The Digestive System - AP Biology

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Transcript The Digestive System - AP Biology

The Digestive System
Addison Martin
of the Digestive System
The gastrointestinal tract
(abbreviated here as GI) is
the system of organs that
takes in and mechanically
and chemically breaks down
food so that it can be
absorbed and used in the
body, and then expels the
unused material.
In humans, the GI has
several organs, the oral
cavity, pharynx, esophagus,
stomach, small intestine,
and the large intestine.
Phases of Digestion
Cephalic, Gastric, and Intestinal phase
Cephalic Phase – This is the phase
before food enters the stomach.
The sight, smell, taste, and
sensation of food causes the brain
to prepare the body for digestion
Gastric Phase – This phase can
take 3-4 hours and takes place in
the stomach. The presence of
food in the stomach causes the it
to secrete enzymes and acids to
break down food.
Intestinal Phase – This phase
begins with food entering the
intestines. This is when nutrients
are absorbed.
Organs & Parts
Of the Digestive System
The Oral Cavity
The function of the oral
cavity is to mechanically
break down food and to
begin breaking it down.
The oral cavity houses the
tongue and teeth, which aid
in the mechanical break
down of food and helps to
create balls of food called a
The salivary ducts also lead
into the oral cavity.
The Salivary Glands
There are four major salivary
glands with hundreds of
minor salivary glands
throughout the mouth cavity.
The salivary glands produce
saliva, which keeps food moist,
begin digestion, help create a
food bolus, and keep the oral
cavity moist and lubricated.
Saliva is composed of mostly
water but also electrolytes,
cheek cells, mucus, various
enzymes, and other small
biochemical compounds.
Saliva begins the digestive
process because of enzymes in
the saliva which begin
breaking materials down.
Here are a list of some of the
enzymes found in saliva and
their function:
Amylase – breaks down starch
and lipase fat
Lysozyme – an enzyme that
causes bacteria to lyse
Saliva contains other enzymes.
Some enzymes are designed so
that they do not function
unless in the acidic
environment of the stomach.
The Pharynx
The pharynx is the throat.
The pharynx’s function is
to separate the windpipe
from the esophagus.
The glottis, or opening to
the windpipe is blocked by
the cartilaginous flag called
the epiglottis.
The pharynx ensures that a
food bolus will go down the
esophagus and not the
windpipe, protecting the
respiratory system.
The Esophagus
The esophagus is the tube
leading food from the oral
cavity and pharynx through
the diaphragm and into the
Although the control of the
pharynx and the act of
swallowing is voluntary, the
transportation of a bolus to
the stomach is involuntary.
The esophagus moves a food
bolus to the stomach in
through rhythmic contractions
of the smooth muscle called
The Stomach
The elasticity and large capacity of
the stomach allows the stomach to
store food, and thus we do not
nee to constantly eat.
An acidic mixture of several
liquids, called gastric juice, is
secreted by the stomach to disrupt
the extracellular matrix between
plant and animal cells and kill
bacterial cells.
Gastric juice also contains pepsin.
Pepsin is an enzyme that begins to
hydrolyze polypeptides. This
enzyme is unique because of its
functionality in highly acidic
The inactive form of pepsin is
secreted by specialized cells in the
stomach and is called pepsinogen.
Similar specialized cells secrete
HCl, one of the main components
of gastric juice, which activates the
Mucus coating the stomach wall,
rapid mitosis regenerating
stomach wall cells, and the fact
that pepsin is secreted in its
inactive form keeps the stomach
from digesting itself.
About every 20 seconds the
stomach involuntarily churns its
The enzymatic activity, and
other chemical reactions
within the stomach turns a
bolus of food into a
nutrient broth known as
acid chyme.
The stomach is normally
closed at both ends. The
orifice from the esophagus
opens when food travels
down the esophagus. The
pyloric sphincter regulates
the passage of acid chyme
into the intestine.
Ulcers can form in the GI
tract (often in the stomach,
esophagus, or most often in
the duodenum). Ulcers found
anywhere in the GI tract are
called peptic ulcers.
An ulcer is defined as mucosal
erosions equal or greater that
.5 cm. This erosion of the
mucosa, or mucus membrane,
can sometime expose the
submucosa which is the dense
irregular connective tissue that
connects the mucosa to the
muscles of the GI tract.
The Small Intestine
Acid chyme enters the small
intestine by the pyloric sphincter.
The small intestine is the site of
the majority of the break down of
macromolecules in food and
absorption of nutrients. The
hollow part of the intestine is the
The duodenum, the first section
of the small intestine, is the small
section of intestine where digestive
juices from the pancreas, liver,
gallbladder, various glands, and
the small intestine itself
enzymatically break down the acid
chyme and neutralizes it.
The less acidic, and broken
down material is easier for the
rest of the small intestine to
absorb and less harmful. (The
duodenum is protected by
mucus secreted by glands
along its wall).
Enzymes within the
duodenum, such as tryspin
and chymotryspin, break
down specific bonds within
The jejunum and ileum are
the next sections of the small
intestine, and are very similar.
The jejunum marks the beginning
of mats of villi, finger-like
projections to increase surface
area, and microvilli, microscopic
finger-like projections on the
surface of villi, which absorb
Within each villi are many
capillaries and lacteal, small
lymphatic vessels.
Two layers of cells separate the
capillaries and lacteal from the
lumen, allowing for easy
absorption through passive or
active transportation.
The Liver
The major functions of the
liver include protein
metabolism and synthesis as
well as bile production. But it
also plays a role in
detoxification, production of
digestive juices, storing
glycogen, hormone
production, decomposition of
red blood cells, and plasma
protein synthesis.
The liver is the only organ
that can regenerate itself,
mostly due to hepatocytes, or
liver cells, re-entering the cell
(Hepato-/hepatic means liver)
The blood vessels with newly
absorbed nutrients from the
villi and the rest of the small
intestine converge at the
hepatic portal vessel which
leads directly to the liver. This
ensures that the liver gets first
access to the sugars and amino
acids absorbed from a meal,
due to the liver’s metabolic
versatility and so the liver can
regulate the level of sugars and
other nutrients and remove
the toxins in newly nutrientenriched blood.
The liver is a vital organ
present in all vertebrates.
The Gallbladder
The gallbladder’s (or
cholecyst) main function is to
store and concentrate the bile,
a green fluid produced by the
liver containing bile salts, in
preparation for digestion of
Bile salts/acids created by the
liver from cholesterol and
stored in the gallbladder serve
to coat tiny fat droplets and
keep them from coalescing, a
process called emulsification.
The emulsification of fat
allows for a greater surface
area of fat to be exposed to
The Pancreas
The pancreas is both an exocrine
gland and endocrine gland. This
means that it creates important
hormones (endocrine) as well as
digestive enzymes (exocrine).
The exocrine pancreas secretes
lipase, amylase, and alkaline
fluids (to help neutralize the acid
chyme) as well as
carboxypeptidase, trypsin, and
chymotrypsin in inactive forms
into the duodenum by the
pancreatic duct.
The endocrine pancreas produces
glucagon, insulin, somatostatin,
and other polypeptides.
The Biliary Tract
The biliary tract, (or biliary
tree) is the name of the
various bile ducts that
converge and lead from the
liver and gallbladder to the
common bile duct, which
is the duct that secretes bile
into the duodenum.
The biliary tract is not an
organ, but rather the series
of ducts.
The Large Intestine
The large intestine, or colon, is
where food enters after exiting the
small intestine. At this T-shaped
intersection of the small and large
intestine, is a small pouch called a
cecum. A finger-like projection
from the cecum is the appendix.
Peristalsis moves the waste from
the small intestine through the
large intestine as most of the water
that was used as a solvent of
various digestive juices.
Many (mostly harmless) bacteria
live in the colon as well and feed
on byproducts that would
otherwise be discarded.
Some of these bacteria
produce gasses (i.e. methane
and hydrogen sulfide) and
others even produce vitamins
(i.e. biotin, folic acid, vitamin
K and some kinds of vitamin
B) which is then absorbed.
The colon can also secrete
various material that is not
needed to the body such as
salts, iron, or calcium that is
later expelled.
The rectum is the terminal
portion of the colon where
fecal matter is stored until it
can be eliminated.
& Regulation
The digestive system is controlled
by various hormones. The brain,
stomach, duodenum, and
pancreas all secrete hormones into
the circulatory system that initiate
different reactions.
Gastrin, released from the
stomach, causes the stomach wall
to secrete HCl.
Secretin, released from the
duodenum when acid chyme
enters the small intestine, signals
the pancreas to produce
bicarbonate to neutralize the
acidic chyme.
Cholecystokinin (CCK), released
from the duodenum if there are
amino acids or lipids in the
chyme, causes the gallbladder to
contract which release bile into
the small intestine.
CCK and secretin and other
hormones secreted from the wall
of the duodenum are collectively
called enterogastrones.
Enterogastrones can also control
the speed at which material
moves through the digestive
system depending on the
contents of the chyme entering
the duodenum.
Nervous System
The section of the nervous
system that regulates the GI
tract is a very complex is
considered like a second
brain. In fact, it contains
more neurons than the
spinal chord. This section
of the nervous system is
called the enteric nervous
system. It is an offshoot of
the autonomic nervous
system which acts to control
autonomous actions such
peristalsis or sphincter