Chapter 24
The Digestive System
The Alimentary Canal
Accessory Digestive Organs
Functions of Digestive System
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Ingestion - selective intake of food.
Digestion – mechanical and chemical breakdown of
food into a form the body can use.
Absorption – uptake of nutrients into blood and
lymph.
Compaction – absorption of water and consolidation
of wastes into fecal mass.
Defecation – the elimination of the fecal mass.
Alimentary canal
[Gastrointestinal (GI) tract]
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Consists of: Mouth→ Pharynx→ Esophagus →
Stomach→ Duodenum→ Jejunum → Ileum→ Caecum→
Colon (ascending, transverse, descending, sigmoid) → Rectum
→ Anus .
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GI tract is a tube that is actually outside of the body.
Accessory Digestive Organs
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Teeth, salivary glands, pancreas, liver, gall
bladder.
The digestive system
Histology of the Alimentary Canal
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Defined: Beginning at the esophagus and continuing
through the alimentary canal to the anus, there are four basic
layers or tunics..
Listed: from the inner-most to the outer-most layer.
1. mucosa – moist epithelial membrane; functions in secretion,
absorption and protection against infection.
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This layer is more complex than the other tunics of the GI tract in
that it has three sublayers.
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Three layers or linings of the mucosa:
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epithelial lining – simple columnar with goblet cells
lamina propria – areolar CT (some reticular CT)
muscularis mucosae – a thin layer of smooth muscle that helps
move the mucosa.
Secretes mucous, digestive enzymes & hormones and absorbs end
products of digestion.
Histology of the Alimentary Canal
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2. submucosa – loose CT containing blood vessels, nerves
(submucosal nerve plexus), lymph vessels.
3. muscularis externa – consists of an inner circular layer and an
outer longitudinal layer of smooth muscle. These muscles are
responsible for the peristalsis and segmentation of the GI tract
and are controlled by the myenteric plexus (a group of nerves
between the layers). Thickenings of this layer create the
sphincters or valves that separate sections of the tract.
4. serosa – the same layer as the visceral peritoneum. Consists
of areolar CT covered by a single layer of epithelial cells.
Should be noted that the esophagus is somewhat different in
that it is not covered by the visceral peritoneum.
GI tract innervation
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Tongue, pharynx, and esophagus are innervated by CN V, VII,
IX-XII regulate swallowing and mastication . Superior
sympath. cervical gang. and CN VII and IX regulate salivary
secretions.
Both sympathetic and parasympathetic nerves regulate GI
tract.
 Sympathetic stimulation causes ↓ in motility, ↓ in secretions
and keep sphincters contracted.
 Parasympathetic ↑ motility, secretions and relaxes
sphincters and promotes digestion.
Enteric nervous system regulate GI function irrespective of
PSym. or Sym. Input. It is located in the submucosa
(Meissners plexus) and between 2 layers muscularis mucosa as
myenteric (Auerbach’s plexus)
Enteric nervous system
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Auerbachs nerve plexus
Monitor stretch
Chem cond.
Meissners plexus
Vascular supply to GI Tract
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Above the diaphragm there are numerous branches
off of the aorta and carotid arteries to the esophagus,
pharynx, mouth etc.
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Below the diaphragm the celiac trunk supplies the
stomach (gastric), spleen (splenic) and liver (hepatic)
as well as superior and inferior mesenteric arteries.
Vascular supply to GI Tract
Vascular supply to GI Tract
Hepatic portal veins
Peritoneum and mesentery
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Peritoneum- serous membrane lining the abdominal
cavity and most of the visceral organs
Parietal peritoneum
 Visceral peritoneum
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Mesentery- is a double layer of peritoneum that
extends from the abdominal wall to the digestive
organs. It keeps the organs in place and allows for
their active contractions to occur without the guts
becoming twisted and entangled.
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Greater and lesser omentum; falciform ligament
Serous membranes
Lesser omentum
Greater omentum
Mesenteries
The Mouth
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Initial point of entry of food into GI tract
Is the first site for beginning the breakdown of food
into its constituent molecules (lipids → fatty acids,
carbohydrates → simple sugars & protieins → amino
acids) for absorption into the body.
Consists of: lips, cheeks, tongue, teeth, salivary glands,
palate.
Lined with a thick stratified squamous epithelium
The mouth
Tongue
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Muscular organ for
manipulating and
moving food in
mouth and aid in
swallowing of
foods.
Contains taste
buds chemoreceptors
The Palate
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Separates the mouth from the nasal cavity.
Allows you to chew food while breathing
Anterior portion is hard palate formed palatine
process of mandible and palatine bones.
Posterior portion is composed of mainly sk. m.
Uvula hangs down from soft palate
Teeth
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Adults: 32 total
( 8) incisors- blade shaped for clipping and
cutting food
(4) canines- conical with sharp ridgeline and
pointed tip for tearing food
( 20) molars/premolars- flattened crowns for
crushing and grinding food
Each tooth is embedded in a socket = alveolus
Teeth
Adults=32
Children =20
Salivary glands
Parotid-serous/watery
Sublingual-serous and
mucous
Submandibular- serous
and mucous
The Pharynx
Three regions defined by location and function:
Nasopharynx- air passage way only: composed of
pseudostratified ciliated columnar epithelium
Oropharynx- back of oral cavity down to epiglottis:
-composed of stratified squamous epithelium.
Laryngopharynx- air passageway, food is deflected into
esophagus and does not enter this region
-composed of stratified squamous epithelium
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Esophagus
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Defined/Location: straight muscular tube ~ 10”
long extends from laryngopharynx to stomach.
Histology:
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mucosa – nonkeratinized stratified squamous epithelium
with mucous glands
muscularis externa – consists of circular and longitudinal
layers of skeletal muscle and smooth muscle: (upper 1/3
is sk.m.; mid 1/3 is sk/sm. m; and lower 1/3 is all sm.m.)
fibrous adventitia that blends with surrounding tissues
Esophagus
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Lower esophageal sphincter is a physiologic control
valve that opens to release food into stomach, but
constricts to prevent gastric regurgitation into
esophagus.
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Movement of food is by segmentation = peristalsis
Courses through the thorax posterior to trachea and
penetrates diaphragm at esophageal hiatus.
Peristaltic movement
Food moves as a
bolus in segmented
manner with alternate
segments of esophagus contracting then relaxing
to move food downward.
Esophagus X-section
The Stomach
Defined: Temporary storage tank where chemical
breakdown of proteins begins and a creamy paste
“chyme” is formed. pH~ 1-2.
Location-Lies in upper left quadrant of abdominal cavity,
inferior to diaphragm and partially covered by the liver.
Histology: Lined with simple columnar epithelium and
numerous gastric glands
Four regions:
Cardia
Fundus
Body
Pyloric
Stomach
Size: Varies from 15 to 25 cm. (6-10”) in length.
empty has a volume of ~ 50 ml; full capacity ~ 4 l.
 empty it collapses onto itself and throws its mucosa
and submucosa into longitudinal folds called “rugae”
 Three layers of smooth muscle:
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Longitudinal
Circular
Oblique
Pyloric region joins the duodenum at pyloric
sphincter.
Cross section of rugae and stomach wall
Small Intestine
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Three sections: Duodenum, Jejunum, and Ileum
Duodenum: shortest (~ 10”) and widest; buffers low pH of
chyme. First 2” is intraperitoneal, remainder is retroperitoneal.
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Contains Plicae circularis = circular folds → spiraling of chyme
Jejunum : ~8’ long; bulk of digestion and absorption occurs
here.
Ileum : ~ 12’ long; controls movement of mass into caecum
of large intestine
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Peyer’s Patches- aggregated lymphoid follicles near ileum; increase in
abundance as you get nearer large intestine
Joins large intestine at ileocecal junction (ileocecal valve)
Histology of Small Intestine
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mucosa –simple columnar epithelium with absorptive
cells bound by tight junctions and numerous mucussecreting goblet cells.
microvilli –tiny projections of plasma membrane
(brushed border) contain enzymes that complete final
stages of CHO and protein digestion
3-D view of small intestinal Microvilli
- note the lacteal within the core of the microvilli
Route of absorbed nutrients
Plicae circularis in Jejunum
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plicae circularis – deep permanent folds of mucosa
and submucosa of duodenum and jejunum; force
chyme to spiral through the intestinal lumen; slows
movement and increases absorptive surface area
Large Intestine divisions of:
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Begins at caecum, a blind pouch with appendix
attached and ends at anal canal.
Ascending colon: proceeds upward on right side after
anastomosing with ileum at ileocecal junction.
(Retroperitoneal)
Transverse colon: curves medially at right colic flexure
and crosses over superior part of abdominal cavity.
Descending colon: begins at left colic flexure off of
transverse colon and descends the left side of
abdomen until it reaches the iliac fossa to join
sigmoid. (retroperitoneal)
Large intestine
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diameter is greater than small intestine
length is only 1.5 m (5.5 ft) vs 6 m for small intestine
Large Intestine divisions in pelvic cavity
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Sigmoid colon: S-shaped segment starts at iliac fossa
and travels behind bladder to empty into the rectum
(retroperitoneal).
Rectum: ~ 15” long expandable end portion of L.I. at
level of S3 and is for temporary storage of fecal
material .
Rectal mocosa is smoother than colon; Has three rectal
folds (rectal valves) that enable it to pass gas (flatulence)
without releasing fecal matter.
 Upon fecal matter entering rectum it triggers the defecation
reflex to expel fecal matter from rectum.
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Large Intestine divisions in pelvic cavity
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Anal canal: Final portion of L.I. before excreting fecal
matter out of anal opening.
Mucosa of anal canal contains longitudinal columns called
anal columns with depressions called anal sinuses between
the columns. As feces passes through anal canal they press
against the sinuses causing them to release more mucous to
lubricate the canal during defecation.
 Anal sphincters internal and external
 Internal sphincter is smooth muscle of muscularis
externa
 External sphincter is skeletal muscle
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Anal Canal
Large intestine
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Large intestine absorbs mostly water from fecal
mass.
Lacks villi and has relatively thin wall.
Interior wall is relatively smooth
mucosa – contains numerous goblet cells which
produce copious amounts of mucus to lubricate
the fecal mass.
lymphoid nodules (Peyers’ patches) extend in
submucosa.
Large intestine
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Muscularis externa layer is reduced in thickness
Wall contains:
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Tenia coli- small bands of longitudinal muscle
occurring just beneath submucosa.
Tenia coli contract and pull Large Intestine
longitudinally to form bulges called haustra.
- Haustra- pocket like sacs along L.I.; result in
segmentation of L.I. Churning action occurs here
which turns fecal material into a sludgy paste.
Large intestine histology
Liver
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Largest gland in the body.
Redish brown and located in upper right quadrant of
abdomen.
Multiple functions only 1 of which has to do with
digestion, manufactures bile for breakdown of fats
which are then acted upon by pancreatic lipases.
Consists of 4 lobes: right, left, caudate and quadrate.
Contains gall bladder = storage bag for bile.
Liver
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Falciform ligament separates right lobe from left lobe anchors
liver to anterior wall of abdomen.
Caudate and quadrate lobes are visible from posterior only.
Gall bladder is lateral to quadrate lobe
Porta hepatis is where vessels and nerves enter liver.
Liver anterior view
Liver posterior view
Liver histology
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The liver is composed
of millions of
hexagonal lobules
about the size of a
sesame seed and made
up of hepatocytes.
Lobule has a central
vein in middle and at
each corner a portal
triad (arteriole, venule
and bile duct)
Gall Bladder
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Storage bag (~ 10 cm long) for bile produced by liver
Bile enters gall bladder via left and right hepatic ducts
which empty into common hepatic duct → cystic duct
→ gall bladder for storage.
Bile leaves gall bladder via cystic duct and joins
pancreatic duct to form hepatopancreatic ampulla
before it empties into duodenum.
Release is controlled by hepatopancreatic sphincter of
“Oddi”
Bile salts act as detergents in breaking down fats in
small intestine.
Pancreas
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Tucked under stomach and liver in a bend of the
duodenum has a lobulated appearance.
Consists of a head, body, and a tail.
Combination endocrine and exocrine gland.
Functions as an endocrine and digestive organ
Endocrine pancreatic islets → insulin and glucagon →
regulation of blood glucose
 Exocrine → pancreatic enzymes and zymogens that aid in digestion.
Pancreatic juices are very alkaline and neutralize acidic chyme coming
from stomach when released into duodenum.
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Pancreas