The Digestive System
Chapter 24
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 o 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
– Simply the process of taking food into the
digestive tract, usually 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 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
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 via
the anus
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 24.5b) 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 Tongue
The Tongue
The Salivary Glands
Composition of Saliva
Control of Salivation
The Teeth
The Tongue
Dentition and the Dental
Formula
The Teeth
Tooth Structure
Tooth and Gum Disease
The Pharynx
The Esophagus
Digestive Processing Occurring
in the Mouth, Pharynx
Digestive Processes
Digestive Processes
Digestive Processes
Digestive Processes
Digestive Processes
The Stomach
The Stomach
Gross Anatomy
Microscopic Anatomy
Microscopic Anatomy
Microscopic Anatomy
Digestive Processes Occurring
in the Stomach
Regulation of Gastric Secretion
Gastric Motility and Emptying
Gastric Motility and Emptying
The Small Intestine and
Associated Structures
Gross Anatomy
Microscopic Anatomy
Intestinal Juice: Composition
and Control
The Liver and Gallbladder
Regulation of Release into the
Small Intestine
Requirements for Optimal
Intestinal Digestive Activity
Motility of the Small Intestine
The Large Intestine
Large Intestine
Gross Anatomy
Microscopic Anatomy
Bacterial flora
Digestive Processes Occurring
in the Large Intestine
Motility of the Large Intestine
Defecation
Food Poisoning
Developmental Aspects of the
Digestive System