vaccination (A)
• Avoid unsafe sex practices (B and C)
• Do not share needles, razors, tooth brushes, etc (B and C)
• Keep alcohol intake very moderate, or abstain (C and
cirrhosis)
• Also a vaccine now for Hepatitis B
Gall Stones
• What is it?
• Gall stones are tiny, pebble-like stones that
develop in the gall bladder
• They form when the bile harden to form
stone-like
• The hormones, secretin and CCK