Untitled Flashcards Set

CHAPTER 15-THE DIGESTIVE SYSTEM
I. THE DIGESTIVE SYSTEM-takes in food, breaks it down into nutrients, absorbs the
nutrient
molecules into the bloodstream and then rids the body of the indigestible remains.
A. The organs of the Digestive System are classified into two groups:
1. Those of the Alimentary Canal-which is also known as the GI Tract.
a. The alimentary canal is a continuous, muscular tube that winds
through the
body. This canal specifically functions by breaking food down into
small
fragments and absorbing the fragments through its lining into the
blood.
to this, many people refer to the digestive system as a “disassembly
line.”
2. The Accessory Digestive Organs-which includes the teeth, tongue,
gallbladder,
salivary glands, liver and pancreas.
II. SIX MAJOR DIGESTIVE PROCESSES
A. Ingestion-taking food into the digestive tract.
B. Propulsion-movement of food through the alimentary canal. This includes
swallowing and
peristalsis (alternate waves of contraction and relaxation of smooth muscles in
the
walls of digestive organs). This squeezes food along through the digestive
system.
C. Mechanical Digestion-physically prepares food for chemical digestion. This
includes
chewing, mixing food with enzymes, churning food in the stomach and
segmentation
(rhythmic, local constrictions of the intestines).
D. Chemical Digestion-occurs as enzymes break down food materials into their
primary
components. Much of this occurs in the lumen of the alimentary canal.
E. Absorption-the passage of digested end products with vitamins, mineral and water,
from
the lumen of the GI Tract into the blood or lymph.
F. Defecation-the process through which indigestible wastes are removed from the
body in
the form of feces.
III. HISTOLOGY OF THE ALIMENTARY CANAL
A. The walls of the alimentary canal (from the esophagus to the anus) contain the
same four
basic layers. Each of these layers is involved in a specific function in food
breakdown.
B. The Four Basic Layers of the Alimentary Canal:

1. The Mucosa (Mucous Membrane)-the innermost layer, is composed of
epithelial
tissue. This layer lines the lumen from the mouth to the anus.
a. Its Major Functions are:
1) Secretion of mucus, digestive enzymes and hormones.
2) Absorption of the end products of digestion into the blood.
3) Protection against infectious disease
2. The Submucosa-external to the mucosa. Composed of connective tissue
that
contains blood vessels, nerve fibers, elastic fibers and lymphatic vessels.
3. The Muscularis Externa-is involved in segmentation and peristalsis. This
layer is
composed of two layers of smooth muscle (an inner circular layer and an
outer
longitudinal layer).
4. The Serosa-also known as the Visceral Peritoneum, this is the outermost,
protective
layer around a digestive organ. It composed o areolar tissue.
IV. THE MOUTH-the only part of the digestive system that is involved in ingestion.
A. The mouth plays a key role in beginning the digestive process.
B. The Major Structures of the Mouth Include:
1. The Lips and Cheeks-composed of a skeletal muscle core that is covered by
skin.
a. These two structures keep food between the teeth.
b. The Labial Frenulum-a fold of tissue that joins the internal aspect of
each lip
to the gum.
2. The Palate-forms the roof of the mouth.
a. Two Parts to the Palate:
1) The Hard Palate-anterior portion, separates the nasal cavity from
the
oral cavity. Is composed of bone tissue.
2) The Soft Palate-composed of mostly skeletal muscle.
a) The Uvula-projects downward from the free edge of the
soft palate.
During swallowing, the uvula moves backward to keep
food out of
the nasal cavity.
3. The Tongue-occupies the floor of the mouth. It is composed of skeletal
muscle fibers.
a. It is composed of bundles of skeletal muscle fibers.
b. During chewing, the tongue holds and positions the food so that saliva
can be
mixed with the food. This process produces a mass called a bolus,
which the

tongue can push into the pharynx.
c. The Lingual Frenulum-a fold of mucosa that attaches the tongue to the
floor of
the mouth and limits posterior movements of the tongue.
d. Recall that the tongue is covered by papillae which store taste buds.
4. The Salivary Glands-produce and secrete saliva into the oral cavity.
a. The Major Salivary Glands include:
1) The Parotid Gland-large, is anterior to the ear between the skin
and
masseter muscle. Saliva is carried from this gland to the mouth
via the
Parotid Duct which opens in the mouth near the second molar.
a) Viral infection of this gland can cause the Mumps.
2) The Submandibular Gland-lies near the mandibular body. Its duct
opens
near the lingual frenulum.
3) The Sublingual Gland-under the tongue.
a) Connects to the floor of the oral cavity via 10-12 ducts.
c. Saliva is slightly acidic (6.75-7.00) and it is composed mostly of water
(99.5%).
1) Saliva also contains:
a) Electrolytes (Such as K+, Na+)
b) Salivary amylase-a key digestive enzyme that breaks
down starch.
c) Lysozome-enzyme that inhibits bacterial growth on the
teeth.
2) Specific Functions of Saliva
a) Cleanses the mouth
b) Moistens food for digestion
c) Contains various digestive enzymes
5. The Teeth-located in sockets (alveoli) of the mandible and maxillae.
a. The teeth allow us to grind and tear food as we chew.
b. Humans have two sets of teeth:
1) Primary Teeth (Baby or Milk Teeth)-20 of these total. These
loosen and
fall out between the ages of 6 and 12.
2) Permanent Teeth-there are typically 32 teeth in the permanent
set.
a) Types of Teeth: Incisors (biting off pieces of food),
Canines (tear food),
Premolars and Molars (crushing food).
c. Tooth Structure
1) Crown-exposed portion of a tooth above the gingiva (gums).
2) Enamel-hardest compound in the body, forms the outer covering
over a

tooth. It is primarily composed of mineral salts and mineral
crystals. The
cells that produce enamel erupt when the tooth is damaged.
3) The Root-the portion of the tooth that is embedded in the
jawbone.
4) Dentin-bonelike material, underlies the enamel of the tooth and
forms the
bulk of the tooth. It surrounds a pulp cavity that stores blood
vessels,
nerves and connective tissue (which is collectively referred to
as Pulp).
C. In the Digestive Process, the Mouth functions by:
1. Ingesting food
2. Starting mechanical digestion by chewing (Mastication).
3. Initiating propulsion via swallowing
4. Releasing Salivary Amylase
V. THE PHARYNX-a passageway that connects the oral cavity to the larynx and esophagus
VI. THE ESOPHAGUS-a muscular tube that carries food materials from the pharynx to the
stomach.
A. The esophagus runs essentially straight through the mediastinum of the thorax and
enters the
stomach at the esophageal or cardiac sphincter (a ring-shaped muscle).
1. The esophageal sphincter acts as a valve that opens when food passes into
the
stomach and closes when food is not present.
2. The esophagus is collapsed when food is not passing through.
VII. THE STOMACH-organ where chemical breakdown of proteins begins and food is
converted into a
material known as chyme.
A. Anatomy of the Stomach
1. The stomach ranges from 15-25 cm in length and it can hold up to 4L (1 gallon)
of food
when full. When the stomach is empty, it collapses throwing its mucosa into
folds
known as Rugae.
B. Major Regions of the Stomach:
1. Cardiac Region (Cardia)-“near the heart.” This area surrounds the cardiac
orifice
through which food passes as it enters the stomach from the esophagus.
2. The Fundus-dome-shaped portion of the stomach, beneath the diaphragm.
3. The Body-midportion
4. The Pyloric Region-becomes continuous with the duodenum of the small
intestine.
a. The Pyloric Sphincter separates the pylorus and the duodendum. This
muscle
regulates and controls stomach emptying.

C. Microscopic Anatomy of the Stomach
1. The stomach contains the four typical layers of the alimentary canal, but its
muscularis
and mucosa are specialized for the specific functions that occur in the
stomach.
2. The Muscularis of the Stomach-contains smooth muscle arranged in the normal
circular
and longitudinal fashion. However, there is an extra layer of fibers arranged in
an
oblique fashion in the muscularis of the stomach.
3. The Mucosa of the Stomach is composed primarily of simple columnar
epithelium that
contains nothing but goblet cells that produce mucus.
a. Secretory Cells Found in the Stomach Mucosa
1) Mucous Neck Cells-found in the upper or neck of gastric glands.
These
secrete an acidic mucus during digestion.
2) Parietal Cells-secrete hydrochloric acid which is involved in the
chemical
breakdown of food materials. This acid also kills pathogenic
bacteria.
3) Chief Cells-produce pepsinogen (the inactive form of the protein-
digesting
enzyme pepsin).
4) Enteroendocrine Cells-releases chemical messengers
(Histamine), and
hormones (such as Gastrin) into the stomach during digestion.
c. The stomach mucosa is exposed to an extremely acid environment. To
protect
itself, the mucosa produces a Mucosal Barrier that is created by:
1) Bicarbonate mucus layer that covers the stomach wall.
2) Tight connections between the cells of the mucosa to prevent
acid leakage. 3) Rapid replacement of the cells in the mucosal layer of the
stomach. D. Digestive Processes That Occur in the Stomach
1. In general, the stomach is involved in breaking down food into chyme and then
moving
the chyme into the small intestine. No absorption occurs in the stomach.
VIII. THE SMALL INTESTINE
A. The small intestine is the body’s major digestive organ. This is the site where
digestion is
completed and most absorption occurs.
B. Gross Anatomy of the Small Intestine
1. It extends from the pyloric sphincter to the ileocecal valve at the large
intestine.
2. It is the longest organ in the alimentary canal (about 20 feet long).
3. 3 Subdivisions of the Small Intestine:

a. Duodenum-immovable, retroperitoneal region of the small intestine.
1) Curves around the pancreas.
2) Hepatopancreatic Ampulla-in the duodenum. This is the site where
the
bile duct and pancreatic duct meet to empty their secretions into
the small
intestine. The ampulla actually opens into the duodendum via the
major
duodenal papilla. The Hepatopancreatic Sphincter is a
muscular valve that
controls the flow of bile (from the liver) and pancreatic juice into the
duodenum.
b. Jejunum-extends from the duodenum to the ileum, about 8 feet long.
c. Ileum-about 12 feet long, attaches to the large intestine at the ileocecal
valve.
The jejunum and ileum occurs as rounded coils that are held in place by
mesentery.
C. Microscopic Anatomy of the Small Intestine
1. It is highly adapted for nutrient absorption. The length of the small intestine
offers a great
surface area for this to occur in. Three structural modifications in the small
intestine
greatly increase the surface area of the organ for greater absorption to occur.
2. Structural Modifications That Are Involved In Absorption In The Small Intestine
a. Plicae Circulares (Circular Folds)-deep folds of the mucosa and
submucosa.
These folds force chyme to spiral through the lumen, slowing its
movement and
allowing time for full nutrient absorption to occur.
b. Villi-fingerlike projections of the mucosa.
1) The epithelial cells that make up the villi are primarily absorptive
columnar
cells.
2) The core of each villus contains:
a) A Capillary bed
b) A Lacteal-a wide lymph capillary.
3) Digested food materials are absorbed through the epithelial cells into
both
the capillary bed and lacteal. This material is absorbed via diffusion,
osmosis
and by active transport.
c. Microvilli-tiny projections of the cell membrane of the absorptive cells of
the

mucosa. These structures give the mucosa a fuzzy appearance known
as the
brush border. This membrane produces enzymes that complete the
digestion of
carbohydrates and proteins.
D. Intestinal Juice
1. Intestinal glands normally secrete 1-2L of intestinal juice daily.
2. Intestinal juice is slightly alkaline (pH of 7.4-7.8).
3. Intestinal juice is largely water but it also contains some mucus. Few enzymes
are
present in intestinal juice.
IX. THE LIVER AND GALLBLADDER-these are considered to be accessory organs
associated with the
small intestine.
A. The Liver-serves many roles but from a digestive point of view, the liver is responsible
for
producing the fat emulsifier Bile.
1. The liver is the largest gland in the body (weighs 1.4Kg or 3 Lbs.). It is located
under the
diaphragm and the largest portion of the liver is to the right of the midline. It is
somewhat
protected by the ribcage.
2. 4 Major Lobes of the Liver:
a. The Right Lobe-largest lobe of the liver.
b. The Left Lobe-separated from the right lobe by a deep fissure.
c. The Caudate Lobe-most posterior of the lobes.
d. The Quadrate Lobe-inferior to the left lobe.
3. The Falciform Ligament-mesentary, separates the right and left lobes of the
liver and
suspends the liver from the diaphragm and abdominal wall.
4. The Round Ligament (Ligamentum teres)-located along the inferior edge of the
falciform
ligament, is a remnant of the fetal umbilical vein.
5. Bile leaves the liver via several ducts that fuse to form the Common Hepatic
Duct. This
duct fuses with the Cystic Duct, which drains the gallbladder of bile, to form
the
Bile Duct.
7. Microscopic Structure of the Liver
a. The liver is composed of functional units known as Liver Lobules. Each
lobule
consist of liver cells (Hepatocytes) that are stacked like bricks.
b. Remember that the primary function of the liver is to filter and process
blood that
contains a substantial supply of nutrients.

c. Portal Triad-at the tip of each of the corners of the lobule. Three
structures are
always present at a Portal Triad:
1) A branch of the Hepatic Artery-carries blood into the liver.
2) A branch of the Hepatic Portal Vein-carries blood out of the liver.
3) A Bile Duct.
d. Liver Sinusoids-located between plates of hepatocytes, blood passes
from the
hepatic portal vein, through these sinusoids into the central vein. Blood
is filtered
by Hepatocytes in the sinusoids.
8. Bile-a yellow-green solution that contains salts, pigments, cholesterol, fats,
phospholipids and electrolytes. Bile plays a role in fat emulsification
(preparation for
digestion and absorption of fats). Bile is also a waste product since it contains
bilirubin,
a breakdown product of red blood cells.
B. The Gallbladder-a green, thin-walled organ located in a shallow fossa on the ventral
surface of
the liver. It is roughly about the size of a kiwi fruit.
1. The gallbladder functions by storing bile that is not immediately needed for
digestion or
absorption.
X. THE PANCREAS-gland extending from the stomach to the small intestine, where it is
encircled by the
duodenum. Most of the pancreas lies deep to the stomach.
A. The pancreas is an accessory digestive organ produces enzymes that breakdown
organic
molecules found in food. These enzymes are delivered to the duodenum.
B. Pancreatic Juice-produced by the pancreas, contains the above digestive enzymes.
XI. THE LARGE INTESTINE
A. The large intestine frames and surrounds the small intestine on three sides and
extends from
the ileocecal valve to the anus. It is about 7cm in diameter and approximately 1.5m
long.
B. The Large Intestine is primarily involved in: absorbing the last amounts of water from
indigestible food and storing and eventually removing the waste materials generated
as food is
broken down in the digestive process via feces.
C. Three Unique Features of the Large Intestine:
1. Teniae coli-three bands of longitudinal smooth muscle located in the muscularis
of the
large intestine.
2. Haustra-pocketlike sacs that make up the large intestine.
3. Epiploic Appendages-fat-filled pouches on the large intestine.
D. The Major Subdivisions of the Large Intestine Include:

1. The Cecum-first part of the large intestine, saclike structure which lies below
the
Ileocecal valve in the right iliac fossa.
2. The Vermiform Appendix-attached to the posteromedial surface of the large
intestine.
structure contains masses of lymphoid tissue, and it may play a role in
immunity.
3. The Colon-major regions of the colon include:
a. The Ascending Colon-travels up the right side of the abdominal cavity to
the level
of the right kidney where it makes a right turn. From here, the colon
travels across
the abdominal cavity as the Transverse Colon. Next, the colon turns
downward
to form the Descending Colon. The descending colon enters the
pelvis, where it
becomes the Sigmoid Colon.
4. Anal Canal-last portion of the large intestine. This canal opens to the exterior of
the body
via the anus. Two Sphincters in the Anal Canal:
a. Internal Anal Sphincter-composed of smooth muscle, is involuntary.
b. External Anal Sphincter-composed of skeletal muscle, is voluntary. Both
muscles function to open and close the anus. They are closed except
during
defecation.
E. Microscopic Anatomy of the Large Intestine
1. The walls of the Large Intestine differ from that of the Small Intestine in several
ways:
a. The mucosa is composed of simple columnar epithelium. In the large
intestine,
there are no folds, no villi and very few cells that secrete digestive
enzymes.
1) Goblet cells are abundant in the large intestine. The mucus
secreted by
these cells eases the passage of feces through the large
intestine.
F. Bacterial Flora-refers to the bacteria that inhabit the large intestine.
1. These bacteria ferment many of the indigestible carbohydrates that we eat to
produce a
mixture of gases known as flatus. About 500ml of flatus is produced each day.
2. The bacterial flora also helps to synthesize B complex vitamins and most of the
Vitamin
K that is used by the liver to synthesize clotting factors.
G. Digestive Processes Occurring in the Large Intestine
1. Essentially no food breakdown occurs in the large intestine (except for that

associated with the bacterial flora).
2. The large intestine does absorb a few vitamins, water and electrolytes.
XII. PHYSIOLOGY OF CHEMICAL DIGESTION AND ABSORPTION
A. Chemical Digestion-catabolic process in which large food molecules are broken
down
to monomers that are small enough to be absorbed by the GI tract lining. This is
accomplished by enzymes that are secreted by cells and glands associated with
the lumen
of the alimentary canal. This use of enzymes in food breakdown is referred to as
hydrolysis.
This ends in the small intestine-no chemical digestion occurs in the large intestine.
B. Chemical Digestion of Carbohydrates
1. Salivary amylase-present in saliva, breaks starch down into glucose
molecules.
a. This enzyme works best in the slightly acidic environment of the
mouth.
b. This enzyme continues to digest starch until it is broken down itself
by the
acid of the stomach.
C. Chemical Digestion of Proteins
1. Proteins are usually broken down into amino acids.
2. Protein digestion usually begins in the stomach when pepsinogen is
converted into
the enzyme pepsin. Pepsin functions extremely well in the acidic
conditions of the
stomach. Pepsin digests about 15% of the incoming protein.
3. Rennin-enzyme that breaks down the protein in milk and dairy products.
D. Chemical Digestion of Lipids
1. Lipases are responsible for digesting many lipids components in the body.
2. Michelles-collections of fatty elements clustered together with bile salts.
These are
very small and can easily carry their fat components through the intestinal
wall. Once
fats are inside of cells, they are resynthesized to produce triglycerides.
E. Chemical Digestion of Nucleic Acids
1. These compounds are broken down in the small intestine. The sugars from
the various
nucleic acids are carried through the small intestine wall by carrier
molecules. These
sugars are then transported to the blood.
F. Absorption-about 80% of the food and drink materials that enter the alimentary canal
are
absorbed in the small intestine.
1. Most nutrients are absorbed by active transport processes at the mucosa
layer of the
small intestine. This overall process requires ATP to occur.

G. Absorption of Carbohydrates-sugars are generally moved through the small
intestine mucosa
by active transport. Carbohydrates then diffuse into capillaries for distribution
throughout the
body.
H. Absorption of Proteins-involves the use of carrier molecules that attach to and
physically
move the amino acids through the wall of the small intestine.
I. Absorption of Lipids
1. Micelles are used to carry lipids through the wall of the small intestine.
J. Absorption of Vitamins
1. Fat soluble vitamins (A, D, E, K) become incorporated into micelles and are
carried
across the mucosa layer of the small intestine.
2. Most water-soluble vitamins (B and C) diffuse through the small intestine.
K. Electrolyte Absorption
1. Many electrolytes are carried through the mucosa of the small intestine via
active
transport. Electrolyte absorption is often regulated by hormones and
vitamins.
L. Water Absorption-95% of the water in chyme is absorbed in the small intestine via
osmosis.
XIII. DISORDERS OF THE DIGESTIVE SYSTEM
A. Appendicitis-acute inflammation of the appendix. Sugrcal removal is the typical
treatment.
B. Inflammatory Bowel Disease (IBD)-chronic disorders that cause painfully swollen
intestines.
This is typically an Autoimmune disorder. Two types of IBD:
1. Crohn Disease-typically affects the last part of the ileum.
2. Ulcerative Colitis-affects the large intestine and/or rectum.
C. Diverticulitis-disorder in which small pouches (called diverticula) develop along the
length of
the colon. Pain, diarrhea are signs of this illness.
D. Hemorrhoids-occurs when veins in the anal canal become enlarged and inflamed.
E. Hepatitis-inflammation of the liver. This can be caused by viruses, drugs, alcohol.
F. Gallstones-crystallized cholesterol stones that form from bile in the gallbladder. The
stones
block the flow of bile out of the gallbladder.