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• Digestive system extends about 8 meters from
the mouth to the anus, consisting of several
accessory organs, which secrete substances
that aid in the process of digestion.
• Digestion- is the chemical and mechanical
breakdown of foods and the absorption of
nutrients by cells.
– Mechanical digestion- breaking larger pieces into
smaller ones without altering their chemical
– Chemical digestion- breaks food into simpler
chemicals. The organs of the digestive system carry
out these processes.
• At its simplest, the digestive system it is a tube
running from mouth to anus. This tube is like
an assembly line, or more properly, a
dissembly line.
• It is the process of turning food substances
into sugars, amino acids, and fatty acids
• The alimentary canal includes the mouth,
pharynx, esophagus, stomach, small intestine,
large intestine, rectum and anus.
• Lumen- the opening/hole of the digestive tract
• Peristalsis- propelling movement of food
through the digestive tracts. When peristalsis
occurs, a ring of contraction appears in the wall
of the tube. At the same time, the muscular wall
just ahead of the ring contracting ring relaxes.
As the peristaltic waves move along, it pushes
the contents ahead.
• The peristaltic movement also aids in the
process of digestion.
• Remarkably diverse and specialized processes
take place in different sections of the digestive
tract, but there is a fundamental consistency in
the architecture of the tubular digestive tract.
With few exceptions, the wall of the digestive
tube from the mouth to the anus is composed of
four basic layers or tunics.
Layers of the GI Tract
• Mucosa- this innermost layer is responsible for
absorption of nutrient. This layer is made of simple
columnar epithelium and has numerous folds that
increase the surface area for maximum absorption
• Submucosa- the 2nd innermost layer that provides the
blood supply. When nutrient are absorbs by the
mucosa layer, they are transported to the submucosal
layer where the will enter the blood supply.
• Muscularis- the layer that is made of smooth muscle
and is responsible for carrying out peristalsis. There
are two layers of muscles; longitudinal and circular.
• Serosa- the outer layer that is binds and protects the
entire GI tract
The lining of the epithelium of the outer lining of the
mucosa function in absorbing nutrients and secreting
mucus and digestive enzymes. Absorption of nutrients
from food into the cells of the epithelium will eventually
be transported through the circulatory system.
Organs of the Digestive System and their
• Mouth
• Digestion begins in the mouth. The mouth
receives the food and begins digestion by
mechanically reducing the size of solid
particles and mixing them with saliva.
• The tongue helps move food under the teeth
as well as mix the food with saliva.
• Salivary glands- secrete saliva, which
contains enzymes, such as amylase, that
initiate the breakdown of carbohydrates.
• The mouth and the tongue roll the mixture into a
mass, known as a bolus, and forces it into the
• Once the flood leaves the mouth and is
swallowed, the pharynx is responsible in
connecting, the oral and nasal cavities to the
esophagus in which the food will enter next.
• Esophagus
• The bolus will move through the esophagus and
pass through the esophageal sphincter and
empty into the stomach. The esophageal
sphincter closes after the food passes through
the stomach and remains that was until the food
leaves the stomach preventing regurgitation.
• Stomach
• The stomach is a Jshaped organ that
function to store food,
initiate the digestion of
proteins, kill bacteria with
the strong acidity of
gastric juices, and move
the food into the small
intestine as a pasty
material known as chyme.
• There are three regions of
the stomach: fundus,
body, and pylorus.
• The stomach on average has a capacity of 1 liter.
• The inner surface of the stomach is lined with long folds
known as rugae.
• The stomach churns the chyme mixing it thoroughly
with gastic juices. The stomach secretes around 2-4
liters of gastric juices a day.
• These gastric juices comes from microscopic holes
found in the rugae known as gastric pits.
Gastric “Juicy”
• As food remains in the stomach for periods of
time the stomach churns the food and mixes it
with gastric juices that help liquefy the solid
food into chyme; also initiating the breakdown
of proteins
• There are three main types of gastric glands:
– Chief Cells
– Parietal Cells
– Mucous Cells
• The combination of all of these chemicals is
referred to as gastric juice, and has a pH of less
than 2.0
What do these Gland Do?
• Chief Cells- secrete digestive enzymes, pepsin
• Parietal Cells- secrete HCl acid.
– HCl acid and Pepsin work together to ultimately
digest proteins
• Mucous Cells-serve to protect the stomach
from digesting itself
• As these enzymes begin digestion and are
being churned by the stomach, peristalsis
forces the chyme begins to move toward the
pyloric region of the stomach. The pyloric
sphincter begins to relax enabling food to enter
the small intestine a little at a time.
The Food Now Enters the S.I.
• The small intestine is approximately 18 feet
long in a living person.
• There are three regions of the small intestine:
– Duodenum- makes up the first foot of the S.I.
– Jejunum- makes up approximately the next 7
– Ileum- makes up the last 10 feet of the S.I.
• Since the S.I.’s major function is to absorb
nutrients, further digestion of proteins,
carbohydrates, and fats begin.
Lets Break it on Down
• The S.I gets help from three important organs
that will secrete more enzymes and juices to aid
in the digestive process.
– Liver
– Gallbladder
– Pancreas
• These three organs secrete substances into the
duodenum of the S.I. to breakdown food
substances even more.
• The liver and gallbladder have two ducts that
join together to form the common bile duct.
This duct empties bile into the duodenum of the
• The liver is found in the upper left quadrant of
the abdominopelvic cavity produces bile, which
is a yellowish-green color.
• The gallbladder has a duct connected directly
to the live. The bile that is produced in the liver
is stored in the gallbladder.
• Furthermore, the pancreas makes andsecretes
enzymes that help digest fats, carbs, proteins
and nucleic acids. These enzymes exit the
pancreas and enter the duodenum of the S.I. at
the same place that the common bile duct
enters. The duct that carries this pancreatic
juice is called the pancreatic duct.
• Page 398 and 399
How the Liver, Gallbladder and Pancreas
Work Together
• The small intestine is held in place and
supported by a double layered peritoneal
membrane known as mesentery
• The mesentery suspends the S.I.’s from the
posterior wall as well as supplies the S.I. with its
nerve and blood supply.
• The chyme leaves the S.I through the ileum by
passing through the ileocecal sphincter into the
cecum of the large intestine.
• The L.I. is about 4.5-5 feet long and is made of
five segements:
– Cecum
– Ascending Colon, Transverse Colon, Descending
– Sigmoid Colon
• The L.I.’s, (a.k.a “colon”), function is to absorbs
water, electrolytes, and vitamins.
• What the L.I. doesn’t absorb is known as feces.
Its color and odor comes from the products of
bacteria and bile.