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The Digestive System
Chapter 22
The Digestive System

The digestive system
– Takes in food
– Breaks it down into nutrient molecules
– Absorbs the nutrient molecules into the
bloodstream
– Rids the body of indigestible remains
The Digestive System

The organs of the digestive system can be separated
into two main groups; those of the alimentary canal
and the accessory organs
The Digestive System

The alimentary canal or gastrointestinal (GI) tract is
the continuous muscular digestive tube that winds
through the body
The Digestive System

The organs of the alimentary canal are
– Mouth, pharynx, esophagus, stomach, small
intestine and large intestine
– Food in this canal is technically out of the
body

The accessory digestive organs are
– Teeth, tongue, gallbladder, salivary glands,
liver and pancreas
– The accessory organs produce saliva, bile and
digestive enzymes that contribute to the
breakdown of foodstuffs
Digestive
Processes

The digestive tract
can be viewed as a
process by which
food becomes less
complex at each step
of processing and
nutrients become
available to the
body
Ingestion

Ingestion is simply
the process of
taking food into
the digestive tract
via the mouth
Propulsion


Propulsion is the
process that moves
food through the
alimentary canal
It includes
swallowing
(voluntary process)
and peristalsis
(involuntary
process)
Propulsion



Peristalsis involves
alternate waves of
contraction and
relaxation of
muscles in the
organ walls
Its main effect is to
squeeze food from
one organ to the
next
Some mixing occurs
as well
Mechanical Digestion

Mechanical
digestion physically
prepares food for
chemical digestion
by enzymes
Mechanical Digestion


Mechanical processes
include chewing,
mixing of food with
saliva by the tongue,
churning of food by
the stomach, and
segmentation
Segmentation mixes
food with digestive
juices and increases
the rate of absorption
by moving food over
the intestinal wall
Chemical Digestion

Chemical digestion
is a series of
catabolic steps in
which complex
food molecules are
broken down to
their chemical
building blocks
Chemical Digestion


Chemical digestion is accomplished by
enzymes secreted by various glands into
the lumen of the alimentary canal
The enzymatic breakdown of foodstuffs
begins in the mouth and is essentially
complete in the small intestine
Absorption

Absorption is the
passage of digested
end products (plus
vitamins, mineral
and water) from
the lumen of the
GI tract into the
blood or lymph
capillaries located
in the wall of the
canal
Absorption


For absorption to occur these substances
must first enter the mucosal cells by
active or passive transport processes
The small intestine is the main absorption
site
Defecation

Defecation is the
elimination of
indigestible
substances from
the body as feces
Basic Functional Concepts



Most organ systems respond to changes in
the internal environment either by
attempting to restore some plasma variable
or by changing their own function
The digestive system creates an optimal
environment for its functioning in the lumen
of the GI tract
Essentially all digestive tract regulatory
mechanisms act to control luminal conditions
so that digestion and absorption can occur
there as effectively as possible
Basic Functional Concepts

Digestive activity is provoked by a range
of mechanical and chemical stimuli
– Receptors are located in the walls of the tract
organs
– These receptors respond to several stimuli
– The most important being the stretching of
the organ by food in the lumen, osmolarity
(solute concentration) and pH of the contents
and the presence of substrates and end
products of digestion
Basic Functional Concepts

When appropriately
stimulated, these
receptors initiate
reflexes that
– Activate or inhibit
glands that secrete
digestive juices into the
lumen or hormones
into the blood
– Mix lumen contents
along the length of the
tract by stimulating
the smooth muscle of
the GI tract walls
Basic Functional Concepts

Controls of digestive activity are both
extrinsic and intrinsic
– Another novel trait of the digestive tract is
that many of its controlling systems are
intrinsic - a product of in-house nerve
plexuses or local hormone-producing cells
– The walls of the alimentary canal contain
nerve plexuses
– These plexuses extend essentially the entire
length of the GI tract and influence each
other both in the same and in different
organs
Digestive Processes



Two kinds of reflex
activity occur
Short reflexes are
mediated entirely by
the local enteric
plexuses in response
to GI tract stimuli
Long reflexes are
initiated by stimuli
arising from within or
outside of the GI tract
and involve CNS
centers and ANS
Digestive Processes


The stomach and small intestine also
contain hormone-producing cells that,
when stimulated by chemicals, nerve
fibers, or local stretch, release their
products to the extracellular space
These hormones circulate in the blood
and are distributed to their target cells
within the same or different tract organs,
which they prod into secretory or
contractile activity
Digestive System Organs



Most of the digestive
organs reside in the
abdominal-pelvic
cavity
All ventral body
cavities contain
serous membranes
The peritoneum of
the abdominal cavity
is the most extensive
serous membrane of
the body
Digestive System Organs


The visceral peritoneum
covers the external surface
of most digestive organs
and is continuous with the
parietal peritoneum that
lines the walls of the
abdomino-pelvic cavity
Between the two layers is
the peritoneal cavity, a
slitlike potential space
containing fluid secreted
by the serous membranes
Digestive System Organs

The serous fluid
lubricates the mobile
digestive organs,
allowing them to glide
easily across one
another as they carry
out their digestive
activities
Digestive System Organs

A mesentery is a double layer of peritoneum - a sheet
of two serous membranes fused back to back - that
extends to the digestive organ from the body wall
Digestive System Organs

Mesenteries provide routes for blood vessels,
lymphatics and nerves to reach the digestive viscera
Digestive System Organs

Mesenteries also suspend the visceral organs in place
as well as serving as a site for fat storage
Digestive Processes


Not all alimentary canal organs are suspended with
the peritoneal cavity by a mesentery
Some parts of the small intestine originate the cavity
but then adhere to the dorsal abdominal wall
(Figure 22.5) above
Digestive Processes


Organs that adhere to the dorsal abdominal wall lose
their mesentery and lie posterior to the peritoneum
These organs, which also include most of the pancreas
and parts of the large intestine are called retroperitoneal organs
Digestive Processes


Digestive organs like the stomach that keep their
mesentery and remain in the peritoneal cavity are
called interperitoneal or peritoneal organs
It is not known why some digestive organs end up in
the retroperitoneal position
Blood Supply



The splanchnic circulation includes those
arteries that branch off the abdominal
aorta to serve the digestive organs and
the hepatic portal circulation
The hepatic, splenic and left gastric
branches of the celiac trunk serve the
spleen, liver, and stomach
The mesenteric arteries (superior and
inferior) serve the small and large
intestine
Blood Supply



The arterial supply to the abdominal organs
is approximately one quarter of the cardiac
output
The hepatic portal circulation collects
nutrient-rich venous blood draining from the
digestive viscera and delivers it to the liver
The liver collects the absorbed nutrients for
metabolic processing or for storage before
releasing them back to the bloodstream for
general cellular use
Histology of the Alimentary Canal

From the esophagus to the anal canal, the
walls of every organ of the alimentary
canal are made up of the same four basic
layers or tunics
–
–
–
–

Mucosa
Submucosa
Muscularis externa
Serosa
Each tunic contains a predominant tissue
type that plays a specific role in food
breakdown
Histology of the Alimentary Canal

From internal to
external the four
layers of the
alimentary canal
are
– Mucosa
– Submucosa
– Muscularis
Externa
– Serosa
Histology: Mucosa


The mucosa is the
moist epithelial
membrane that lines
the length of the
lumen of the
alimentary canal
Major functions are
– Secretion of mucus,
digestive enzymes
and hormones
– Absorption
– Protection
Histology: Mucosa


The mucosa is the
moist epithelial
membrane that lines
the length of the
lumen of the
alimentary canal
Major functions are
– Secretion of mucus,
digestive enzymes
and hormones
– Absorption
– Protection
Histology: Mucosa

More complex than most other mucosae
the typical digestive mucosa consists of
three sublayers
– A surface epithelium
– A lamina propria
– A deep muscularis mucosae
Histology: Mucosa

The epithelium of
the mucosa is a
simple columnar
epithelium that is
rich in mucus
secreting goblet cells
Histology: Mucosa



The slippery mucus it produces protects
certain digestive organs from digesting
themselves by enzymes working within
their cavities and eases food passage
In the stomach and small intestine the
mucosa contain both enzyme-secreting
and hormone-secreting cells
Thus, in such sites, the mucosa is a
diffuse kind of endocrine organ as well as
part of the digestive organ
Histology: Mucosa


The lamina propria
which underlies the
epithelium is loose
areolar connective
Note lymph nodule
Histology: Mucosa



Its capillaries nourish the epithelium and
absorb digested nutrients
Its isolated lymph nodules are part of the
mucosa associated lymphatic tissue (MALT)
which collectively act as a defense against
bacteria and other pathogens
Large collections of lymph nodules occur at
strategic locations such as within the
pharynx (tonsils) and appendix
Histology: Mucosa

The muscularis
mucosae is a scant
layer of smooth
muscle cells that
produces local
movements of the
mucosa
Histology: Mucosa


The twitching of this muscle layer dislodges
food particles that have adhered to the
mucosa
In the small intestine, it throws the mucosa
into a series of small folds that immensely
increase its surface area
Histology: Submucosa


The submocosa is a
moderately dense
connective tissue
containing blood and
lymphatic vessels,
lymph nodules, and
nerve fibers
Its rich supply of
elastic fibers enables
the stomach to
regain its normal
shape after storing a
large meal
Histology: Submucosa


The submocosa is a
moderately dense
connective tissue
containing blood and
lymphatic vessels,
lymph nodules, and
nerve fibers
Its rich supply of
elastic fibers enables
the stomach to
regain its normal
shape after storing a
large meal
Histology: Muscularis Externa



The muscularis
externa is
responsible for
segmentation and
peristalsis
It mixes and propels
foodstuffs along the
digestive tract
This thick muscular
layer has an inner
circular and an
outer longitudinal
layer
Histology: Muscularis Externa


In several places along the GI tract, the
circular layer thickens to form sphincters
Sphincters act as valves to prevent backflow
and control food passage from one organ to
the next
Histology: Serosa


The serosa is the
protective outermost
layer of interperitoneal organ
This visceral
peritoneum is
formed of areolar
connective tissue
covered with mesothelium, a single
layer of squamous
epithelial cells
Histology: Serosa



In the esophagus, which is located in the
thoracic instead of the abdominopelvic
cavity, the serosa is replaced by an
adventitia
The adventitia is an ordinary fibrous
connective tissue that binds the esophagus to
surrounding structures
Retroperitoneal organs have both a serosa
(on the side facing the peritoneal cavity) and
an adventitia (on the side abutting the dorsal
body wall)
Enteric Nervous System


The alimentary
canal has its own inhouse nerve supply
Enteric neurons
communicate widely
with each other to
regulate digestive
system activity
Intrinsic
Nerve
Plexes
Enteric Nervous System

These enteric
neurons constitute
the bulk of the two
major intrinsic
nerve plexuses
found within the
walls of the
alimentary canal
– Submucosal nerve
plexus
– Myenteric nerve
plexus
Myenteric
plexus
Submucosal
plexus
Enteric Nervous System

A smaller third
plexus is found
within the serosa
layer
– Subsersora nerve
plexus
Subserosa
nerve
plexus
Enteric Nervous System


The submucosal
nerve plexus chiefly
regulates the
activity of glands
and smooth muscle
in the mucosa tunic
The myenteric
nerve plexus lies
between the circular
and longitudinal
layers of smooth
muscle of the
muscularis externa
Myenteric
plexus
Submucosal
plexus
Enteric Nervous System



Via their communication with each other,
with smooth muscle layers, and with
submucosal plexus, the enteric neurons of
the myenteric plexus provide the major
nerve supply to the GI tract
This plexus controls GI tract mobility by
controlling the patterns of segmentation
and peristalsis
Control comes from local reflex arcs
between enteric neurons in the same or
different plexus or organs
Enteric Nervous System


The enteric nervous system is also linked
to the CNS by afferent visceral fibers and
sympathetic and parasympathetic
branches of the ANS
Digestive activity is subject to extrinsic
control exerted by ANS which can speed
up or slow secretory activity and mobility
Digestive System
Mouth, Pharynx, and Esophagus



The mouth is the only part of the digestive
system that is involved in the ingestion of
food
Most digestive function of the mouth
reflect the activity of accessory organs
chewing the food and mixing it with salvia
to begin the process of chemical digestion
The mouth also begin the propulsive
process by which food is carried through
the pharynx and esophagus to the
stomach
The Mouth




The oral cavity is a
lined with mucosa
It bounded by the
lips anteriorly, and
the tongue inferiorly
and the cheeks
laterally
Its anterior opening
is the oral orifice
Posteriorly the oral
cavity is continuous
with the oropharynx
The Mouth



The walls of the
mouth are lined with
stratified squamous
epithelium
The epithelium is
highly ketatinized
for extra protection
against abrasion
during eating
The mucosa also
produces defensins
to fight microbes in
the mouth
The Lips and Cheeks




The labia and the
cheeks have a core
of skeletal muscle
covered by skin
The orbicularis oris
muscle forms the
bulk of the lips
The cheeks are
formed largely by
the buccinators
The area between
the teeth and gums
is the vestibule
The Lips and Cheeks



The lips extend
from the inferior
margin of the nose
to the superior
boundary of the
chin
The reddened area
is called red margin
The labial frenulum
is a median fold
that joins the
internal aspect of
each lip to the gum
The Palate

The palate which
forms the roof of
the mouth has two
distinct parts
– Hard palate
– Soft palate
The Palate



The hard palate is
underlain by bone
and is a rigid surface
against which the
tongue forces food
during chewing
There exists a
centerline ridge
called a raphe
The mucosa is
corrugated for
friction
The Palate



The soft palate is a
mobile fold formed
by skeletal muscle
Projecting down
from its free edge is
the uvula
The soft palate rises
reflexively to close
off the nasopharynx
when swallowing
The Palate


The soft palate is
anchored to the
tongue by the
palantoglossal
arches and to the
wall of oropharynx
by the
palantopharyngeal
arches
These arches form
the boundary of the
facuces
The Tongue




The tongue occupies the floor of the
mouth and fills most of the oral cavity
when closed
The tongue is composed of interlacing
masses of skeletal muscle fibers
The tongue grips the food and constantly
repositions it between the teeth
The tongue also mixes the food with
salvia and form it into a mass called a
bolus and then initiates swallowing by
moving the mass into the pharynx
The Tongue



The tongue has both
intrinsic and extrinsic
skeletal muscles
The intrinsic muscles
are confined within
the tongue and are
not attached bone
The fibers allow the
tongue to change its
shape for speech and
swallowing but not its
position
The Tongue



The extrinsic muscles
extend the tongue
from their points of
origin
The extrinsic muscles
allow the tongue to be
protruded, retracted
and moved side to
side
The tongue is divided
by a median septum
of connective tissue
The Tongue



A fold of mucosa
called the lingual
frenulum secures the
tongue to the floor of
the mouth
This frenulum limits
the posterior movement of the tongue
You cannot swallow
your tongue
The Tongue




The conical filaform
papillae give the
tongue surface a
roughness that aids
in manipulating
foods in the mouth
They align in
parallel rows on the
dorsum
They contain keratin
which stiffens them
House taste buds
The Tongue



The mushroom
shaped fungiform
palillae are scattered
over the surface
Each has a vascular
core that gives it a
reddish hue
Houses taste buds
The Tongue


The circumvallate
are located in a Vshaped row at the
back of the tongue
Appear similar to
the fungiform
papillae but with an
additional
surrounding furrow
The Salivary Glands


A number of glands both inside and
outside the oral cavity produce and
secrete saliva
Saliva functions to
– Cleanses the mouth
– Dissolves food chemical so that they can be
tasted
– Moistens food and aids in compacting it into
a bolus
– Contains enzymes that begin the chemical
breakdown of starches
The Salivary Glands

Most saliva is
produced by three
pairs of extrinsic
salivary glands
– Parotid
– Submandibular
– Sublingual

These glands lie
outside the oral
cavity and empty
their secretions into
it
The Salivary Glands

The intrinsic
salivary glands are
small and are
scattered throughout
the oral cavity
The Salivary Glands



The salivary glands are composed of two
types of secretory cells; mucus and serous
The serous cells produce a watery
secretion containing enzymes and the
ions of saliva
The mucus cells produce mucus a stringy
viscous solution
The Teeth


The teeth lie in sockets in the gum
covered margins of the mandible and
maxilla
Teeth function to tear and grind food and
begin the mechanical process of digestion
Dentition



Ordinarily we have two sets of teeth the
primary and permanent dentitions
The primary dentition consists of
deciduous teeth
The first teeth appear at six months and
additional teeth continue to erupt until
about 24 months when all 20 teeth have
emerged
Dentition


As the deeper permanent teeth enlarge
and develop, the root of the milk teeth are
resorbed from below causing them to
loosen and fall out between the ages of 6
and 12 years
Generally, all the teeth of the permanent
dentition have erupted by adolescence
The Teeth

Teeth are classified
according to their
shape and function
–
–
–
–

Incisors / cutting
Canines / tear
Premolars / grind
Molars / crush
There are 20 milk
teeth and 32
permanent teeth
Tooth Structure



Each tooth has two
major regions; the
crown and the root
The crown represents
the visible portion of the
tooth exposed above the
gum
The root is the portion
of the tooth that is
imbedded in the
jawbone
The Pharynx



From the mouth, the
food passes
posteriorly into the
oropharnyx
The mucosa consists
of stratified
squamous epithelium
The epithelium is
supplied with mucus
producing glands for
lubrication
The Pharynx




The external muscle
layer consists of two
skeletal muscle layers
The cells of the inner
layer run
longitudinally
The outer layer of
muscles pharyngeal
constrictor muscles,
encircle the wall
Sequential
contractions propel
food into esophagus
The Esophagus

The esophagus takes a fairly straight course through
the mediastinum of the thorax, pierces the diaphragm
at the esophageal hiatus to enter the abdomen
The Esophagus


The esophagus joins
the stomach at the
cardiac orifice
The cardica orifice is
surrounded by the
cardiac esophogeal
sphincter
The Pharynx



The esophageal mucosa contains a nonketatinized stratified squamous epithelium
which changes abruptly simple columnar
epithelium upon reaching the stomach
When empty the esophagus is empty with its
mucosa drawn into folds which flatten out
when food is in passage
The mucosa contains mucus secreting
esophageal glands which are compressed by
a passing bolus of food resulting in the
glands secreting a lubricant
The Pharynx


The muscularis externa changes from
skeletal muscle to a mix of skeletal and
smooth to finally all smooth as it
approaches the stomach
Instead of a serosa, the esophagus has a
fibrous adventitia composed entirely of
connective tissue, which blends with
surrounding structures along its route
Digestive Processes

The mouth and its accessory digestive
organs are involved in most digestive
processes
–
–
–
–
The mouth ingests food
Begins mechanical digestion by chewing
Initiates propulsion by swallowing
Starts the process of chemical digestion
– The pharynx and the esophagus serve as conduits to
pass food from the mouth to the stomach
Digestive Processes: Mastication




Mastication is the mechanical process of
breaking down food
The cheeks and closed lips hold the food
between the teeth
The tongue mixes the food with saliva to
soften it
The teeth cut and grind food into smaller
pieces
Digestive Processes: Deglutition


In deglutition, food is
first compacted by the
tongue into a bolus and
swallowed
Swallowing is a process
that requires the
coordination of tongue
soft palate, pharynx,
esophagus and 22
separate muscles
Digestive Processes: Deglutition


In deglutition, food is
first compacted by the
tongue into a bolus and
swallowed
Swallowing is a process
that requires the
coordination of tongue
soft palate, pharynx,
esophagus and 22
separate muscles
Digestive Processes: Deglutition


Food passage into
respiratory
passageways by rising
of the uvula and larynx
Relaxation of the upper
esophageal sphincter
allows food entry into
the esophagus
Digestive Processes: Deglutition


The constrictor muscles
of the pharynx
contract, forcing food
into the esophagus
inferiorly
The upper esophageal
sphincter contracts
after entry
Digestive Processes: Deglutition

Food is conducted
along the length of the
esophagus to the
stomach by peristaltic
waves
Digestive Processes

The gastroesophageal
sphincter enters opens
and food enters the
stomach
The Stomach



The stomach functions as a temporary
storage tank where the chemical
breakdown of protein begins and food is
converted to a creamy paste called chyme
The stomach lies in the upper left
quadrant of the abdominal cavity
Though relatively fixed at both ends, it is
free to move in between
The Stomach: Gross Anatomy


The stomach varies
from 6 to 10 inches in
length, but its
diameter and volume
depend on how much
food it contains
Empty it may
contain on 50 ml but
can expand to hold
about 4 liters of food
The Stomach: Gross Anatomy

When empty, the stomach collapses inward, throwing
its mucosa into large, longitudinal folds called rugae
The Stomach: Gross Anatomy

The major region of the stomach are the cardia region,
the fundus, body, pyloric region, and the greater and
lesser curvatures
The Stomach: Gross Anatomy

The lesser omentum runs from the liver to the lesser
curvature where it becomes continuous with the
visceral peritoneum of the stomach
The Stomach: Gross Anatomy

The greater omentum drapes inferior from the greater
curvature of the stomach to cover the coils of the small
intestine
Stomach: Microscopic Anatomy



The stomach wall exhibits the four tunics
of most of the alimentary canal but its
muscularis and mucosa are modified for
the special roles of stomach
The muscularis externa has an extra
oblique layer of muscle that enables it to
mix, churn and pummel food
The epithelium lining the stomach
mucosa is simple columnar epithelium
composed entirely of goblet cells, which
produce a protective coating of mucus
Microscopic Anatomy

The four tunics
typical of the
alimentary canal
–
–
–
–
Mucosa
Submucosa
Muscularis Externia
Serosa
Microscopic Anatomy


The otherwise smooth
lining is dotted with
millions of gastric pits
which lead to gastric
glands that produce
gastric juice
The glands of the
stomach body are
substantially larger
and produce the
majority of the
stomach secretions
Microscopic Anatomy


Mucus neck cells
produce a different
type of mucus from
that secreted by the
mucus secreting cells
of the surface
epithelium
The special function of
this unique mucus is
not yet understood
Microscopic Anatomy



Parietal cells scattered
among the chief cells
secrete hydrochloric
acid (HCl) and intrinsic
factor
The parietal cells have
a large surface area
adapted for secreting
HCl in the stomach
Intrinsic factor is
required for absorption
of B12 in the small
intestine
Microscopic Anatomy



Chief cells produce
pepsinogen, the inactive
form of the proteindigesting enzyme
pepsin
The cells occur mainly
in the basal regions of
the gastric glands
Pepsinogen is activated
by HCl
Microscopic Anatomy



Parietal cells scattered
among the chief cells
secrete hydrochloric
acid (HCL) and
intrinsic factor
The parietal cells have
a large surface area
adapted for secreting
HCL in the stomach
Intrinsic factor is
required for absorption
of B12 in the small
intestine
Microscopic Anatomy


Enteroendocrine
release a variety of
hormones directly into
the lamina propria
These products diffuse
into capillaries and
ultimately influence
several digestive system
target organs which
regulate stomach
secretion and mobility
Mucosal Barrier


Gastric juice is 100,000 more concentrated
than that found in the blood
Under such harsh conditions the stomach
must protect itself from self digestion with a
mucosal barrier
–
–
–
–
Bicarbonate rich mucus is on the stomach wall
Epithelial cells are joined by tight junctions
Glandular cells are impermeable to HCl
Surface epithelium is replace every 3 to 6 days
Digestive Processes: Stomach

The stomach is involved in the whole
range of digestive activities
– It serves as a holding area for ingested food
– Breaks down food further chemically and
mechanically
– It delivers chyme to the small intestine at a
controlled rate
Digestive Processes: Stomach



Protein digestion is initiated in the
stomach and is essentially the only type of
enyzmatic digestion that occurs there
The most important protein digesting
enzyme produced by the gastric mucosa
is pepsin
In children, the stomach glands also
secrete rennin, an enzyme that acts on
milk protein converting it to a curdy
substance appearing like sour milk
Digestive Processes: Stomach



Despite its many functions in the
digestive system the only one that is
essential for life is secretion of intrinsic
factor
Intrinsic factor is required for intestinal
absorption of vitamin B12, needed to
produce mature erythrocytes
Without B12 the individual will develop
prenicious anemia unless administered by
injection
Regulation of Gastric Secretion



Gastric secretion is controlled by both
neural and hormonal mechanisms
Under normal conditions the gastric
mucosa creates as much as 3 liters of
gastric juice every day
Gastric juice is an acid solution that has
the potential to dissolve nails
Regulation of Gastric Secretion



Nervous control is regulated by long
(vagus nerve mediated) and short (local
enteric) nerve reflexes
When the vagus nerves actively stimulate
the stomach, secretory activity of
virtually all of its glands increase
The sympathetic nerves depress secretory
activity
Regulation of Gastric Secretion



Hormonal control of gastric secretion is
largely from the presence of gastrin
Gastrin stimulates the secretion of both
enzymes and HCL in the stomach
Hormones produced by the small
intestine are largely gastrin antagonists
Regulation of Gastric Secretion



Stimuli acting at three distinct sites, the
head, stomach, and small intestine,
provoke or inhibit gastric secretory
activity
Accordingly the three phases are called
cephalic, gastric, and intestinal phases
However, the effector site is the stomach
in all cases and once initiated, one or all
threephases may be occurring at the
same time
Phase 1: Cephalic reflex



The cephalic reflex phase of gastric
secretion occurs before food enters the
stomach
It is triggered by the aroma, taste, sight,
or though of food
During this phase the brain gets the
stomach ready for food
Phase 1: Cephalic reflex


Inputs from activated olfactory receptors
and taste buds are relayed to the
hypothalamus which in turn stimulates
the vagal nuclei of the medulla oblongata,
causing motor impulses to be transmitted
via the vagus nerves to the
parasympathetic nerve ganglia
Eneteric ganglionic neurons in turn
stimulate the stomach glands
Phase 1: Cephalic reflex


The enhanced secretory activity that
results when we see or think of food is a
conditioned reflex and occurs only when
we like or want the food
If we are depressed or have no appetite,
this part of the cephalic reflex is
suppressed
Phase 2: Gastric reflex



Once food reaches the stomach, local
neural and hormonal mechanisms initiate
the gastric phase
This phase provides about two-thirds of
the gastric juice released
The most important stimuli are distension,
peptids, and low acidity
Phase 2: Gastric reflex



Stomach distension activates stretch
receptors and initiates both local
(myentertic) reflexes and the long
vagovagal reflexes
In vagovagal reflex, impulses travel to the
medulla and then back to the stomach via
vagal fibers
Both types of reflexes lead to acetylcholine
(ACH) release, which in turn stimulates the
output of more gastric juice by cells
Phase 2: Gastric reflex


Though neural influences initiated by
stomach distension are important, the
hormone gastrin probably plays a greater
role in stimulating stomach gland
secretion during the gastric phase
Chemical stimuli provided by partially
digested proteins (peptids)caffine (colas,
coffee) and rising pH directly active
gastrin secreting entoendocrine cells
called G cells
Phase 2: Gastric reflex


Although gastrin also stimulates the
release of enzymes, its main target is the
HCL secreting parietal cells, which it
prods to spew out even more HCL
Highly acidic (pH below 2) gastric
contents inhibit gastrin secretion
Phase 2: Gastric reflex


When protein foods are in the stomach,
the pH of the gastric contents generally
rises because proteins act as buffers to tie
up H+
The rise in pH stimulates gastrin and
subsequently HCL release, which in turn
provides the acidic conditions needed for
protein digestion
Phase 2: Gastric reflex



The more protein in the meal, the greater
the amount of gastrin and HCL released
As proteins are digested, the gastric
contents gradually become more acidic,
which again inhibits the gastrin secreting
cells
This negative feedback mechanism helps
maintain optimal pH and working
conditions for the gastric enzymes
Phase 2: Gastric reflex


G cells are also activated by the neural
reflexes already described
Emotional upsets, fear, anxiety, or
anything that triggers the fight-or-flight
response inhibits gastric secretion
because (during such times) the
sympathetic division overrides
parasympathetic controls of digestion
Phase 2: Gastric reflex


The control of the HCL secreting parietal
cells is unique and multifaceted
Basically, HCL secretion is stimulated by
three chemicals, all of which work
through second-messenger systems Ach
released by parasympathetic nerve fibers
and gastrin secreted by G cells
Phase 2: Gastric reflex

Ach released by
parasympathetic
nerve fibers and
gastrin secreted
by G cells bring
about their
effects by
increasing
intercellular
Ca++ levels
Phase 2: Gastric reflex

Histamine
released by
mucosal cells
called
histaminocytes
acts through
cyclic AMP
(cAMP)
Phase 2: Gastric reflex


When only one of the three chemicals
binds to the parietal cells, HCL secretions
are minimul
When all three of the chemicals bind to
the parietal cells volumes of HCL pour
forth as if pushed out under pressure
Phase 2: Gastric reflex


The process of HCL formation within the
parietal cells is complicated and poorly
understood
The consensus is that H+ is actively
pumped into the stomach lumen against a
tremendous concentration gradient
Phase 2: Gastric reflex


As hydrogen ions are secreted, chloride
ions (Cl-) are also pumped into the lumen
to maintain an electrical balance in the
stomach
The Cl- is obtained from blood plasma,
while the H+ appears to come from a
breakdown of carbonic acid formed by
the combination of carbon dioxide and
water and within the parietal cells
Phase 2: Gastric reflex


CO2 + H2O 
H2CO3  H+ +
HCO3As H+ is
pumped from
the cell and
HCO3- is ejected
through the
basal cell
membrane into
the capillary
blood
Phase 2: Gastric reflex


The result of ejection of the bicarbonate
ion into the capillary blood is that blood
draining from the stomach is more
alkaline than the blood serving it
The phenomenon is called the alkaline
tide
Phase 3: Intestinal

The intestinal phase of gastric secretion
has two components
– One excitatory
– One inhibitory
Phase 3: Intestinal


The excitatory aspect is set into motion as
partially digested food begins to fill the
initial part (duodenum) of the small
intestine
This stimulates intestinal mucosal cells to
release a hormone that encourages the
gastric glands to continue their secretory
activity
Phase 3: Intestinal



The effects of this hormone imitate those
of gastrin, so it has been named intestinal
(enteric) gastrin
However, intestinal mechanisms stimulate
gastrin secretion only briefly
As the intestine distends with chyme
containing large amounts of H+, fats,
partially digested proteins, and irritating
substances, the inhibitatory component is
triggered in the form of the enterogastric
reflex
Phase 3: Intestinal

The enterogastric reflex is actually a trio
of reflexes that
– Inhibit the vagal nuclei in the medulla
– Inhibit local reflexes
– Activate sympathetic fibers that cause the
pyloric sphincter to tighten and prevent
further food entry into the small intestine

As a result, gastric secretory activity
declines
Phase 3: Intestinal

These inhibitions on gastric activity
product the small intestine to harm due
to excessive acidity and match the small
intestine’s processing abilities to the
amount of chyme entering it at a given
time
Phase 3: Intestinal

In addition, the factors just named trigger
the release of several intestinal hormones
collectively called enterogastrones which
include
–
–
–
–

Secretin
Cholecystokinin (CCK)
Vasoactive intestinal peptide (VIP)
Gastric inhibitory peptide (GIP)
All of these hormones inhibit gastric
secretion when the stomach is very active
Gastric Motility and Emptying


Stomach contractions, accomplished by the
tri-layered muscularis, not only cause its
emptying but also compress, knead, twist,
and continually mix the food with gastric
juice to produce chyme
Because the mixing movements are
accomplished by a unique type of peristalis
(bidirectional) the process of mechanical
digestion and propulsion are inseparable in
the stomach
Gastric Motility: Stomach Filling


Although the stomach stretches to
accommodate incoming food, internal
stomach pressure remains constant until
about 1 liter of food has been ingested
The relatively unchanging pressure in the
filling stomach is due to 1) reflex mediated
relaxation of the stomach muscle and 2)
plasticity of visceral smooth muscle
Gastric Motility: Stomach Filling



Reflexive relaxation of stomach muscle in
the fundus and body occurs both in
anticipation of and in response to food
entry into the stomach
As food travels through the esophagus,
the stomach muscles relax
This receptive relaxation is coordinated
by the swallowing center in the brain
stem and mediated by the vagus nerves
Gastric Motility:Stomach Filling


The stomach also actively dilates in
response to gastric filling, which activates
stretch receptors in the wall
The phenomenon called adaptive
relaxation appears to depend on local
reflexes involving nitric oxide (NO)
releasing hormones
Gastric Motility: Stomach Filling

Plasticity is the intrinsic ability of visceral
smooth muscle to exhibit the stressrelaxation response, that is, to be
stretched without greatly increasing its
tension and contractile strength
Gastric Motility and Emptying

After a meal peristalsis begins near the
cardiac sphincter, where it produces only
gentle rippling movements of the stomach
wall
Gastric Motility and Emptying

As contractions approach the pylorus,
where the stomach musculature is thicker,
the contractions become more powerful
Gastric Motility and Emptying

Consequently, the contents of the fundus
remain relatively undisturbed, while the
foodstuffs close to the pylorus receive a
very active mixing
Gastric Motility and Emptying

The pyloric region of the stomach, which
holds about 30 ml of chyme, acts as a
“dynamic filter” that allows only liquids
and small particles of food to pass
Gastric Motility and Emptying

Normally, each peristaltic wave reaching
the pyloric muscle squirts 3 ml or less of
chyme into the small intestine
Gastric Motility and Emptying

While the stomach delivers small amounts
of chyme into the doudenum it also
simultaneously forces most of the
contained material backward into the
stomach for further mixing
Gastric Motility and Emptying


Although the intensity of the stomach’s
peristaltic waves can be modified, the rate
is always constant at around 3 per minute
The contractile rhythm is set by the
spontaneous activity of pacemaker cells
located at the margins of the longitudinal
smooth muscle layer
Gastric Motility and Emptying


The pacemaker cells, are believed to be
muscle-like noncontractile cells called
interstitial cells of Cajal which depolarize
the repolarize spontaneously three times
each minute
This depolarization and repolarization
establish the so-called cyclic slow waves
of the stomach or its basic electrical
rhythm (BER)
Gastric Motility and Emptying



Since the pacemakers are electrically
coupled to the rest of the smooth muscle
sheet by gap junctions, their “beat” is
transmitted efficiently and quickly to the
entire muscularis
The pacemakers set the maximum rate of
contraction, but they do not initiate the
contractions or regulate their force
They generate subthreshold depolarization
waves, which are then enhance by neural
and hormonal factors
Gastric Motility and Emptying


Factors that increase the strength of
stomach contractions are the same
factors that enhance gastric secetory
activity
Distension of the stomach wall by food
activates stretch receptors and gastric
secreting cells, which both ultimately
gastric smooth muscle and so increase
gastric motility
Gastric Motility and Emptying



Thus, the more food there is in the
stomach, the more vigorous the stomach
mixing and emptying movements will be
evident
The stomach usually empties completely
within four hours after a meal
However, the larger the meal (greater
distension) and the more liquid the meal,
the faster the stomach empties
Gastric Motility and Emptying


Fluids pass quickly through the stomach
Solids linger, remaining until they are
well mixed with gastric juice and
converted to a liquid state
Gastric Motility and Emptying




The rate of emptying depends as much on the
contents of the duodenum as on whats
happening in the stomach
The stomach and duodenum act in tandem
As chyme enters the duodenum, receptors in
its wall respond to chemical signals and to
stretch, initiating the enterogastric reflex and
hormonal mechanisms described earlier
These factors inhibit gastric secretory activity
and prevent further duodenal filling by
reducing the force of pyloric contractions
Gastric Motility and Emptying


A carbohydrate-rich meal moves through
the duodenum rapidly, but fats form an
oily layer at the top of the chyme and are
digested more slowly by enzymes acting
in the intestines
Thus, when chyme entering the
duodenum is fatty, food may remain in
the stomach six hours or more
The Small Intestine and
Associated Structures



In the small intestine, usable food is
finally prepared for its journey into the
cells of the body
However, this vital function cannot be
accomplished without the aid of
secretions from the liver (bile) and
pancreas (digestive enzymes)
Thus the accessory organ are discussed in
this section
Small Intestine

The small
intestine is a
convoluted tube
extending from
the pyloric
sphincter in the
epigastric
region to the
iliocecal valve
where it joins
the large
intestine
Small Intestine



It is the longest part of the alimentary
tube, but its diameter is only about 2.5 cm
In the cadaver, the small intestine is 6 - 7
meters long because of loss of muscle tone,
while it is only 2 - 4 meters long in the
living individual
The small intestine has three subdivisions
– Duodenum
– Jejunum
– Ileum
Gross Anatomy

The relatively immovable duodenum which
curves about the head of the pancreas
Small Intestine


The duodenum is about 10 inches long
Although it is the shortest subdivision,
the duodenum has the most features of
interest
–
–
–
–
The bile duct
Main pancreatic duct
Hepatopancreatic ampulla
Major duodenal papilla
Gross Anatomy


The bile duct, delivering bile from the liver
The main pancreatic duct, carries pancreatic juice from
the pancreas
Gross Anatomy


The hepatopancreatic ampulla is where these two ducts
unite in the wall of the duodenum
The papilla is where this sphincter enters the duodenum
Small Intestine


The jejunum is
about 8 ft long
and extends
from the
duodenum to
the ileum
This central
section twists
back and forth
within the
abdominal
cavity
Small Intestine


The ileum is
approximately
12 ft. in length
It joins the
large intestine
at the ileocecal
valve
Small Intestine

The jejunum
and ileum
hang in coils
in the central
and lower
part of the
abdominal
cavity
Small Intestine

The jejunum
and ileum are
suspended
from the
posterior
abdominal
wall by the
fan shaped
mesentery
Small Intestine


Nerve fibers serving the small intestine
include the parasympathetics from the
vagus nerves and sympathetics from the
long splanchic nerves
These are relayed through the superior
mesenteric and celiac plexus
Small Intestine



The arterial supply is primarily from the
superior and mesenteric artery
The veins run parallel to the arteries and
typically drain into the superior
mesenteric vein
From the mesenteric vein, the nutrient
rich venous blood from the small
intestine drains into the hepatic portal
vein which carries it to the liver
Microscopic Anatomy



The small intestine is highly adapted for
nutrient absorption
Its length provides a huge surface area
for absorption
There are three structural modifications
which increase the surface area for
absorption
– Plicae circulares
– Villi
– Microvilli
Microscopic Anatomy
Digestive System Organs

In this view you can see the plicae circulares
and the villi of the small intestine
Microscopic Anatomy



Structural modifications increase the
intestinal surface area tremendously
It is estimated that the surface area of the
small intestine is equal to 200 square
meters or roughly equivalent to the floor
space of a two story house
Most absorption occurs in the proximal
part of the small intestine, with these
structural modifications decreasing
toward the distal end
Microscopic Anatomy


The circular
folds or plicae
circularis are
deep permanent
folds of the
mucosa and
submucosa
These folds are
nearly 1 cm tall
Microscopic Anatomy

The folds force
chyme to spiral
through the
lumen, slowing
its movement
and allowing
time for full
nutrient
absorption
Microscopic Anatomy


Villi are finger
like projections
of the mucosa
Over 1 mm tall
they give a
velvety texture
to the mucosa
Microscopic Anatomy

The epithelial
cells of the villi
are chiefly
absorptive
columnar cells
called
enterocytes
Microscopic Anatomy



In each villus is a capillary
bed and a wide lymphatic
capillary called a lacteal
Digested food is absorbed
through the epithelial cells
into both the capillary blood
and the lacteal
Villi become gradually
narrower and shorter along
the length of the sm. intestine
Enterocyte
Microscopic Anatomy


Microvilli are tiny
projections of the
plasma membrane of
the absorptive cells of
the mucosa
It gives the mucosal
surface a fuzzy
appearance
sometimes called a
brush border
Microscopic Anatomy


Beside increasing the absorptive surface,
the plasma membrane of the microvilli
bear enzymes referred to as the brush
border enzymes
These enzymes complete the final stages
of digestion of carbohydrates and
proteins in the small intestine
Histology of the Wall

The four tunics of
the digestive tract
are modified in the
small intestine by
variations in
mucosa and submucosa
Histology of the Wall



The epithelium of the mucosa is largely
simple columnar epithelium serving as
absorptive cells
The cells are bound by tight junctions
and richly endowed with microvilli
Also present are many mucus-secreting
goblet cells
Histology of the Wall



Scattered among the epithelial cells of the
wall are T cells called intraepithelial
lymphocytes
These T cells provide an immunological
component
Finally, there scattered enteroendocrine
cells which are the source of secretin and
cholecystokinin
Histology of the Wall

Between villi the
mucosa is
studded with pits
that lead into
tubular intestinal
glands called
intestinal crypts
or crypts of
Lieberkuhn
Histology of the Wall


The epithelial cells that line these crypts
secrete intestinal juice
Intestinal juice is a watery mixture
containing mucus that serves as a carrier
fluid for absorption of nutrients from chyme
Histology of the Wall



Located deep on the crypts are
specialized secretory cells called Paneth
cells
Paneth cells fortify the small intestine by
releasing lysozyme an antibacterial
enzyme
The number of crypts decreases along the
length of the wall of the small intestine,
but the number of goblet cells becomes
more abundant
Histology of the Wall



The various epithelial cells arise from
rapidly dividing stem cells at the base of
the crypts
The daughter cells gradually migrate up
the villi where they are shed from the
villis tips
In this way the villus of the epithelium is
renewed every three to six days
Histology of the Wall



The rapid replacement of the intestinal
(and gastric) epithelial cells has clinical
as well as physiological implications
Treatments for cancer, such as radiation
therapy and chemotherapy preferentially
target the cells in the body that divide
most quickly
This kills cancer cells, but it also nearly
obliterates the GI epithelium causing
nausea, vomiting, and diarrhea after each
treatment
Histology of the Wall


The submucosa is typical areolar
connective tissue, and it contains both
individual and aggregated lymphoid
follicles (Peyer’s patches)
Peyer’s patches increase in abundance
toward the end of the small intestine,
reflecting the fact that the large intestine
contains huge numbers of bacteria that
must be prevented from entering the
bloodstream
Histology of the Wall

A set of
elaborated
mucus-secreting
duodenal glands
(Brunner’s) is
found in the
submucosa of the
duodenum only
Histology of the Wall


These glands produce an alkaline
(bicarbonate-rich) mucus that helps
neutralize the acidic chyme moving in
from the stomach
When this protective mucus barrier is
inadequate, the intestinal wall is eroded
and duodenal ulcers results
Histology of the Wall


The muscularis is typical and bilayered
Except for the bulk of the duodenum,
which is retroperitoneal and has an
adventitia, the external intestinal surface
is covered by visceral peritoneum
(serosa)
Intestinal Juice


The intestinal glands normally secrete
between 1 and 2 liters of intestional juice
daily
The major stimulus for its production is
distension or irritation of the intestinal
mucosa by hypertonic or acidic chyme
Intestinal Juice



Normally, the pH range of intestinal juice
is slightly alkaline (7.4-7.8), and it is
isotonic with blood plasma
Intestinal juice is largely water but it also
contains some mucus, which is secreted
both by the duodenal glands and by
goblet cells of the mucosa
Intestinal juice is relatively enzyme poor
because intestinal enzymes are largely
limited to the bound enzymes of the
brush border
The Liver and Gallbladder





The liver and gallbladder are accessory
organs associated with the small intestine
The liver has many metabolic and
regulatory roles
Its digestive function is to produce bile
for export to the duodenum
Bile is a fat emulsifier which breaks up
fat into tiny particles so that they are
more accessible to digestive enzymes
The gallbladder is a storage site for bile
The Liver

The ruddy, blood rich liver is the largest
gland in the body weighing about 1.4 kg in the
average adult
The Liver

Shaped like a wedge, it
occupies most of the right
hypochondriac and
epigastric regions
extending farther to the
right of the body midline
than the left
The Liver


Located under the diaphragm, the liver
lies almost entirely within the rib cage
The location of the liver within the rib
cage offers this organ some degree of
protection
The Liver

The liver has four primary lobes; right, left,
caudate and quadrate
The Liver

A mesentery, the falciform ligament,
separates the right and left lobes anteriorly
and suspends the liver from the diaphragm
The Liver

Running along the free inferior edge of the
falciform ligament is the ligamentum teres a
remnant of the fetal umbilical vein
The Liver

Except for the superiormost liver area, which
is fused to the diaphragm, the entire liver is
enclosed by a serosa (visceral peritoneum)
The Liver

A dorsal
mesentery, the
lesser omentum,
anchors the liver
to the lesser
curvature of the
stomach
The Liver

The hepatic artery and hepatic portal vein,
enter the liver at the porta hepatis
The Liver

The common bile duct, which runs inferiorly
from the liver, travels through the lesser
omentum
The Liver

The gallbladder rests in a recess of the
inferior surface of the right lobe of the liver
The Liver

Bile leaves the liver through several bile ducts
that ultimately fuse to form the large common
hepatic duct which travels to the duodenum
The Liver

The common hepatic duct and the cystic
duct fuse to form the bile duct
Microscopic Anatomy of Liver


The liver is
composed of
seed sized
structural &
functional
units called
liver lobules
Each lobule
is roughly
hexagonal
Microscopic Anatomy of Liver

Hepatocytes
or live cells are
organized to
radiate out
from a central
vein running
the length of
the
longitudinal
axis of the
lobule
Microscopic Anatomy of Liver


To make a rough “model” of a liver
lobule, open a paperback book until the
two covers meet
The pages represent the plates of
hepatocytes and the hollow cylinder
formed by the rolled spine represents the
central vein
Microscopic Anatomy of Liver


The liver’s main function is to filter and
process the nutrient rich blood delivered
to it
At each of the six corners of a lobule is a
portal triad so named because three basic
structures are always present there:
– A branch of the hepatic artery
– A branch of the hepatic portal vein
– A bile duct
Microscopic Anatomy of Liver

The hepatic artery supplies oxygen rich
arterial blood to the liver
Microscopic Anatomy of Liver

The hepatic vein carries blood laden with
nutrients from the digestive viscera
Microscopic Anatomy of Liver

A bile duct to carry secreted bile toward
the common bile duct and ultimately to
the duodenum
Microscopic Anatomy of Liver

Between the
hepatocyte plates
are enlarged,
very leaky
capillaries, the
liver sinusoids
Microscopic Anatomy of Liver

Blood from both
the hepatic portal
vein and the
hepatic artery
percolates from
the triad regions
through these
sinusoids and
empties into the
central vein
Microscopic Anatomy of Liver

From the central
vein blood
eventually enters
the hepatic veins,
which drain the
liver, and empty
into the inferior
vena cava
Microscopic Anatomy of Liver

Inside the
sinusoids are star
shaped hepatic
macrophages,
also called
Kupffer cells,
which remove
debris such as
bacteria and
worn-out blood
cells
Microscopic Anatomy of Liver


The hepatocytes (liver cells) are virtual
organelle storehouses with large amounts
of both rough and smooth endoplasmic
reticulum, Golgi apparatuses, peroxisomes,
and mitochondria
Thus equipped, the hepatocytes produce
not only bile but also
– Process blood borne nutrients
– Store Fat-soluble vitamins
– Detoxify the blood
Microscopic Anatomy of Liver




In processing nutrients the hepatocytes
store glycogen and make plasma proteins
Fat soluble vitamins are stored until such
time as they are needed for metabolism
Detoxification occurs are the hepatocytes
rid the blood of ammonia by converting it
to urea
The net result is that the blood leaving the
liver contains fewer nutrients and waste
materials than the blood that entered
Microscopic Anatomy of Liver

Secreted bile
flows through
tiny canals, called
bile canaliculi
that run between
adjacent hepato
cytes toward the
bile branch ducts
in the portal triad
Microscopic Anatomy of Liver


Note that the bile
and the blood
flow in opposite
directions in the
liver lobule
Bile entering the
bile ducts
eventually leaves
the liver via the
common hepatic
duct
Microscopic Anatomy of Liver

Bile is a yellow-green, alkaline solution
containing
–
–
–
–
–
–

Bile salts
Bile pigments
Cholesterol
Neutral fats
Phospholipids (lecithin and others)
Electrolytes
Only bile salts and phospholipids aid the
digestive process
Microscopic Anatomy of Liver



Bile salts, primarily cholic acid and
chenodeoxycholic acids are cholesterol
derivatives
Their role is to emulsify fats which
distributes them throughout the watery
intestinal contents
As a result, large fat globules entering the
small intestine are physically separated
into millions of small fatty droplets
Microscopic Anatomy of Liver


Millions of tiny fat droplets vastly
increase the surface area for the fat
digesting enzymes to work on
Bile salts also facilitate fat and cholesterol
absorption and help solubilize
cholesterol, both that contained in bile
and that entering the small intestine for
food
Microscopic Anatomy of Liver



Although many substances secreted in bile
leave the body in feces, bile salts are not
among them
Bile salts are conserved by a means of a
recycling mechanism called enterohepatic
circulation
In this process bile salts are
– Reabsorbed into the small intestine
– Returned to the liver via the hepatic portal vein
– Resecreted in newly formed bile
Microscopic Anatomy of Liver


The chief bile pigment is bilirubin, a
waste product of hemoglobin (heme)
during the breakdown of worn-out
erythrocytes
The globin and iron parts of hemoglobin
are saved and recycled, but bilirubin is
absorbed from the blood by the liver cells
and actively excreted into the bile
Microscopic Anatomy of Liver



Most of the bilirubin in bile is metabolized
in the small intestine by resident bacteria
A breakdown by-product is urobilirubin
which give feces its brown color
In the absence of bile, feces are grey-white
in color and have fatty streaks because
essentially no fats are digested or
absorbed
Microscopic Anatomy of Liver


The liver produces 500 to 1000 ml of bile
daily, and bile production is stepped up
when the GI tract contains fatty chyme
Bile salts themselves are a major stimulus
for enhance bile secretion
Microscopic Anatomy of Liver

The single most
important
stimulus of bile
secretion is an
increased level of
bile salts in the
enterohepatic
circulation
The Gallbladder


The gallbladder is
a thin-walled,
green muscular
sac, rouhgly the
size of a kiwi fruit
It snuggles in a
shallow fossa on
the ventral
surface of the
liver
The Gallbladder

The gallbladder stores bile that is not
immediately needed for digestions
The Gallbladder




Bile that is not needed is concentrated by
absorbing some of its water and ions
When empty, its mucosa adopts the ridge
like folds or rugae of the stomach
Its muscular walls can contract to expell
its contents into the cystic duct which
then flows into the bile duct
Like most of the liver it is covered by
visceral peritoneum
The Gallbladder

When digestion is not occurring, the
hepatopancreatic sphincter is tightly closed
The Gallbladder

Bile then backs up the cystic duct into the
gallbladder where it is stored until needed
The Gallbladder



Although the liver makes bile continuously
bile does not usually enter the small
intestine until the gallbladder contract
The major stimulus for gallbladder
contraction is the intestinal hormone
cholecystokinin (CCK)
CCK is released to the blood when acidic,
fatty chyme enters the duodenum
The Gallbladder

Besides causing the gallbladder to
contract, CCk has two other important
effects
– It stimulates secreation of pancreatic juice
– It relaxes the hepatppancreatic sphincter so
that bile and pancreatic juice can enter the
duodenum

Parasympathetic impulses delivered by
the vagus nerves have a minor impact on
stimulating gallbladder contraction
The Pancreas

The pancreas is a soft, tadpole-shaped
gland that extends across the abdomen
The Pancreas

Most the pancreas is retroperitoneal and lies
deep to the greater curvature of stomach
The Pancreas



An accessory organ, the pancreas is
important to the digestive process
because it produces a broad spectrum of
enzymes
These enzymes break down all categories
of foodstuffs, which the pancreas then
delivers to the duodenum
This exocrine product is called pancreatic
juice
The Pancreas

Pancreatic juice drains from the pancreas via
the centrally located main pancreatic duct
The Pancreas

The pancreatic duct generally fuses with the
bile duct just as it enters the duodenum
The Pancreas

A smaller accessory pancreatic duct empties
directly into the main duct
The Pancreas

Within the
pancreas are
the acini,
clusters of
secretory cells
surrounding
ducts
The Pancreas

The acini cells
are full of
rough
endoplasmic
reticulum and
exhibit deeply
staining
zymogen
granules
containing
digestive
enzymes
The Pancreas


The pancreas
also has an
endocrine
function
Scattered
amidst the
acini are the
more lightly
staining
pancreatic
islets
The Pancreas

These Islets of
Langerhans
release insulin
and glucagon,
hormones that
regulate
carbohydrate
metabolism
Pancreatic Juice




Approximately 1200 to 1500 ml of clear
pancreatic juice is produced daily
It consists mainly of water and contains
enzymes and electrolytes
The acinar cells produce the enzyme rich
pancreatic juice
The epithelial cells lining the smallest
pancreatic ducts release the bicarbonate
ions that make it alkaline (pH 8)
The Pancreas



The high pH enables pancreatic fluid to
neutralize the acid chyme entering the
duodenum
It also provides the optimal environment
for activity of intestinal and pancreatic
enzymes
Like pepsin of the stomach, pancreatic
protein digesting enzymes are produced
and released in active forms, which are
then activated in the duodenum
The Pancreas


Within the duodenum trypsinogen is
activated to trypsin by enterokinase an
intestinal brush border enzyme
Trypsin in turn activates two other
pancreatic enzymes
– Procarboxypeptidase > carboxypeptidase
– Chymotrypsinogen > chymotrypsin

Other pancreatic enzymes (amylase, lipase,
and nucleases) are secreted in active form
but require ions in the bile for activity
Regulation of Pancreatic Secretion

Secretion of pancreatic juice is regulated
both by local hormones and by the
parasympathetic nervous system
Regulation of Pancreatic Secretion


Secretin is
released in
response to the
presence of HCL
in the intestine
Cholecystokinin
is released in
response to the
entry of proteins
and fats
Regulation of Pancreas Secretion


Both hormones act on the pancreas, but
secretin targets the duct cells, prompting
their release of watery bicarbonate-rich
pancreatic juice, Whereas CCK stimulates
the acini to release enzyme-rich pancreatic
juice
Vagal stimulation causes release of
pancreatic juice primarily during the
cephalic and gastric phases of gastric
secretion
Regulation of Pancreatic Secretion


Normally, the amount of HCL produced
in the stomach is exactly balanced by the
amount of bicarbonate (HCO3) actively
secreted by the pancreas
HCO3 is secreted into the pancreatic
juice, and H+ enters the blood
Regulation of Pancreatic Secretion

Consequently, the pH of venous blood
returning to the heart remains relatively
unchanged because alkaline blood
draining from the stomach is neutralized
by the acidic blood draining the pancreas
Digestion: Small Intestine



Although food reaching the small intestine
is unrecognizable, it is far from being
digested chemically
Carbohydrates and proteins are partially
degraded, but virtually no fat digestion has
occurred to this point
The process of food digestion is accelerated
during the chyme’s journey of 3 to 6 hours
through the small intestine, it is here that
virtually all nutrient absorption occurs
Optimal Intestinal Activity


Although the primary functions of the small
intestine are digestion and absorption,
intestinal juice provides little of what is
needed to perform these functions
Most substances required for chemical
digestion - bile, digestive enzymes (except
for brush border enzymes) and bicarbonate
ions (to provide the proper pH for
enzymatic catalysis) are imported from the
liver and pancreas
Optimal Intestinal Activity

Anything that impairs liver or pancreatic
function or delivery of their juices to the
small intestine severely hinders the
individual’s ability to digest food and
absorb nutrients
Optimal Intestinal Activity



Optimal digestive activity in the small
intestine also depends on a slow, measured
delivery of chyme from the stomach
The small intestine can process only small
amounts of chyme at one time
Chyme enter the small intestine is usually
hypertonic
Optimal Intestinal Activity



If large amounts of chyme were rushed
into the small intestine, the osmotic water
loss from the blood into the intestinal
lumen would result in dangerously low
blood volume
Additionally, the low pH of entering
chyme must be adjusted upward and the
chyme must be well mixed with bile and
pancreatic juice for digestion to continue
These adjustments take time
Optimal Intestinal Activity

Food movement into the small intestine is
carefully controlled by the pumping
action of the stomach pylorus which
prevents the duodenum from being
overwhelmed
Motility of the Small Intestine


Intestinal smooth muscle mixes chyme
thoroughly with bile and pancreatic and
intestinal juices and moves food residues
through the ileocecal valve and into the
large intestine
In contrast to the peristaltic waves of the
stomach, which both mix and propel
food, segmentation is the most common
motion of the small intestine
Motility of the Small Intestine

In segmentation,
chyme is moved
backward and
forward a few
centimeters at a
time by alternating
contraction and
relaxation of rings
of smooth muscles
Motility of the Small Intestine

These segmenting
movements of the
intestine are
initiated by
intrinsic
pacemaker cells
(interstitial cells of
Cajal) in the
longitudinal
smooth muscle
layer
Motility of the Small Intestine


Unlike the somach pacemakers, which have
only one rhythm, the pacemakers in the
duodenum depolarizes more frequently
(12-14 contractions per minute) than those
of the ileum (8-9 contractions per minute)
As a result, segmentation moves intestinal
contents slowly and steadily toward the
ileocecal valve at a rate that allows ample
time to complete digestion and absorption
Motility of the Small Intestine

The intensity of the segmentation is
altered by hormones and long and short
reflexes
– Parasympathetic enhances segmentation
– Sympathetic decreases segmentation

The more intense the contractions, the
greater the mixing effect, however the
basic contractile rhythms of the various
intestinal regions remain unchanged
Motility of the Small Intestine


True peristalsis
occurs only
after most
nutrients have
been absorbed
Segmentation
movements
wane, and
peristaltic
waves begin
Motility of the Small Intestine



Peristaltic waves initiated in the duodenum
begin to sweep slowly along the intestine,
moving 10 - 70 cm before dying out
Each successive wave is initiated a bit more
distally, and this pattern of peristaltic
activity is called the migrating mobility
complex
A complete migration from the duodenum
to the ileum takes about two hours and
then repeats itself
Motility of the Small Intestine



Peristalsis serves to sweep out the last
remnants of the meal plus bacteria,
sloughed-off mucosal cells, and other
debris into the large intestine
This “housekeeping” function is critical
for preventing the overgrowth of bacteria
that migrate from the large intestine to
the small intestine
As food enters the stomach with the next
meal segmentation replaces peristalsis
Motility of the Small Intestine


The local enteric neurons of the GI tract
wall coordinate intestinal mobility patterns
The physiological diversity of the enteric
neurons allows a variety of effects to occur
depending on which neurons are activated
or inhibited
Motility of the Small Intestine

A given ACh-releasing (cholinergic)
sensory neuron in the small intestine, once
activated, may simultaneously send
messages to several different interneurons
in the myenteric plexus that regulate
peristalsis:
– Impulses sent proximally by cholingeric
neurons cause contraction and shortening of
the circular muscular layer
Motility of the Small Intestine

…interneurons in the myenteric plexus
that regulate peristalsis:
– Impulses sent distally to certain interneurons
cause shortening of the longitudinal muscle
layers and distension of the intestine, in
response to Ach-releasing neurons
– Other impulses sent distally by activated VIP
or NO-releasing enteric neurons cause
relaxation of the circular muscle
Motility of the Small Intestine

As a result, as the proximal area constricts
and forces chyme along the tract, the
lumen of the distal part of the intestine
enlarges to receive it
Motility of the Small Intestine



Most of the time, the ileocecal sphincter is
constricted and closed
Two mechanisms, one neural and one
hormonal , cause it to relax when ileal
mobility increases and allow food
residues to entry the cecum
Enhance activity of the stomach initiates
the gastroileal reflex, a long reflex than
enhances the force of segmentation in the
ileum
Motility of the Small Intestine


In addition, gastrin released by the
stomach increases the motility of the
ileum and relaxes the ileocecal sphincter
Once the chyme has passes through, it
exerts backward pressure that closes the
valve’s flaps, preventing regurgitation
into the ileum
Large Intestine

The large intestine frames the small
intestine on three sides and extends from
the ileocecal valve to the anus
Large Intestine


Its diameter is greater than that of the
small intestine, but is less than half as
long 1.5 meters
Its major function is to absorb water
from indigestible food residues (delivered
to it in fluid state) and eliminate them
from the body as semisolid feces
Large Intestine

Over most of its length, the large intestine
exhibits three features not seen elsewhere;
teniae coli, haustra, epiploic appendages
Large Intestine

Teniae coli are three bands of smooth
muscle which are the remnants of the
smooth muscle layer
Large Intestine

The muscle tone of the teniae coli cause
the wall of the large intestine to form
pocketlike sacs called haustra
Large Intestine

Epiplocic appendages are small fat-filled
pouches of visceral peritoneum that hang
from its surface. Significance is not known
Large Intestine

The large intestine has the following
subdivisions; cecum, appendix, colon,
rectum, and anal canal
Large Intestine

The saclike cecum, or blind pouch, lies
below the ileocecal valve is the first part of
the large intestine
Large Intestine

Attached to the cecum is the blind,
wormlike, vermiform appendix
Large Intestine


The appendix contains masses of lymphoid
tissue, and as part of the MALT it plays an
important role in body immunity
It has a significant structural problem in
that its twisted tissue provides an ideal
location for enteric bacteria to accumulate
and multiply
Large Intestine

The colon has several distinct regions;
ascending, transverse, and descending colon
segments connected by flexures
Large Intestine

The ascending colon travels up the right
side of the abdominal cavity to the level of
the right kidney
Large Intestine

At the level of the kidney the colon makes a
right-angle turn, the right colic, or hepatic
flexure
Large Intestine

The transverse colon travels across the top
of the abdominal cavity
Large Intestine

Directly anterior to the spleen, it bends
downward to form the left colic or splenic
flexure
Large Intestine

The descending colon descends down the
left side of the abdominal cavity
Large Intestine

As the descending colon enters the pelvis it
forms the S-shaped sigmoid colon
Large Intestine

The transverse
and sigmoid
portions of the
colon are
anchored to the
posterior
abdominal wall
by mesentary
sheets called
mesocolons
Large Intestine

In the pelvis, at the level of the third sacral
vertebra, the sigmoid colon joins the rectum,
which is positioned anterior to the sacrum
Large Intestine

The natural
orientation of
the rectum
allows for a
number of
pelvic organs to
be examined
digitally during
a rectal exam
Large Intestine

The rectum has three lateral curves or bends
represented internally are transverse folds
called rectal valves
Large Intestine

Rectal valves
separate feces
from flatus, thus
allowing gas to
passed
Large Intestine


The anal canal
lies entirely
external to the
abdominopelvic
cavity
About 3 cm long
the canal begins
where the rectum
penetrates the
muscles of the
pelvic floor
Large Intestine

The anal canal
has two
sphincters
– External anal
sphincter
– Internal anal
sphincter
Large Intestine



The involuntary internal anal sphincter is
composed of smooth muscle
The voluntary external anal sphincter is
composed of voluntary muscle
These sphincters which act rather like
purse strings to open and close the anus,
are ordinarily closed excepts during
defecation
Large Intestine: Microscopic



The wall of the large intestine differs in
several ways from that of the small
intestine
The colon mucosa is simple columnar
epithelium except in the anal canal
Because most food is absorbed before
reaching the large intestine, there are no
circular folds, no villi, and no cells that
secrete digestive enzymes
Large Intestine: Microscopic


Its mucosa is thicker, its abundant crypts
are deeper, and there are tremendous
numbers of goblet cells in the crypts
Lubricating mucus produced by goblet
cells eases the passage of feces and
protects the intestinal wall from irritating
acids and gases released by resident
bacteria in the colon
Large Intestine: Microscopic

The mucosa of
the anal canal is
different from
the rest of the
colon, reflecting
the greater
abrasion that
this region
receives
Large Intestine: Microscopic

The mucosa
hangs in long
ridges or folds
called anal
columns and
contains
stratified
squamous
epithelium
Large Intestine


The anal sinuses
are recesses
between the anal
columns which
exude mucus
when
compressed by
feces
This aids in the
emptying of the
canal
Large Intestine


The horizontal
lines that
parallels the
inferior margin
of the anal
sinuses is called
the pectinate line
The line
separates areas
of visceral and
somatic sensory
innervation
Pectinate line
Large Intestine: Microscopic


The mucosa superior to the line is
innervated by visceral sensory fibers and
so are relatively insensitive to pain
The are inferior to the pectinate line is
innervated by somatic sensory fibers and
is very sensitive to pain
Large Intestine: Microscopic


Two superficial venous plexuses are
associated with the anal canal, one with
the anal columns and the other with the
anus itself
Where these veins (hemorhoidal) are
inflamed, itchy varicosities called
hemorrhoids result
Large Intestine: Microscopic


In contrast to the more proximal regions
of the large intestine, teniae coli and
haustra are absent in the rectum and anal
canal
Consistent with its need to generate
strong contractions to perform its
expulsive role, the rectum’s muscularis
muscle layers are complete and well
developed
Bacterial Flora


Although most bacteria entering the
cecum from the small intestine are dead
having been killed by the action of
lysozyme, defensins, HCL, and protein
digesting enzymes
The bacteria that survive, together with
the bacteria that enter the GI tract via
the anus, constitute the bacterial flora of
the large intestine
Bacterial Flora

The bacterial flora colonize the colon and
ferment some of the indigestible carbohydrates (cellulose and others) releasing
irritating acids and a mixture of gases
– Dimethyl sulfide, H2, N2, CH4, and CO2


About 500 ml of gas is produced each day
with much more when certain
carbohydrate rich foods are eaten
The bacterial flora also synthesize B
complex vitamins and most of vitamin K
Processes: Large Intestine


What is finally delivered to the large
intestine contains few nutrients, but still
has 12 to 24 hours more digestive system
Except for the small amount of digestion
of residue by the enteric bacteria, no
further food breakdown takes place in
the large intestine
Processes: Large Intestine


Although the large intestine harvests
vitamins made by the bacterial flora and
reclaims most of the remaining water and
some of the electrolytes (particularly
sodium and chloride) absorption is not a
major function of this organ
The primary concern of the large
intestine are propulsive activities that
force the fecal material toward the anus
and then eliminate it from the body
Processes: Large Intestine


While the large intestine is undeniably
essential for our comfort, it is not
essential for life
Several different surgical procedures
remove a part or all of the large intestine
in order to save life
Motility: Large Intestine


The large intestine musculature is
inactive much of the time, and when it is
mobile, its contractions are sluggish and
of short duration
The most frequent movements seen in the
colon are haustral contractions, which
are slow segmenting movements that
occurs every 30 minutes or so
Motility: Large Intestine



Haustral contractions reflect local controls
of smooth muscle within the walls of
individual haustra
As a haustrum fills with food residue, the
distension stimulates its muscle to contract,
which propels the luminal contents into the
next haustrum
These movements also mix the residue
which aids in water absorption
Motility: Large Intestine


Mass movements (mass peristalsis) are long,
slow-moving, but powerful contractile
waves that move over large areas of the
colon three or four times daily and force the
contents toward the rectum
Typically these movements occur during or
just after eating when the presence of food
in the stomach activates the gastroileal
reflex in the small intestine and the
propulsive gastrocolic reflex in the colon
Motility: Large Intestine

Bulk, or fiber, in the diet increases the
strength of colon contractions and softens
the stool, allowing the colon to act more
efficiently
Defecation

The rectum is
usually empty,
but when feces
are forced into it
by mass
movements,
stretching of the
rectal walls
initiates the
defecation reflex
Defecation

This is a spinal
cord mediated
reflex that causes
the walls of the
sigmoid colon
and the rectum to
contract and the
anal sphincters to
relax
Defecation

Distension or
stretch of the
rectal walls
triggers a
depolarization of
sensory (afferent)
fibers which
synapse with the
spinal cord
Defecation

Parasympathetic
motor (efferent)
fibers, in turn,
stimulate
contraction of the
rectal walls and
relaxation of the
internal anal
sphincter
Defecation

If it is convenient
to defecate,
voluntary signals
stimulate the
relaxation of the
external anal
sphincter
Defecation



As feces are forced into the anal canal,
impulses reach the brain allowing us to
decide whether the external(voluntary) anal
sphincter should remain open or closed
If defection is delayed, the reflex
contractions end within a few seconds and
the walls relax
With the next mass movement, the reflex is
initiated again and again until one chooses to
defecate