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THE DIGESTIVE SYSTEM
What are the two reasons that food is vital to life?
1.
2.
It provides the energy that drives the chemical reactions of the body cells.
It provides matter that is useful in building new tissues or repairing old
ones.
As it is normally consumed, food is not in a form that is immediately useful to the body.
What must happen to the food before it is useful? What is this process called?
It must first be broken down into molecules small enough to be absorbed across
cell membranes.
This breaking down process is called digestion. The organs that accomplish this
process form the digestive system.
A.
DIGESTIVE PROCESSES
Name and give a brief definition for each of the five basic processes of digestion.
ingestion -- taking food and drink by mouth
movement of food -- passage of food through the gastrointestinal tract by
peristalsis
digestion -- breakdown of foodstuffs
absorption -- passage of simple molecules from the digestive tract into
either blood or lymph
defecation -- elimination of indigestible substances from the system
Name and give a brief description for each of the two types of digestion.
mechanical digestion -- various movements specific for a particular organ
that moves and churns food, breaking it down mechanically.
chemical digestion -- consists of a series of catabolic reactions that break
down the foodstuffs enzymatically
B.
ORGANIZATION
What are the two main groups of digestive organs?
The organs of the digestive system are divided into two main groups, the
gastrointestinal (GI) tract (alimentary canal) and the accessory organs.
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List the organs of the gastrointestinal tract.
The organs of the GI tract include the mouth, esophagus, stomach, small
intestine, and large intestine. This continuous tube contains the food from
the time it enters the mouth until it exits via the anus.
How does the food move through this tube?
Muscular contraction of the wall of the GI tract break down the food
physically and move it through the tract by peristalsis, a wave-like
contraction of the smooth muscle.
What are the accessory organs of the digestive system?
The accessory organs are those structures which are not a direct part of
the GI tract, but which aid in the breakdown of food, either mechanically or
by adding secretions to the material as it moves down the tube (teeth,
tongue, salivary glands, pancreas, liver, and gallbladder).
1.
GENERAL HISTOLOGY OF THE GI TRACT
Name, and then describe, the four main layers of the GI tract.
In general, the wall of the GI tract, from esophagus to anus, has
four main layers:
1.
mucosa (mucous membrane)
2.
submucosa
3.
muscularis
4.
serosa
a.
MUCOSA
Describe the mucosa by listing and describing its three
components.
The mucosa, the inner lining of the tract, consists of three
parts, listed from innermost to outermost.
1.
epithelium
2.
lamina propria
3.
muscularis mucosa
Describe the epithelium.
The epithelium of the GI tract is in direct contact with
the GI contents and is involved in protection, secretion of enzymes and hormones, and absorption of
nutrients. From the stomach to the rectum, the
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epithelium is simple columnar. All other parts of the
tract are stratified squamous.
Describe the lamina propria.
The middle layer of the mucosa is the lamina propria.
This is a loose connective tissue rich in blood and
lymphatic vessels and scattered lymphatic nodules.
Describe the muscularis mucosa.
The third layer of mucosa is the muscularis mucosae,
a thin layer containing the smooth muscle cells
arranged circumferentially around the tube. It throws
the mucosa into folds that increase its surface area.
This is particularly true in the stomach.
b.
SUBMUCOSA
Describe the submucosa.
The second major level of the GI tract is the submucosa, a
loose connective tissue that binds the mucosa to deeper
structures. It is highly vascular and contains autonomic
ganglia of the parasympathetic system, and may contain
some exocrine glands that secrete into the gut.
c.
MUSCULARIS
Describe the muscularis.
The third layer of the GI tract is the muscularis, generally
consisting of inner circular and outer longitudinal smooth
muscle. This muscle is responsible for GI motility
(movement of food through the gut).
d.
SEROSA
Describe the serosa.
The fourth, and outermost, layer of the GI tract is the serosa.
This is a serous membrane composed of epithelium and
connective tissue. It is also known as the visceral
peritoneum.
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2.
PERITONEUM
What is the peritoneum?
The peritoneum consists of a simple squamous epithelium called
the mesothelium and an underlying connective tissue layer. It is
divided into two layers with a potential space between them.
Differentiate between parietal peritoneum, visceral peritoneum, and the
peritoneal cavity.
The parietal peritoneum lines the muscular wall of the abdominal
cavity while the visceral peritoneum covers some of the organs as
their serosa. The peritoneal cavity is the potential space between
the two layers.
What does retroperitoneal mean?
Some organs of the abdominopelvic cavity lie between the parietal
peritoneum and the muscular posterior abdominal wall. They are
therefore said to be retroperitoneal.
What are the mesentery and the mesocolon?
In some areas, the peritoneum contains large folds that weave
between the viscera and bind organs to the posterior wall, suspending them within the abdominal cavity. The folds contain blood vessels, lymphatics, and nerves supplying the suspended organs. The
mesentery surrounds the small intestine, and the mesocolon
surrounds part of the large intestine.
C.
MOUTH (ORAL CAVITY)
Describe the mouth as follows:
boundaries -- The mouth (oral or buccal cavity) is formed by the cheeks
laterally, hard and soft palates superiorly, and the tongue inferiorly.
hard palate -- The hard palate is the anterior portion of the roof of the
mouth. It is formed by the two maxillae and the two parietal bones.
It also serves as the floor of the nasal cavity.
soft palate -- The soft palate is the posterior portion of the roof of the
mouth. It consists of skeletal muscle covered by mucous membrane. Hanging from the middle of its free border is the uvula, a
cone-shaped muscular process.
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palatal arches -- Extending from the uvula are two muscular folds.
Anteriorly, the palatoglossal arches extend to the base of the
tongue. Posteriorly, the palatopharyngeal arches project to the
anterior surface of the pharynx.
mucosa -- The inside of the mouth is lined with nonkeratinized stratified
squamous epithelium.
vestibule -- The vestibule of the mouth is the space bounded externally by
the cheeks and lips and internally by the gums and teeth.
fauces -- The oral cavity itself extends from the gums and teeth, to the
fauces, the passageway posterior to the palatopharyngeal arches
that open into the oropharynx.
1.
TONGUE
Describe the tongue as follows:
location and structure -- The tongue, with its associated muscles,
forms the floor of the oral cavity. The tongue itself is composed of interlacing skeletal muscles, called the intrinsic
muscles, covered by stratified squamous epithelium.
extrinsic muscles -- The extrinsic muscles of the tongue originate
outside the mouth and insert into the tongue’s base. They
are used to move the tongue side-to-side and in-and-out,
and to help maneuver food and shape it within the mouth.
function -- Once a rounded mass of chewed food, called a bolus,
has been formed, the intrinsic muscles bow the tongue so
that the bolus is moved from the front of the mouth, through
the fauces, and into the oropharynx. The tongue is also
used to alter sounds in the creation of speech.
innervation -- The tongue muscles, both intrinsic and extrinsic, are
innervated by cranial nerve XII, the hypoglossal nerve.
2.
SALIVARY GLANDS
What is saliva?
Saliva is a fluid constantly secreted into the mouth by the salivary
glands located in or near the mouth. There are four sets of salivary
glands.
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Describe each of the following salivary glands?
buccal -- The buccal glands are the small glands located in the
mucous membrane of the mouth itself. They are responsible
for keeping the mouth and pharynx moist, even when food is
not in the mouth.
parotid -- The paired parotid glands are located inferior and anterior
to the ears, between the skin and masseter muscle. They
empty their secretions into the mouth via the parotid
(Stensen’s) ducts that open into the vestibule of the mouth
opposite the upper second molars.
submandibular -- The paired submandibular glands are located just
inside the mandible and beneath the base of the tongue on
either side. They empty via the submandibular (Wharton’s)
ducts on either side of the lingual frenulum, opposite the two
lower central incisors.
sublingual -- The paired sublingual glands are located superior to
the submandibular glands just under the mucosa of the floor
of the mouth. They empty via a number of small ducts
directly into the floor of the oral cavity proper.
a.
COMPOSITION OF SALIVA
What are the basic components of saliva?
Saliva is composed of 99.5% water, used to dissolve foods,
and 0.5% solutes.
Name, and then describe, each of the four basic solutes or solute
groups?
salivary amylase --This enzyme is used to initiate
carbohydrate chemical digestion by catalyzing the
breakdown of starch (a polymer of glucose) to the
disaccharide maltose (glucose-glucose).
ions – Na+, K+, C-l, HCO3-, and HPO4-2 are the main ions
found in saliva. They are used to activate salivary
amylase and to buffer acidic foods, keeping salivary
pH between 6.35 - 6.85.
mucous -- Mucin mixed with water forms mucous. It is used
to lubricate food so that it can be easily swallowed
and to facilitate speech.
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lysozyme -- Lysozyme is a nonspecific antimicrobial enzyme
used to maintain some degree of cleanliness in the
mouth.
b.
SECRETION OF SALIVA
How is salivation controlled?
Salivation is completely under the control of the parasympathetic nervous system, via cranial nerves VII (facial) and IX
(glossopharyngeal).
Name, and then describe, the three types of stimuli that initiate
salivation.
psychic -- The anticipation, sight, sound, smell, or memory of
food, working through the cerebral cortex, initiates
reflexive salivation. (think Pavlov).
chemical -- Stimulation of taste buds by food components or
the ingestion of irritants, working through the taste
buds, initiates reflexive salivation.
tactile -- Stimulation of touch receptors within the mouth and
pharynx initiates reflexive salivation. (Think about
holding a pencil in your mouth.)
3.
PHYSIOLOGY OF DIGESTION IN THE MOUTH
a.
MECHANICAL DIGESTION
Describe the mechanical digestion that occurs in the mouth.
Through chewing (mastication), food is reduced to a soft,
shredded, and ground moist mass. This mass is shaped by
the combined action of the cheeks, tongue, and teeth into a
flexible bolus that can be easily swallowed.
b.
CHEMICAL DIGESTION
Describe the chemical digestion that occurs in the mouth.
Salivary amylase enzymatically converts starch (a polymer
[a chemical compound consisting essentially of repeating
structural units] of glucose) into the disaccharide maltose
(GLU-GLU). There is a lingual lipase, a lipid- digesting
enzyme but its effects are very minor. Other dietary sugars,
as well as proteins and fats, are unaffected chemically.
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4.
PHYSIOLOGY OF DEGLUTITION
Define swallowing, and then describe the three phases by which the
process occurs.
Swallowing (deglutition) moves food from the mouth, through the
pharynx and esophagus, to the stomach. It involves three phases
and requires 4-8 seconds for solids, 1 second for liquids.
In the voluntary stage, the bolus of food is forced to the back of the
mouth and into the oropharynx by the movement of the tongue
upward and back against the palate.
The pharyngeal stage begins when tactile receptors in the
oropharynx are stimulated by the presence of the bolus. The
sensory information is carried to the deglutition center of the
medulla.
From the deglutition center, motor information via cranial nerves IX,
X, XI causes contraction of the pharyngeal constrictors, contraction
of the soft palate, and contraction of the laryngeal muscles.
As a result, food is moved down the pharynx to the esophagus, the
soft palate blocks access to the nasal cavity, and the larynx is
pulled up against the epiglottis, blocking access to the trachea.
The esophageal stage begins when the bolus is pushed into the
proximal end of the esophagus. In response, the upper esophageal
sphincter relaxes and the bolus moves into the esophagus.
Stretch of the esophagus initiates reflexive peristalsis, a wavelike
contraction of the esophageal muscularis that pushes the bolus of
food to the stomach.
At the distal end of the esophagus, the gastroesophageal sphincter
(lower esophageal sphincter) is reflexively opened and the bolus is
pushed into the stomach. The gastroesophageal sphincter immediately closes.
D.
ESOPHAGUS
1.
HISTOLOGY
2.
PHYSIOLOGY
Describe the esophagus as follows:
gross anatomy -- The esophagus is a muscular, collapsible 10-inch
long tube that lies posterior to the trachea. It begins at the
inferior end of the laryngopharynx, passes through the
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mediastinum anterior to the vertebral column, pierces
through the diaphragm at the esophageal hiatus, and ends
at the stomach.
sphincters -- There are two esophageal sphincters. The upper
esophageal sphincter lies at the point where the esophagus
exits from the laryngopharynx. The lower esophageal
(gastroesophageal) sphincter lies at the distal end of the
esophagus, at its junction with the stomach. They are
normally closed and open only in response to the deglutition
reflex.
mucosa/submucosa -- The mucosa of the esophagus is
nonkeratinized stratified squamous for abrasion resistance.
The submucosa contains numerous blood vessels and
mucous-secreting glands.
muscularis -- The muscularis begins as skeletal muscle proximally,
and grades into smooth muscle distally. The inner circular
portions of the muscularis form the two sphincters of the
esophagus.
function -- The esophagus plays no role in digestion other than as a
conduit from the pharynx to the stomach, using peristalsis to
move the food. There are no enzymatic secretions from the
esophagus.
E.
STOMACH
1.
ANATOMY
Describe the gross anatomy of the stomach as follows:
location -- The stomach is a J-shaped enlargement of the GI tract
directly posterior to the diaphragm in the left upper quadrant
(left hypochondriac region) of the abdominopelvic cavity.
The superior end is a continuation of the esophagus; the
inferior end becomes the duodenum.
subdivisions -- The stomach is divided into four major areas by
imaginary lines: the cardia, fundus, corpus (body), and
pylorus.
What is the cardia?
The cardia of the stomach is that portion that immediately surrounds the opening of the esophagus into
the stomach.
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What is the fundus?
The fundus is the rounded portion lying above and to
the left of the cardia. It is located above an imaginary
line drawn in the horizontal plane between the opening of the esophagus and far left margin of the
stomach.
What is the corpus?
The corpus (body) of the stomach is the large central
portion below the fundus.
What is the pylorus?
The pylorus is the narrow inferior portion that tapers
to funnel the corpus into the small intestine
(duodenum).
pyloric sphincter -- At the termination of the pylorus, the inner
circular layer of smooth muscle is thickened to form the
pyloric sphincter which regulates the movement of food from
the stomach into the duodenum.
curvature -- The concave medial border of the stomach is the lesser
curvature and the convex lateral border is the greater
curvature.
rugae -- When empty the mucosa of the stomach lies in large folds
called rugae. As the stomach is filled during a meal, the
rugae becomes flattened. This allows the stomach to
increase its size without increasing its tension.
2.
HISTOLOGY
Describe the histology of the stomach as follows:
epithelium -- There are two major differences in the histology of the
stomach compared to the other GI structures. The epithelium of the stomach changes abruptly from the stratified
squamous of the esophagus to simple columnar at the
cardia. This epithelium contains many gastric pits; at the
bottom of each are the gastric glands.
cells of the gastric glands -- The gastric glands consist of four
different cell types:
1.
chief (zygomatic) cells
2.
parietal cells
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3.
4.
mucous neck cells
G cells
What is the function of chief cells?
Chief cells secrete the principal gastric enzyme as an
inactive precursor form called pepsinogen, and an
enzyme of lesser importance, gastric lipase.
What is the function of the parietal cell?
Parietal cells secrete hydrochloric acid (HCl), which
converts pepsinogen to its active form of pepsin, and
intrinsic factor, which binds dietary vitamin B12.
What is the function of the mucous neck cells?
Mucous neck cells, located at the top (neck) of the
gastric glands, secrete mucous which coats the
mucosa, giving it protection from the HCl and pepsin.
What is the function of the G cells?
G cells secrete the hormone gastrin. As will be seen
shortly, gastrin has a number of roles in GI
physiology.
muscularis -- The other major change from the basic structure of
the GI tract is that the muscularis has three layers of smooth
muscle:
1.
inner oblique
2.
middle circular
3.
outer longitudinal
3.
PHYSIOLOGY OF DIGESTION IN THE STOMACH
a.
MECHANICAL DIGESTION
Describe the mixing waves of the stomach.
Mixing waves are gentle, rippling peristaltic movements that
begin in the upper corpus and move toward the pylorus of
the stomach. They occur every 15 - 25 seconds and
become more vigorous with continued stimulation from
gastrin and parasympathetic input via the vagus nerve.
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What do mixing waves accomplish?
These movements macerate (to cause to become soft or
separate into constituent elements) the food, mixing it with
gastric juices to form a thin liquid paste called chyme.
As chyme becomes more liquid, some moves through the
partially closed pyloric sphincter, while the rest is sloshed
back into the corpus for more mixing.
b.
CHEMICAL DIGESTION
Describe each of the following chemical activities in the stomach:
pepsinogen, gastric lipase, and rennin.
HCl
pepsinogen ---> pepsin (requires pH1-3)
proteins ---> polypeptides
Gastric lipase has limited activity in lipid digestion while in
the stomach because it requires pH 5-6. Therefore, its
effects are negligible.
Rennin is present in the infant stomach only. Its function is
to curdle milk so that it is retained in the stomach to allow
time for protein digestion before passing to the small
intestine.
4.
REGULATION OF GASTRIC SECRETION AND MOTILITY
Overview of stomach control
There are three phases of control:
1.
cephalic phase
2.
gastric phase
3.
intestinal phase
a.
CEPHALIC PHASE
Describe the endocrine and nervous mechanisms that occur during
the cephalic phase of gastric control.
The activities of the stomach in digestion are controlled by
hormonal and nervous mechanisms that occur in three
overlapping phases: cephalic, gastric, and intestinal.
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The cephalic phase refers to reflexes initiated by sensory
receptors of the head and are related to the same psychic
stimuli that initiate salivation.
In addition, the feeding center of the hypothalamus, and the
cerebral cortex, send impulses to the medulla that stimulate
the vagus nerve (X).
Parasympathetic impulses from the vagus nerve stimulate
the gastric glands to secrete their products and the smooth
muscle of the stomach to increase its motility.
b.
GASTRIC PHASE
Describe the endocrine and nervous mechanisms that occur during
the gastric phase of gastric control.
The gastric phase of stomach control begins when food
reaches the stomach. Sensory receptors in the stomach
initiate both nervous and endocrine mechanisms causing
gastric secretion and motility to continue.
Food stretches the stomach, stimulating the stretch
receptors, while increased pH of chyme stimulates
chemoreceptors. This sensory input initiates a neural
negative feedback cycle.
The combined effects of this are a localized parasympathetic
reflex that causes increased gastric gland secretion
(lowering pH) and increased mixing wave rate and intensity.
In addition, distention of the stomach and the presence of
proteins in chyme stimulate the G cells to secrete gastrin,
which enters the blood and further stimulates the gastric
glands.
Feedback system
controlled condition -- Food entering the stomach disrupts
homeostasis by causing an increase in gastric juice
pH AND stretch (distention) of the stomach wall.
receptors -- Chemoreceptors and stretch receptors in the
stomach detect the changes in homeostasis and
generate nerve impulses that pass to the control
centers.
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control center -- The submucosal plexus in the wall of the
stomach and neurons of the medulla generate
parasympathetic impulses to the effectors.
effectors -- Parietal cells of the gastric mucosa secrete HCl
and gastric smooth muscle contracts more vigorously
(increased mixing waves)
responses -- In response, there is increased acidity in the
stomach chyme and the mixing waves begin emptying
of the stomach. An empty stomach is a return to
homeostasis.
c.
INTESTINAL PHASE
Describe the endocrine and nervous mechanisms that occur during
the intestinal phase of gastric control.
With increased gastric secretion and motility, chyme
becomes a thin liquid and some is eventually moved through
the pyloric sphincter into the first part of the small intestine,
the duodenum.
The intestinal phase of stomach control is due to activation
of stretch receptors, as well as chemoreceptors, in the
duodenum by chyme.
Stimulation of stretch receptors in the duodenum initiates the
enterogastric reflex, in which sensory input to the medulla
turns off the vagal output to the stomach, resulting in
decreased gastric secretion and motility.
In addition, chyme stimulates enteroendocrine cells of the
duodenum to secrete four hormones: gastrin,
cholecystokinin (CCK), secretin, and gastric inhibitory
peptide (GIP).
CCK, secretin, and GIP circulate through the blood and act
to inhibit the gastric secretion and motility. These hormones
will have other important effects at other locations in the GI
tract.
The net effect of the intestinal phase is to inhibit the stomach
until the duodenum can process the chyme it received and
pass it on within the small intestine.
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5.
REGULATION OF GASTRIC EMPTYING
Summarize the processes that stimulate and inhibit gastric emptying and
describe their net result.
Gastric emptying is regulated by both neuronal (via the vagus
nerve) and endocrine (via four hormones) mechanisms.
Stomach emptying is stimulated by the action of the vagus nerve,
parasympathetic reflexes, and the hormone gastrin.
Stomach emptying is inhibited by the action of the enterogastric
reflex, which inhibits vagal output, and three hormones (CCK,
secretin, and GIP) that directly inhibit the stomach.
The net result is that the stomach is emptied slowly so that only a
small amount of chyme enter the duodenum at any given time; as
chyme is processed by the duodenum, the stomach becomes
turned on again.
Within 2 - 8 hours after a meal, the stomach is completely empty of
chyme. Meals rich in carbohydrates are emptied fastest, while
meals rich in fat are emptied slowest.
6.
ABSORPTION
Discuss absorption by the stomach.
The stomach mucosa is impermeable to most materials, so that
most substances are not absorbed until they reach the small
intestines.
Name 4 substances that are absorbed by the stomach.
water, electrolytes, certain drugs (aspirin), and alcohol
The next steps in digestion occur in the small intestine. Chemical digestion here
depends not only on its own secretions, but also on the activities of three
accessory organs of digestion.
Name the three accessory organs whose secretions are necessary to proper digestion
within the small intestine.
pancreas, liver, and gallbladder.
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F.
PANCREAS
1.
ANATOMY
2.
HISTOLOGY
Describe the pancreas as follows:
location -- The pancreas is an oblong gland about 5 inches long
and 1 inch thick. It consists of a “head” end nestled into the
bend of the duodenum and a “tail” lying next to the spleen.
The gland lays posterior to the greater curvature of the
stomach.
histology -- Histologically, 99% of the pancreas consists of small
clusters (acini) of exocrine secretory cells that produce
enzymes and sodium bicarbonate, then release them into
the organ’s duct work. The other 1%of the cells are clusters
of endocrine cells (islets of Langerhans) that secrete the
hormones insulin and glucagon into the bloodstream.
ducts -- Exocrine pancreatic secretions pass from the secreting
cells into small ducts which unite to form ever larger ducts.
Eventually 2 large ducts are formed: the main pancreatic
duct (of Wirsung) and the smaller accessory duct ( of
Santorini).
hepatopancreatic ampulla -- In most people the main pancreatic
duct exits the pancreas, joins with the common bile duct, and
enters the small intestine about 4 inches distal to the pyloric
sphincter as the hepatopancreatic ampulla(a flask with a
globular body, a saccular anatomic swelling) (of Vater)
The smaller accessory duct, if present, opens into the
duodenum about 1 inch above the ampulla of Vater.
3.
PANCREATIC JUICE
Describe, and name if applicable, the function(s) of each of the following
pancreatic juice components:
sodium bicarbonate -- (NaHCO3) buffers the pH of the small
intestinal contents (chyme) to 7.1 - 8.2. This stops the action
of pepsin and creates the proper environment for the
enzymes that function in the small intestine.
carbohydrate-digesting enzymes -- Pancreatic amylase is the same
as salivary amylase. It functions in the degradation of starch
to maltose.
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lipid-digesting enzymes -- Pancreatic lipase and cholesterol
esterase are fat-digesting enzymes.
nucleotide-digesting enzymes – Nucleases (DNAse and RNAse)
Give the names of the three inactive protein-digesting enzymes found in
pancreatic juice.
trypsinogen, chymotrypsinogen, and procarboxypeptidase
Why are these enzymes secreted in an inactive form?
As with the stomach and pepsinogen, these protein-digesting
enzymes are secreted in an inactive form to prevent self-digestion
of the pancreas.
How, and where, are these enzymes activated?
Enterokinase, an enzyme secreted by the small intestinal mucosal
cells, converts trypsinogen to trypsin, the active form. Trypsin, in
turn, converts chymotrypsinogen to chymotrypsin and procarboxypeptidase to carboxypeptidase.
The activation of the enzymes occurs in the small intestine where
the protein substrates for the active enzymes are found. In this
way, self-digestion of the pancreas is avoided.
4.
REGULATION OF PANCREATIC SECRETIONS
Describe the control of pancreatic secretion as follows:
nervous mechanisms -- During the cephalic gastric phase of
stomach control, parasympathetic impulses are conveyed to
the pancreas via the vagus nerve, resulting in increased
secretion of pancreatic juice.
endocrine mechanisms -- The hormones CCK and secretin,
released during the intestinal phase of stomach control,
affect the pancreas as well.
CCK stimulates the secretion of pancreatic enzymes.
Secretin stimulates the release of NaHCO3.
G.
LIVER
1.
ANATOMY
2.
HISTOLOGY
3.
BLOOD SUPPLY
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Describe the liver as follows:
location -- The liver, the second largest organ of the body, is
located in the right upper quadrant (right hypochondriac and
part of the epigastric regions), tucked under the diaphragm.
lobes -- It is divided into two primary lobes, right and left, by the
falciform ligament, a fold of the peritoneum, and two smaller
lobes, the caudate and the quadrate, located between the
right and left lobes.
bile canaliculi -- The liver is connected to the ampulla of Vater at
the duodenum by a series of ducts through which the main
liver secretion associated with digestion, bile, flows into the
small intestine. Bile is secreted by rows of hepatic parenchymal cells into tiny bile canaliculi to begin the journey out
of the liver.
hepatic ducts -- Bile canaliculi join larger ducts, which eventually
fuse to form ducts that exit the liver as the right and left
hepatic ducts. These two fuse outside the liver to form the
common hepatic duct.
common bile duct -- The common hepatic duct is joined by the
cystic duct from the gallbladder, forming the common bile
duct.
The common bile duct joins the main pancreatic duct at the
ampulla of Vater, as described earlier. In this way bile
makes its way to the small intestine.
Describe the liver lobule as follows:
cell arrangement -- The liver is composed of thousands of functional units called lobules. Each consists of specialized
epithelial cells called hepatocytes arranged as plates of cells
about the central vein. Adjacent plates of hepatocytes are
separated by blood sinusoids lined with endothelium and
reticuloendothelial cells (destroy RBCs and blood-borne
bacteria).
bile flow -- Bile is secreted from the basal surfaces of the hepatocytes, between two juxtaposed rows of cells, and into the
bile canaliculi. It then flows outwards to the periphery of the
lobule to eventually enter the hepatic ducts.
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blood supply -- The blood supply of the liver, which comes from two
different sources, flows in the direction opposite to that of
bile within the lobule (from the periphery towards the central
vein in the center of the lobule).
The hepatic artery, coming from the celiac artery that rises
from the aorta, brings oxygenated blood into the liver.
The hepatic portal vein carries the blood previously found in
the small and large intestines where it absorbed nutrients
from the diet.
portal triad -- Located in several locations around the periphery of a
given lobule are distinct areas that contain a branch of the
hepatic artery and a branch of the hepatic portal vein
(delivering blood to the lobule), and a branch of a bile duct
(taking bile away from the lobule). Each arrangement of
these three tubes is called a portal triad.
blood flow -- Blood flows from the vessels of the portal triads, into
the sinusoids, and towards the central vein, located in the
center of each lobule. All central veins join to form the
hepatic vein that leaves the liver and enters the inferior vena
cava.
4.
BILE
What is bile?
Bile is a yellowish-brown to olive-green liquid consisting of water,
bile salts, bile pigments, cholesterol, and other substances. It is
NOT an enzyme. Rather, it is a detergent.
How is bile used in the digestive process of the small intestine?
Bile is used in the small intestine for the process of fat
emulsification. That is, the breakdown of large fat globules into
smaller fat droplets, resulting in increased surface area for chemical
digestion by the fat-digesting enzymes.
5.
REGULATION OF BILE SECRETION
Describe the control of liver secretion of bile as follows:
nervous mechanisms -- As with the pancreas, increased vagal
stimulation during the cephalic and gastric phases of
stomach control, also leads to increased production of bile
by two-fold over resting levels.
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endocrine mechanisms -- Secretin, released from the duodenum
during the intestinal phase of stomach control, also
stimulates the liver to increase its production of bile.
6.
PHYSIOLOGY OF THE LIVER
Without giving specifics, can you list 9 functions of the liver including bile
production?
1.
2.
3.
4.
5.
6.
7.
8.
9.
carbohydrate metabolism
protein metabolism
lipid metabolism
removal of drugs, hormones, and other chemicals from the
blood for detoxification
synthesis of bile salts
excretion of bile
storage (particularly glycogen, ferritin, certain vitamins, and
toxic heavy metals that cannot be detoxified or excreted)
phagocytosis of worn-out RBCs, WBCs, and some bacteria
activation of vitamin D
Name, and then describe, the three main types of glucose metabolism that
occurs in the liver.
glycogenesis -- In glycogenesis, hepatocytes convert glucose ( a
monosaccharide) to glycogen (a polymer of glucose) when
blood glucose level is high. This is one of the major means
by which blood glucose is lowered back to within homeostatic range after a meal.
glycogenolysis -- In glycogenolysis, stored liver glycogen is broken
down into its component glucose molecules and liberated
into the blood for circulation in the body. This is one of the
major means by which blood sugar level is brought back into
homeostatic range during times of fasting.
gluconeogenesis -- In gluconeogenesis, hepatocytes convert amino
acids and lactic acid to glucose when blood sugar level is
low. This is another means used to raise blood glucose
back into the homeostatic range during times of fast.
Name, and then describe, the three main types of protein metabolism that
occurs in the liver.
deamination -- Hepatocytes remove the amino group (-NH2) from
amino acids (deamination) so they can be used for ATP
production or be converted to carbohydrates or fats. The
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amino groups are then converted to ammonia (NH3).
Without this deamination process, death would occur within
just a few days.
urea formation -- Hepatocytes convert toxic ammonia (NH3) into
the much less toxic urea which is excreted from the body in
urine.
plasma protein production -- Virtually all plasma proteins are
produced by hepatocytes.
H.
GALLBLADDER
1.
HISTOLOGY
2.
PHYSIOLOGY
Describe the gallbladder. Include its location, basic histology, how it is
filled with bile, and how and when bile is removed from the gall bladder.
The gallbladder is a pear-shaped sac about 3-4 inches long,
located in a fossa of the visceral (inferior) surface of the liver. It is
connected to the biliary system of ducts by the cystic duct.
It consists of a mucosa of simple columnar cells that are thrown into
rugae when the organ is empty. It has a typical muscularis of inner
circular and outer longitudinal smooth muscle.
The functions of the gallbladder are to store bile excreted from the
liver and concentrate it (up to 10-fold), by removing its water, until it
is needed in the small intestine.
When no food is in the small intestine, bile is produced and
excreted from the liver at a slow rate, filling the biliary system of
ducts.
The bile does not flow into the small intestine because the ampulla
of Vater has a sphincter, the sphincter of Oddi, which is normally
closed. As a result, bile backs up in the ducts and backfills into the
gall bladder.
When chyme enters the duodenum, CCK is secreted. In addition to
its other roles, it also causes the sphincter of Oddi to relax and the
smooth muscle of the gallbladder to contract.
As a result, bile is forcefully ejected from the gallbladder and it is
drained from the biliary system into the duodenum, where it participates in the emulsification of fats in the diet.
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I.
SMALL INTESTINE
1.
ANATOMY
Describe the location of the small intestine and give the names of its three
subdivisions.
The small intestine extends from the stomach to the large intestine,
stretching some 21 feet in the cadaver. It is only about 7 feet long
in the living state because of muscle tone. It is subdivided into
three portions: the duodenum, the jejunum, and the ileum.
Describe the duodenum.
The duodenum, beginning distal to the pyloric sphincter of the
stomach, is the C- shaped 10-inch long initial portion of the small
intestine. It extends from the pyloric sphincter of the stomach to the
second portion, the jejunum. It is retroperitoneal.
Describe the jejunum.
The jejunum, the second portion, is about 8 feet long (cadaver). It
begins where the duodenum emerges from behind the peritoneum,
is wrapped in the mesentery, and terminates at the ileum.
Describe the ileum.
The ileum, the third portion of the small intestine, is about 12 feet
long (cadaver). It begins as a continuation of the jejunum and ends
at the colon. Its distal-most portion forms the ileocecal (AKA-ileocolic) sphincter. Most absorption (90%) occurs in this section.
2.
HISTOLOGY
Describe the small intestine as follows:
basic histology -- The small intestine has the same 4 tunics as the
rest of the GI tract. The mucosa contains pits, like the
stomach, that are lined with simple columnar epithelium
forming simple tubular intestinal glands that secrete
intestinal juice. The muscularis has typical inner circular and
outer longitudinal smooth muscle layers.
mucosal specializations -- Because most absorption occurs in the
small intestine, there are three structural adaptations of the
mucosa that facilitate this process by increasing the mucosal
surface area: plicae circulares, villi, and microvilli.
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What is the plicae circulares?
The plicae circulares is a circularly- arranged permanent longitudinal fold, about 0.4 inches high, that
spirals through the length of the small intestine. It
swirls the chyme as it passes through the tube.
What are villi?
Villi (singular = villus) are finger-like projections of the
mucosa about 1 mm high (40 villi/ sq. mm or 26,000 /
sq. in.), thus greatly increasing surface area for
absorption. The center of each villus contains an
arteriole, a capillary bed, a venule, and a single
lymphatic capillary called a lacteal.
What are microvilli?
The apical (free) surface of each epithelial cell of the
mucosa features microvilli. These are microscopic,
finger-like processes of the cell membrane that further
increase surface area. They are collectively called
the brush border.
Peyer’s patches -- Peyer’s patches are lymphatic nodules
found throughout the submucosa of the ileum in particular. Their purpose is for protection from food-borne
pathogens.
3.
INTESTINAL JUICE AND BRUSH BORDER ENZYMES
What is intestinal juice?
Intestinal juice is a clear yellow fluid, pH 7.6, containing water and
mucous. Together, pancreatic and intestinal juices, and bile
(collectively known as the succus entericus), provide the medium
for the completion of chemical digestion and absorption of the
nutrients.
What are brush border enzymes? Name them.
Brush border enzymes are enzymes of the small intestine located
on the membranes of the microvilli. They cause final chemical
digestion of foodstuffs to occur on the surface of the absorptive
cells, thus facilitating absorption.
Brush border enzymes include maltase, lactase, various
peptidases, dextrinase, nucleosidases, and phosphatases.
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4.
PHYSIOLOGY OF DIGESTION IN THE SMALL INTESTINES
a.
MECHANICAL DIGESTION
Describe the process of segmentation. How does it occur and what
does it accomplish?
The major movement associated with mechanical digestion
in the small intestine is segmentation, a strictly localized
contraction of areas of the tract containing chyme.
Segmentation mixes chyme with all the digestive juices and
up against the absorptive cells of the mucosa. It does not
push chyme along the tract, but rather sloshes it back and
forth.
Segmentation begins with contraction of the inner circular
smooth muscle in an area containing chyme. It results in the
formation of small segments in the length with the chyme.
Next, muscle fibers in the center of a segment contract while
the other fibers relax, dividing the segment again. Finally, it
repeats the sequence 12 - 16 times/minute, causing the
segments to slosh together.
The second major movement of the small intestine is simple
but weak peristalsis, contractions that move chyme forward
through the tract. Chyme is propelled through the small
intestine at a rate of about 1 meter (3 feet) per hour.
b.
CHEMICAL DIGESTION
Review the chemical digestion of carbohydrates.
starch
salivary and pancreatic amylase
maltose
maltase
glucose + glucose
Starch, a polymer of glucose, is reduced to its glucose components
during digestion. The monosaccharide can then be absorbed
across the intestinal mucosa into the blood.
_____________________________________________________
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sucrose
sucrase
glucose + fructose
Sucrose, or table sugar, is reduced to its component monosaccharides by the enzyme sucrase. Glucose and fructose can them
be absorbed across the intestinal mucosa into the blood.
_____________________________________________________
lactose
lactase
glucose + galactose
Lactose, or milk sugar, is reduced to its component
monosaccharides by the enzyme lactase. Glucose and galactose
can then be absorbed across the intestinal mucosa into the blood.
_____________________________________________________
Review the chemical digestion of proteins.
proteins
HCl
pepsin<---- pepsinogen
polypeptides
enterokinase
trypsin<---------------- trypsinogen
trypsin
chymotrypsin<-------- chymotrypsinogen
trypsin
carboxypeptidase <-------- procarboxypeptidase
dipeptides
dipeptidases
amino acids
Amino acids can then be absorbed across the intestinal mucosa
into the blood.
_____________________________________________________
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Review chemical digestion of lipids.
lipids
lingual lipase and gastric lipase (?)
lipids
bile
emulsified lipids
pancreatic lipase
cholesterol esterase
fatty acids
glycerols
glyercides
The products of fat digestion are absorbed into the
small intestinal cells, reassembled as chylomicrons,
then absorbed into the lymph of the small intestine.
_____________________________________________________
5.
REGULATION OF INTESTINAL SECRETION AND MOTILITY
Regulation of small intestinal secretion and motility is accomplished in
three ways. Name and briefly them.
local reflexes -- Local reflexes are based on stretch of the intestinal
wall and work through the parasympathetic ganglia
(submucosal plexi) located there. (segmentation)
parasympathetic reflexes -- Parasympathetic reflexes via the vagus
nerve are based on stretch of the intestinal wall. Stretch
receptors send nervous impulses to the medulla, initiating an
increase in rate of peristalsis (10 cm/sec) and increased
intestinal gland secretion.
gastrin -- Gastrin is secreted by both the gastric and duodenal
mucosae. It stimulated increased peristalsis and increased
secretion from the intestinal glands.
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6.
PHYSIOLOGY OF ABSORPTION
What is absorption?
Passage of digested nutrients from the GI tract into either the blood
or lymph of the intestine is known as absorption. 90% of all
absorption occurs in the small intestine. The remainder occurs in
the stomach and the large intestine. Any undigested or unabsorbed materials left in the small intestine are passed into the large
intestine.
By what process(es) are carbohydrates and proteins absorbed?
For the most part, monosaccharides and amino acids are absorbed
by either active transport or facilitated diffusion.
These products are absorbed into what?
All nutrients, except those derived from fat, are absorbed into the
blood capillaries within the villi. This blood is eventually collected
into the superior mesenteric vein for transport to the hepatic portal
vein (formed by the union of the superior mesenteric vein and the
splenic vein.)
By what process(es) are fats absorbed?
The products of fat digestion are absorbed into intestinal cells by
simple diffusion, and then reassembled within the intestinal
mucosal cell to form triglycerides that are coated with a protein,
forming a chylomicron.
These products are absorbed into what?
Chylomicrons are exocytosed into the intestinal villi, where they
move into a lacteal ( a specialized lymphatic capillary) and are
eventually carried to the bloodstream by the thoracic duct. They
are removed from the blood as it passes through the liver.
How is water absorbed by the small intestine?
Water, approximately 8 liters/day is absorbed by the gut by
osmosis, following the osmotic gradients created as the various
nutrients are moved from the gut into the mucosal cells, then into
the blood or lymph.
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J.
LARGE INTESTINE
1.
ANATOMY
Describe the large intestine as follows:
general functions -- the overall functions of the large intestine are
1.
completion of absorption
2.
manufacture of certain vitamins using bacteria
3.
formation of feces
4.
expulsion of feces from the body
gross anatomy and location -- the large intestine is about 2.5 inches
in diameter and about 5 feet long (cadaver). It extends from
the ileocecal sphincter to the anus. It is attached to the
posterior abdominal wall by the mesocolon.
parts -- The large intestine is divided into six portions. From the
beginning to end they are the:
cecum -- The cecum, the beginning of the large intestine, is
a blind pouch that receives the small intestine at the
ileocecal sphincter. To it is attached the vermiform
appendix.
ascending colon --The ascending colon is retroperitoneal.
From the cecum, it ascends the right posterior
abdominal wall, reaches the liver, and turns abruptly
to the left at the hepatic (right colic) flexure.
transverse colon -- The transverse colon, extending from the
hepatic flexure, hangs from the mesocolon as it
crosses the abdomen anteriorly, then moves
posteriorly to reach the spleen. It then turns
downward at the splenic (left colic) flexure.
descending colon -- The descending colon extends
retroperitoneally from the splenic flexure, along the
left posterior abdominal wall to the left anterior
superior iliac spine, where it moves toward the
posterior midline as the sigmoid colon.
sigmoid colon -- The sigmoid colon moves into the posterior
midline, lying on the anterior surface of the sacrum
and terminates at the level of S-3 as the rectum.
rectum
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Describe the following:
rectum -- The rectum is the last 8 inches of the colon, extending
from the sigmoid colon. It lies just anterior to the curve of
the sacrum and coccyx, and terminates as the anal canal.
anal canal -- The anal canal is the terminal one inch of the GI tract.
It is lined with a mucous membrane found in longitudinal
folds called the anal columns. These folds contain branches
of the hemorrhoidal arteries and veins. The anal canal
opens to the outside of the body as the anus after passing
through 2 sphincters.
internal anal sphincter -- The internal anal sphincter is formed by
the inner circular smooth muscle of the muscularis at the
bottom of the anal canal. It is innervated by parasympathetic
outflow from the sacral spinal cord (involuntary control).
external anal sphincter -- The external anal sphincter is formed by
the skeletal muscles of the pelvic diaphragm (muscular floor
of the pelvis). It is controlled by the sacral spinal nerves
(voluntary control as a learned response).
2.
HISTOLOGY
Describe the histology of the large intestine as follows:
mucosa -- The mucosa has no villi or permanent plicae circulares
so its surface area is greatly reduced from that of the small
intestine. The epithelium is simple columnar with absorptive
cells and a tremendous number of goblet cells.
muscularis -- The outer longitudinal layer of smooth muscle in the
muscularis is modified to form the taenia coli, three bands of
muscle that run the length of the colon. They serve to gather
the colon into a series of pouches called haustra.
epiploic appendages -- The serosa of the large intestine is part of
the visceral peritoneum. It has within it small fat-filled
pouches called epiploic appendages which are used as
energy reserves.
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3.
PHYSIOLOGY OF DIGESTION IN THE LARGE INTESTINE
a.
MECHANICAL DIGESTION
How is chyme moved into the large intestine?
In response to the gastroileal reflex and the hormone gastrin,
the ileocecal sphincter relaxes. Increased motility of the
small intestine causes chyme to move into the cecum.
Name and briefly describe the three types of mechanical digestion
that occurs in the large intestine.
haustral churning -- Chyme entering the cecum initiates
haustral churning. As each haustra fills, it stretches.
This causes a local reflex that causes the haustra to
contract. This pushes chyme to the next haustra,
where the process is renewed.
mass peristalsis -- Stretch of the stomach initiates the
gastrocolic reflex. Massive vagal output to the transverse colon initiates strong peristaltic contractions that
begin in the middle of the transverse colon and
extend to the rectum. This drives feces into the
rectum and makes way for new chyme.
peristalsis -- Regular peristalsis, occurring as weak muscular
contractions, sweep the colon at a rate of 3 - 12 times
per minute, slowly moving chyme through the large
intestine.
b.
CHEMICAL DIGESTION
Describe the role of bacteria in the large intestine.
Chemical digestion in the large intestine is dependent upon
the activity of bacteria that normally reside in the colon.
1.
The bacteria ferment remaining carbohydrates in
colonic chyme, releasing gas (flatus).
2.
The bacteria secrete vitamin K and some of the B
complex vitamins which are absorbed by the large
intestinal mucosa.
4.
ABSORPTION AND FECES FORMATION
What role does the large intestine play in absorption?
Simple molecules that escaped the small intestine without being
absorbed or which were created by bacterial action, including
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vitamins, are absorbed by the large intestine. In addition, most of
the remaining water in the chyme is absorbed, leaving only about
150 ml per day in the feces.
List the components of feces.
Water (about 150 ml/day), undigested foodstuff (plant fibers =
cellulose), bacteria, products of bacterial decomposition, sloughed
epithelial cells.
5.
PHYSIOLOGY OF DEFECATION
Describe the defecation reflex.
Distention of the rectum when it is filled with feces results in
stimulation of its stretch receptors. Sensory input to the sacral
spinal cord causes parasympathetic motor output which elicits: (1)
contraction of the descending colon, sigmoid colon, and rectum,
and (2) reflex relaxation of the internal anal sphincter. With voluntary relaxation of the external anal sphincter, the pressure generated within the colon by contraction of muscularis causes the anus
to open and the feces to be expelled.
K.
REVIEW OF DIGESTIVE PROCESSES
1.
CEPHALIC PHASE OF GASTRIC CONTROL
Review the cephalic phase of gastric control.
1.
2.
3.
4.
2.
The taste, smell, anticipation, thought, or sensation of food in
the mouth promotes cerebral input to the vagal center in the
medulla oblongata.
Parasympathetic sensations are carried by the vagus nerves
to the stomach.
Postganglionic parasympathetics in the stomach wall
stimulate the gastric juice secretion and the secretion of
gastrin from the endocrine cells.
Gastrin is carried through the circulation to all parts of the
stomach, where it also stimulates gastric juice secretion.
GASTRIC PHASE OF GASTRIC CONTROL
Review the gastric phase of gastric control.
1.
2.
Distention of the stomach by food initiates local stretch
reflexes that stimulate gastric juice secretion.
At the same time, distention stimulates afferent vagal input
to the vagal nuclei of the medulla
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3.
3.
This results in efferent vagal impulses that further stimulate
gastric juice secretion.
INTESTINAL PHASE OF GASTRIC CONTROL
Review the intestinal phase of gastric control.
1.
2.
3.
4.
The presence in the duodenum of chyme with a pH > 3 or
containing amino acids and peptides stimulate gastric
secretions through vagus nerve pathways or through
secretion of gastrin.
Stretch of the duodenum by chyme initiates the enterogastric
reflex resulting in decreased gastric secretion and motility.
The presence of chyme in the duodenum which a pH of < 2
also inhibits the vagal pathway and stimulates secretion of
three hormones that enter the circulation for delivery to the
stomach, where they inhibit gastric secretion.
CONTROL OF BILE SECRETION
Review the control of bile secretion.
1.
2.
3.
5.
Secretin, produced by the duodenum and carried through
the circulation to the liver, stimulates bile secretion by the
liver.
Cholecystokinin, produced by the duodenum and carried
through the circulation to the gallbladder and sphincter of
Oddi, stimulates contraction of gallbladder smooth muscle
and relaxation of the sphincter of Oddi. These actions allow
bile to move from the gallbladder into the duodenum.
Vagal nerve stimulation of the liver causes the liver to
secrete bile and the gallbladder to release bile to the
duodenum.
CONTROL OF PANCREATIC SECRETIONS
Review the control of pancreatic secretion.
1.
2.
3.
Secretin, secreted from the duodenum in response to
chyme, travels through the circulation to the pancreas,
where it stimulates release of a watery secretion rich in
bicarbonate arrows.
Cholecystokinin, produced by the duodenum in response to
chyme, travels through the circulation to the pancreas,
where it stimulates release of secretion rich in digestive
enzymes.
Vagal nerve stimulation of the pancreas causes it to secrete
juices rich in digestive enzymes.
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6.
REFLEXES IN THE COLON AND RECTUM
Review the reflexes in the colon and rectum.
1.
2.
3.
4.
The presence of food in the stomach (gastrocolic reflex) and
chyme in the duodenum stimulate mass movement in the
colon
Mass movements in the colon propel its contents towards
the rectum
Mass movements are integrated by local reflexes throughout
the length of the colon.
The presence of feces in the rectum stimulates local stretch
reflexes as well as the parasympathetically-controlled
defecation reflex which operates via the sacral spinal cord.
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