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Chapter 25: Anatomy of
the Digestive System
Dr. Kim Wilson
Digestive System
Consists of two parts:
1. Gastrointestinal tract – a tube from mouth
to anus
• Mouth
• Pharynx
• Esophagus
• Stomach
• Small and large intestine
2. Accessory organs of digestion – teeth,
tongue, salivary glands, liver, gall bladder,
pancreas
Digestive System
Functions of the Digestive
System
• Ingestion – Taking in food and liquids
• Secretion – Water, enzymes, acid or
buffers secreted into the gastrointestinal
tract
• Mixing and propulsion – Food is churned
and moved along the GIT by peristalsis.
• Digestion – Food is mechanically and
chemically broken into smaller particles
• Absorption – Passage of digested
substances into GIT wall and then into
blood & lymph
• Defecation – Elimination of undigested or
unabsorbed waste in the feces.
Walls of the Digestive Tract
Consists of 4 layers of tissue:
1. Mucosa
• Innermost layer made up of the mucous epithelium,
lamina propria and the muscularis mucosa
2. Submucosa
• Contains many glands, blood vessels and
parasympathetic nerves that forms the submucosal
(Meissner’s) plexus
3. Muscularis
• Inner layer of circular muscle and outer layer of
longitudinal smooth muscle
• Contains Auerbach (myenteric) plexus between the 2
muscle layers
4. Serosa
• Outermost layer, same as the visceral peritoneum
• Connected to the parietal peritoneum via the
mesentery
Oral (Buccal) Cavity
• Structure of the mouth
1. Lips
•
Externally covered by
skin, internally by a
mucous membrane
•
Keep food in the mouth
•
Sense temperature and
texture of food
•
Needed to form speech
sounds
2. Cheeks
•
Lateral boundaries of
the oral cavity
•
Formed largely by the
buccinator muscle lined
with mucous membrane
Oral (Buccal) Cavity
3.
Hard and Soft Palate
•
Hard palate: palatine and
maxillary bones
•
Soft palate: muscular arch
separating the mouth from
the nose
•
Uvula projects off the
soft palate
Oral (Buccal) Cavity
4.
Tongue
•
3 parts: root, tip, and body
•
Skeletal muscle covered by mucous membrane
•
Covered with papillae which contain taste buds
•
Lingual frenulum helps anchor the tongue to the
floor of the mouth
•
Rich supply of blood vessels allows for quick
absorption (sublingual medications)
•
Intrinsic muscles: originate and insert in the
mouth, used for mastication and speech
•
Extrinsic muscles: insert into the tongue, but
originate from the hyoid or skull bones, used for
swallowing (deglutition) and speech
Papillae
•
•
Rough elevations; cover upper surface and sides of tongue
3 types
• VALLATE
• Large and mushroomlike
• Form inverted "V" on posterior portion of tongue
• FUNGIFORM
• Located on sides and tip of tongue
• FILIFORM
• Small and white in appearance (very numerous)
• Located on anterior 2/3 of tongue
Circumvallate Papillae
Oral (Buccal) Cavity
5.
Salivary Glands
•
Parotid glands
•
Anterior and inferior to the ear
•
Produce watery saliva containing enzymes; open into the
mouth via the Stenson ducts
•
Inflammation = mumps
•
Submandibular glands
•
At the mandibular angle
•
Produce saliva containing enzymes and mucus
•
Wharton ducts open into the mouth on either side of the
frenulum
•
Sublingual glands
•
In front of the submandibular glands
•
Drained by ducts of Rivinus
•
Produce mucus saliva
Teeth
•
•
•
•
•
•
•
•
•
•
Involved in mastication and speech
Crown: Exposed portion of the tooth
Neck: enameled part of tooth below
gum line
Root: anchors the tooth into the
periodontal membrane
Enamel: Hard, protective outer
covering
Dentin: living, cellular, calcified
tissue within the root, dentin is
covered by cellular bone-like
structure that helps hold tooth in the
socket.
Pulp cavity within the dentin, filled
with blood vessels, nerves, and
connective tissue
Periodontal ligaments: hold tooth
in socket.
Cementum: Anchors the root
Gingiva: dense, fibrous C.T.
covered by stratified squamous
epithelium.
Teeth
• Two sets
• Primary,
deciduous, milk:
Childhood (20)
• Permanent or
secondary: Adult
(32)
• Types
• Incisors, canines,
premolars and
molars
Pharynx and Esophagus
•
•
Swallowing moves the food mass, or bolus, from the
mouth into the pahrynx
Esophagus: a muscular tube which connects the pharynx
to the stomach
– Lies posterior to the trachea
– Pierces through the diaphragm
– 1st segment of the digestive tube where the 4 layers
of tissue can be observed
– Upper 1/3: voluntary, striated muscle
– Middle1/3: involuntary, mixed
– Lower 1/3: Involuntary, smooth
– Upper and lower esophageal sphincters
• Lower AKA cardiac sphincter
– Stretching of the esophageal hiatus may allow
upward bulging of the stomach and lower end of the
esophagus; hiatal hernia
– GERD: stomach acid flows back up through the LES
Stomach Size and Divisions
•
•
Size and Position
– Enlarged after a meal and collapses as the food is pushed through
– Holds 1- 1.5 L
– Under the liver and the diaphragm, most left of midline
Divisions
– Fundus, body, and pyloris
– Cardiac portion: where esophagus meets the stomach
– LES or cardiac sphincter controls movement of food from the esophagus into the
stomach
– Pyloric sphincter controls movement from the stomach into the small intestine
Stomach Wall
•
Gastric mucosa
– Epithelial lining of the stomach
made up of rugae folds and
gastric pits
– Gastric glands, which secrete
HCl and digestive enzymes, are
found within the pits
• Contain 3 major cell types:
1. Chief cells – secrete
pepsin
2. Parietal cells –
secrete HCl and
Intrinsic Factor
3. Endocrine cells –
produce ghrelin and
gastrin
•
Gastric muscle
–
3 layers of muscle:
longitudinal, circular and
oblique
•
Increases contraction and
mixing ability
Stomach Muscles
• Cardiac sphincter
•Located between esophagus and stomach (at entrance)
•Controls materials entering stomach
• Pyloric sphincter
•Located between stomach (pylorus) and small intestine (at exit)
•Controls materials exiting stomach
Functions of the Stomach
1.
2.
3.
4.
5.
6.
7.
Food reservoir – food is stored until it can
be digested
Secrete gastric juice for digestion of food
Churns and mixes food
Secretes intrinsic factor important in B12
uptake
Small amount of absorption; drugs,
water, alcohol, some fats
Secretes gastrin ( regulates digestive
functions) and ghrelin (increases apetite)
HCl destroys pathogens
Small Intestine
•
•
3 divisions: duodenum, jejunum, ileum
Wall of the small intestine
– Made up of circular plicae containing villi
projections
• Each villus contains an arteriole, a venule
and lacteal
• Microvilli line each villus creating a brush
border
• Goblet cells – mucus
• Endocrine cells – hormones
– Intestinal crypts of Lieberkuhn are sites of
mitosis where new enterocytes are formed
• New cells are pushed up and the old ones
shed
• Paneth cells are located at the base of
each crypt and produce enzymes to
inhibit bacterial growth
•
Large Intestine
•
Diameter is greater than the small
intestine, but it is much shorter (5 ft vs 20
ft)
Divisions
– Cecum: blind pouch off the first part of
the LI, located in the LRQ
• Ileocecal valve allows material to
move from SI to LI
– Colon
• Ascending colon moves up the
right side of the abdomen
• Transverse colon moves
horizontally across the abdomen
from the hepatic flexure to the
splenic flexure
• Descending colon moves down
the left side of the abdomen
• Sigmoid colon extends below the
colon and connects to the rectum
Greater and Lesser Omentum
• Greater Omentum
• An extension of visceral peritoneum from the greater curvature of the
stomach
• Hangs over the intestines in a double fold (“Lace Apron”)
• Function: Protection – In inflammation, the greater omentum envelops
the inflamed area and “walls it off” from the rest of the abdomen
• Lesser Omentum
• An extension of visceral peritoneum from the lesser curvature of the
stomach to the liver (Holds stomach to liver)
Large Intestine cont.
• Divisions cont.
• Rectum
• Last part of the colon
which terminates at the
anus; contains the internal
(involuntary) and external
(voluntary) anal sphincters
Large Intestine
• Modifications of the walls of the LI
– Intestinal mucus glands lubricate feces
– Longitudinal muscle group produce taeniae coli
– Circular muscle group produces haustra
Veniform Appendix
•
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Accessory organ whose function is not fully understood
Possible breeding ground for nonpathogenic intestinal bacteria
Contains lymph tissue
Inflammation is known as appendicitis
McBurney’s point: area in the RLQ which is extremely tender to touch with
appendicitis
Peritoneum and
Mesenteries
• Peritoneum – large sheet of serous
membrane
• Visceral: Covers organs
• Parietal: Covers interior surface of
body wall
• Retroperitoneal: Certain organs
covered by peritoneum on only
one surface and are considered
behind the peritoneum; e.g.,
kidneys, pancreas, duodenum
• Mesenteries: two layers of
peritoneum with thin layer of loose
C.T. between
• Routes by which vessels and
nerves pass from body wall to
organs
• Greater omentum: connects
greater curvature of the stomach to
the transverse colon.
• Lesser omentum: connects lesser
curvature of the stomach and the
proximal part of the duodenum to
the liver and diaphragm.
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• Transverse mesocolon, sigmoid
mesocolon, mesoappendix.
Liver
•
•
•
•
Largest organ in the body (3 – 4 pounds)
Lies immediately beneath the diaphragm, within the right hypochondrium
2 lobes separated by the falciform ligament
– Right lobe (5/6 of the liver)
• Right lobe proper, caudate lobe and quadrate lobe
– Left lobe (1/6 of the liver)
Each lobe is separated into lobules and supported by a capsule of Glisson
– A central vein extends through each lobule
– Hepatic cells, sinusoids, bile canaliculi, arteries and veins also make up the
lobules
Hepatic Lobules - "Structural Units of the
Liver"
• Each lobe is divided into
lobules by blood vessels
and fibrous tissue
• Each lobule is a tiny
cylinder that contains 5 - 6
sides
Liver – Hepatic Lobule Function
• Hepatic lobule function
– Blood enters lobule from
hepatic artery
– Blood oxygenates
hepatocytes
– Sinusoids contain phagocytic
Kupffer cells
– Blood continues along the
sinusoids to the central vein
– Central veins lead to the main
hepatic veins which drain into
the inferior vena cava
– Bile formed by hepatocyes
passes through the canaliculi
to join bile ducts
Liver – Bile Ducts
•
Bile Ducts
• Small bile ducts join to form the right and left hepatic ducts which join to form
the common hepatic duct
• The common hepatic duct merges with the cystic duct from the gallbladder to
form the common bile duct
• Bile is emptied into the SI at the duodenum via the major duodenal papilla
Functions of the Liver
• Liver cells detoxify substances
• Liver cells secrete bile which aids in fat
digestion
• Liver cells help metabolize proteins, fats
and carbohydrates
• Liver cells store iron, vitamins A, B12 and
D
• Produces plasma proteins and serves as a
site of hematopoeisis during fetal
development
Gallbladder
• Lies underneath the liver
• Cholecystitis – GB
inflammation
• Cholelithiasis – gallstone
formation
• Cholecytectomy – GB
removal
Gallbladder Functions
• The GB stores bile and concentrates it
• During fat digestion, the GB contracts and ejects bile into the
duodenum
• Jaundice results when an obstruction of bile flow occurs
• Bile cannot be lost through the feces and enters the blood, creating
a yellowish skin hue
Pancreas
• Pancreas both endocrine and
exocrine
• Head, body and tail
• Endocrine: pancreatic islets.
Produce insulin, glucose, and
somatostatin
• Exocrine: groups acini (grapelike cluster) form lobules
separated by septa, produce
digestive enzymes
• Intercalated ducts lead to
intralobular ducts lead to
interlobular ducts lead to the
pancreatic duct.
• Pancreatic duct joins common
bile duct and enters duodenum
at the hepatopancreatic
ampulla controlled by the
hepatopancreatic ampullar
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sphincter
Functions of the Pancreas
• Functions of the
pancreas
• Acinar cells secrete
digestive enzymes
• Beta cells secrete
insulin
• Alpha cells secrete
glucagon