Digestive System
•Digestive Tract  begins with
mouth and ends with anus
•Digestion occurs
outside of cells (extracellular)
•Enzymes for digestion are found in
nearby glands and accessory organs
•Enzymes break down proteins (AA’s),
fats (simple lipids), and carbohydrates (sugars).
•These smaller molecules are
used by the body for energy.
The Mouth
•Mouth is lined with chemical receptors called taste buds,
which are connected to the brain via cranial nerves
•Olfactory receptors in the nose also contribute to our
sense of taste
•Most humans have 32 teeth
•Each tooth consists of a crown which is dense with
calcium, and the pulp, which is rich with nerves and blood
vessels
•Dental caries (cavities) are caused by bacteria on the tooth
that metabolize sugars & give off acids which erode the
enamel
•Fluoride hardens the enamel and protects it from erosion
Parts of the Mouth
Hard palate bony layer of the top of the mouth. Found
toward the anterior of the mouth
Soft palate  Muscular portion of the roof of the mouth
that ends at the uvula
Salivary glands  3 pairs of glands found at the side of
the mouth (below the ears) and under the tongue (little
flaps under the tongue). Saliva contains amylase, an
enzyme which begins the digestion of starch
After physical mastication and some chemical digestion,
food in the mouth is mixed into a soft mass called a bolus,
which is then swallowed
Parts of the Mouth
Parts of the Mouth
Pharynx
The pharynx is the crossroads between the mouth and the
esophagus
Once food is swallowed, the soft palate moves back to cover
the nasal cavities; the trachea presses against the epiglottis,
cutting itself off from food
Esophagus
•The esophagus is a tube that conducts food from the mouth to the
stomach
•The esophagus is covered by multiple mucous membranes that help the
food slide toward the stomach
•Smooth muscles in the esophagus rhythmically contract to push the
food along, these contractions are called peristalsis
•Food passes through to the lower esophagus and enters through the
esophageal sphincter
Esophagus
Esophagus
•Sphincter muscles are valves that open when relaxed
•The valve opens and the food enters into the stomach
•If the valve doesn’t close completely, contents of the
stomach escape into the esophagus, causing heartburn
Esophagus
Stomach
•Stomach is a “J” shaped organ on the left side of
the body below the diaphragm
•The stomach has many folds, which expand when
food is inside (1L)
•Inside the columnar epithelium of the stomach are
many gastric glands, which secrete the enzyme
pepsin; this enzyme digests proteins
•HCl is present in high levels in the stomach (pH
=2). It breaks down connective tissue and also kills
many bacteria present in food.
•HCl stays in the middle of the stomach. Mucus
lines the outer edges of the stomach, which
protects the wall of the stomach from ulcers
Stomach
•Although food is broken down in the
stomach, no food is absorbed into the body
from the stomach.
•Food leaves the stomach (after 2-6 hours) in
a pasty form called chyme. Chyme leaves the
stomach and enters the small intestine via the
pyloric sphincter.
Small Intestine
Diameter: 2.5 cm
Length: 18 feet
Receives bile from the gall bladder
Receives secretions from the pancreas
Absorbs nutrients and sends undigested
food to large intestine
1st 25 cm of the SI is called the duodenum
Pancreatic juice is released into the duodenum and
contains (NaHCO3)
Bicarbonate neutralizes the chyme
SI contains small finger-like projections called
microvilli
Small Intestine
These projections break down chyme into
small, absorbable molecules that are
absorbed into epithelial cells in the SI, which
are then transported throughout the body
Active Transport takes sugars and AA’s and
passes them into the blood stream; fats are
passed into the ducts of the lymphatic system
Large Intestine
•Diameter: 6.5 cm
Length: 4.5 feet
What does it do?
•Absorbs water, salts, and vitamins
Parts of the LI
•Cecum: blind (dead) end of the LI on the
right side of the body
•Appendix: at the end of the
cecum (may play a role in immunity)
The Colon
•Ascending Colon travels up to the
right side of the body to the liver
•Transverse Colon  crosses over
from right to left
•Descending Colon  travels down the
left side of the body
•Last 20cm of the LI is called the
rectum
•Opening at the end of the rectum is
called the anus
•Defecation occurs out the anus
What is in my Feces?
•Non-digestible parts of food
•Bile pigments (color)
•Lots of anaerobic bacteria (bacteria)
that die in the presence of oxygen;
they also provide us with some
vitamins
Diarrhea
•Caused by infection of the lower LI
and/or nervous system stimulation
•In the case of infection, peristalsis
increases, water is not absorbed, all
“bad stuff” is released at once
Constipation
•Feces are dry and hard
•Usually caused by ignoring the urge
to defecate; body habituates to holding
in waste
•Laxatives usually make a bulk of
cellulose in the LI
•Milk of Magnesia prevents water from
being absorbed in LI
Accessory Organs of the
Digestive System
•Pancreas lies deep in the abdominal
cavity; on posterior abdominal wall;
flattened organ
•Produces pancreatic juice, which
contains enzymes that digest
carbohydrates, proteins, and fats
•Pancreatic fluid flows through ducts
(little hallways) into the duodenum of
the SI
Accessory Organs of the
Digestive System
The Liver
Largest gland in the body
Located in the upper right section of
the abdominal cavity
Liver produces almost 2 liters of
bile/day
Bile is sent to the gall bladder (under
the liver) for storage
Bile salts (made from cholesterol)
emulsify (break down) fat molecules
that are broken down my pancreatic
juice
Accessory Organs of the
Digestive System
Liver & Blood Filtration
Once nutrients are have been absorbed
by the small intestine, they are sent to
the liver, through the blood, by the
hepatic portal vein
Liver filters the blood, removing toxins
and excess materials; then sends the
blood to the lungs for oxygen
Accessory Organs of the
Digestive System
Example
Liver removes excess glucose from the
blood, and stores it as glycogen
Liver, in times of need, can convert
amino acids to glucose (deamination)
Liver detoxifies ethanol (toxic) into
acetate (non-toxic)
Cirrhosis
Liver tissue turns to inactivated scar
tissue; no longer detoxifies
Caused by excessive detoxification of
ethanol
Accessory Organs of the
Digestive System
Digestive Enzymes
Amylase: Found in mouth; breaks down starch
Pepsin: found in stomach; breaks down proteins
Pancreatic Amylase: found in pancreas; breaks down starch
Trypsin: made by pancreas; breaks down proteins
Lipase: made by pancreas; breaks down fat
Peptidase & Maltase; secreted by small intestine; breaks
down proteins & carbs
Accessory Organs of the
Digestive System
Hormones of the Digestive System
Gastrin: found in stomach; stimulates production of
pancreatic juices
Secretin: found in SI; stimulates pancreas to secrete
sodium bicarbonate
Cholescystokinin (CCK); found in SI; stimulates gall
bladder to release bile