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Gastrointestinal Tract Activities
• There are six essential activities:
• Ingestion – taking food into the digestive tract
• Propulsion – swallowing and peristalsis
• Peristalsis – waves of contraction and relaxation of
muscles in the organ walls
• Mechanical digestion – chewing, mixing, and churning
food
• Chemical digestion – catabolic breakdown of food
• Absorption – movement of nutrients from the GI tract to
the blood or lymph
• Defecation – elimination of indigestible solid wastes
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Ingestion
Mechanical
digestion
• Chewing (mouth)
• Churning (stomach)
• Segmentation
(small intestine)
Chemical
digestion
Food
Pharynx
Esophagus
Propulsion
• Swallowing
(oropharynx)
• Peristalsis
Stomach (esophagus,
stomach,
small intestine,
large intestine)
Absorption
Lymph
vessel
Small
intestine
Large
intestine
Defecation
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Blood
vessel
Mainly H2O
Feces
Anus
Figure 23.2
Peritoneum and Peritoneal Cavity
• Peritoneum – serous membrane of the abdominal
cavity
• Visceral – covers external surface of most
digestive organs
• Parietal – lines the body wall
• Peritoneal cavity
• Lubricates digestive organs
• Allows them to slide across one another
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Peritoneum and Peritoneal Cavity
• Mesentery – double layer of peritoneum that provides:
• Vascular and nerve supplies to the viscera
• Hold digestive organs in place and store fat
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Peritoneum and Peritoneal Cavity
• Retroperitoneal organs – organs outside the peritoneum
• Peritoneal organs (intraperitoneal) – organs surrounded by
peritoneum
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Figure 23.5b
Histology of the Alimentary Canal
• From esophagus to the anal canal the walls of the GI
tract have the same four tunics
• From the lumen outward they are the
• mucosa
• submucosa
• muscularis externa
• serosa
• Each tunic has a predominant tissue type and a specific
digestive function
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Histology of the Alimentary Canal
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Figure 23.6
Mucosa: innermost layer
• A layer of epithelium that is in direct contact with the content of the
GI tract –
• In the mouth, pharynx and anal canal – stratified squamous
(protection);
• In the rest of GI tract simple columnar (secretion and absorption)
• A layer of connective tissue- lamina propria
• Areolar CT containing blood and lymphatic vessels
• Support epithelium
• Contains most cells of MALT (mucosa-associated lymphatic
tissue)
• Muscularis mucosa – thin layer of smooth muscles that is
responsible for the folding of the GI tract (increased surface area)
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Mucosa: Other Sub layers
• Submucosa – dense connective tissue containing
elastic fibers, blood and lymphatic vessels, lymph
nodes, and nerves
• Muscularis externa – responsible for segmentation and
peristalsis (GI motility)
• Serosa – the protective visceral peritoneum
• Replaced by the fibrous adventitia in the
esophagus
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Enteric Nervous System
• The GI tract has its own nerve supply – the enteric neurons
• The system is made of interconnected ganglia found in the walls
of the GI tract – short reflexes
• Composed of two major intrinsic nerve plexuses:
• Submucosal nerve plexus – regulates glands and smooth
muscle in the mucosa
• Myenteric nerve plexus – Major nerve supply that controls
GI tract mobility
• Linked to the CNS via afferent visceral fibers (long autonomic
reflex arc)
• The motor fibers of the ANS synapse with the enteric neurons
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http://arbl.cvmbs.colostate.edu/hbooks/pathphys/digestion/basics/peristalsis.html
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Mouth
• The mouth is lined with stratified squamous epithelium
• The gums, hard palate, and dorsum of the tongue are
slightly keratinized
• Hard palate – made of the palatine bones and palatine
processes of the maxillae
• Assists the tongue in chewing
• Soft palate – mobile fold formed mostly of skeletal
muscle
• Closes off the nasopharynx during swallowing
• Uvula projects downward from its free edge
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Salivary Glands
• Parotid – lies anterior to the ear between the masseter
muscle and skin
• Parotid duct opens next to second upper molar
• Submandibular – lies along the medial aspect of the
mandibular body
• Its ducts open at the base of the lingual frenulum
• Sublingual – lies anterior to the submandibular gland under
the tongue
• It opens via 10-12 ducts into the floor of the mouth
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Saliva: Source and Composition
• Secreted from serous and mucous cells of salivary glands
• 97-99.5% water, hypo-osmotic, slightly acidic solution
containing
• Electrolytes – Na+, K+, Cl–, PO42–, HCO3–
• Digestive enzyme –
• salivary amylase – begins starch digestion in the oral
cavity
• lingual lipase – activated by stomachacid to digest fat
after food is swollowed
• Proteins – mucin, lysozyme, defensins, and IgA
• Metabolic wastes – urea and uric acid
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Salivary glands function
• Lubrication and binding
• the mucus in saliva binds masticated food into
bolus that slides easily through the esophagus
without inflicting damage to the mucosa.
• Saliva also coats the oral cavity and esophagus,
and food basically never directly touches the
epithelial cells of those tissues.
• Solubilizes dry food
• in order to be tasted, the molecules in food must
be solubilized.
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Salivary glands function
• Oral hygiene:
• The oral cavity is almost constantly flushed with
saliva, which floats away food debris and keeps
the mouth relatively clean.
• Saliva contains lysozyme, an enzyme that lyses
many bacteria and prevents overgrowth of oral
microbial populations.
• Initiates starch digestion
• the serous cells secrete an alpha-amylase which
can begin to digest dietary starch into maltose.
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Teeth
• Primary – 20 deciduous teeth that erupt at intervals
between 6 and 24 months
• Permanent – enlarge and develop causing the root of
deciduous teeth to be reabsorbed and fall out between
the ages of 6 and 12 years
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Tooth Structure
• Crown: the exposed part above the gingiva (gum)
• Covered by enamel
• Enamel is not a tissue but cell secretions
• It is produced before the tooth erupts – if
damaged can not regenarate
• the hardest substance in the body (calcium salts
and hydroxyapatite crystals)
• Root: portion embedded in the jawbone
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Tooth Structure
• Cementum: calcified connective tissue that contains cell –
cementocytes in lacunae
• Covers root and attaches it to the periodontal ligament
• Dentin: bonelike material under enamel
• Maintained by odontoblasts that line the pulp cavity
• Periodontal ligament
• Forms fibrous joint called a gomphosis
• Pulp cavity: cavity surrounded by dentin
• Pulp: connective tissue, blood vessels, and nerves
• Root canal: extends from pulp cavity to the apical foramen of
the root
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The esophagus
• Carries solids and liquids from the pharynx to the stomach
• Passes through esophageal hiatus in diaphragm
• Travels through the mediastinum and pierces the diaphragm
• Joins the stomach at the cardiac orifice
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Histology of the esophagus
• Esophageal mucosa contains stratified squamous
epithelium
• Changes to simple columnar at the stomach
• Esophageal glands in submucosa secrete mucus to aid
in bolus movement
• Muscularis: skeletal superiorly; smooth inferiorly
• Adventitia instead of serosa
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Microscopic Anatomy of the Stomach
• Epithelial lining is composed of:
• Simple columnar ET with goblet cells that produce
a coat of alkaline mucus
• Gastric pits contain gastric glands that secrete gastric
juice, mucus, and gastrin
• Muscularis – has an additional oblique layer that:
• Allows the stomach to break and mix, food
physically
• Breaks down food into smaller fragments
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Digestion in the Stomach
• The stomach:
• Holds ingested food
• Breaks this food both physically and chemically
• Delivers chyme to the small intestine
• Enzymatically digests proteins with pepsin
• Secretes intrinsic factor required for absorption of
vitamin B12
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Digestion and absorption in the stomach
• Preliminary digestion of proteins by pepsin (not completed).
Pepsin breaks down complex proteins into smaller peptides
• Permits digestion of carbohydrates - until pH fall below 4.5 the
salivary amylase continue its function.
• Very little absorption of nutrients:
• The epithelial cells are covered with mucus
• Epithelial cells lack transport mechanisms
• Gastric lining is relatively impermeable to water
• Most carbohydrates, lipids and proteins are too big (not
completely broken down)
• Some drugs, however, are absorbed (ethyl alcohol,
aspirin)
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Small Intestine: Microscopic Anatomy
• Structural modifications of the small intestine wall
increase surface area
• Plicae circulares: deep circular folds of the mucosa
and submucosa
• Villi – fingerlike extensions of the mucosa
• Microvilli – tiny projections of absorptive mucosal
cells’ plasma membranes (brush border)
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Vein carrying blood to
hepatic portal vessel
Muscle
layers
Circular
folds
Villi
Lumen
(a)
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Figure 23.22a
Structure of Small Intestine
1
5
4
2
3
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Histology of the small intestine and stomach mucosa comparison
stomach
Small intestine
http://www.lab.anhb.uwa.edu.au/mb140/CorePages/GIT/git.htm#topicsmallintestine
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Small Intestine - regional specializations
Duodenum
• Main function is to receive chyme and neutralize it
• Duodenal glands in the mucosa secrete part of the intestinal
juice in response to local reflexes, enterocrinin hormone,
vagus nerve stimulation.
• Local reflexes and hormonal mechanisms start working
only after chyme enters the duodenum.
• The vagus nerve activates the duodenal glands during the
cephalic phase of gastric secretion
• Brunner’s glands in the submucosa
• produce mucus, buffers, hormone urogastrone that inhibit
gastric acid production and stimulates ET division
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Small Intestine - regional specializations
Ileum
• Distal region lack plicae
• aggregated lymphoid nodules (Peyer’s patches)
that protect the small intestine from bacteria
populations that are normally found in the large
intestine
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Intestinal Juice
• Secreted in response to distension or irritation of the
mucosa
• Slightly alkaline and isotonic with blood plasma
• Largely water, enzyme-poor, but contains mucus
• Facilitates transport and absorption of nutrients
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Liver
• The largest gland in the body
• The falciform ligament:
• Separates the right and left lobes anteriorly
• Suspends the liver from the diaphragm and anterior abdominal
wall
• The lesser omentum anchors the liver to the stomach
• The gallbladder rests in a recess on the inferior surface of the right
lobe
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The hepatic portal system begins in the capillaries of the digestive organs
and ends in the portal vein. Consequently, portal blood contains
substances absorbed by the stomach and intestines. Portal blood is passed
through the hepatic lobules where nutrients and toxins are absorbed,
excreted or converted.
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Liver: Microscopic Anatomy
• Liver lobules
• Hexagonal structural and functional units
• Filter and process nutrient-rich blood
• Composed of plates of hepatocytes (liver cells)
• Longitudinal central vein
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Liver: Microscopic Anatomy
• Portal triads consist of a
• bile duct
• Hepatic artery – supplies
oxygen-rich blood to the liver
• Hepatic portal vein – carries
venous blood with nutrients from
digestive viscera
• Liver sinusoids – enlarged, leaky
capillaries located between hepatic
plates
• Kupffer cells – hepatic macrophages
found in liver sinusoids
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Figure 23.24d
Liver lobule
Central
vein
Bile
canaliculus
Hepatic
artery
Hepatic
portal vein
Portal triad
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Liver: Microscopic Anatomy
• Hepatocytes’ functions include:
• Production of bile
• Processing bloodborne nutrients
• Storage of fat-soluble vitamins
• Detoxification
• Secreted bile flows between hepatocytes toward the bile
ducts in the portal triads
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Liver physiology
• Removing and excreting body wastes, hormones, drugs and
other foreign substances Enzymes in the liver alter some toxins
so they can be more easily excreted in urine.
• Synthesizing plasma proteins, including those necessary for
blood clotting Most of the 12 clotting factors are produced by the
liver. Other plasma proteins produced by the liver include
albumin, fibrinogen and certain globulins which transport
substances such as cholesterol and iron.
• Producing bile to aid in digestion Bile salts aid in fat digestion
and absorption. Bile is continuously secreted by the liver and
stored in the gallbladder until a meal, when bile enters the
beginning of the small intestine.
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Liver physiology
• Excretion of bilirubin Bilirubin is one of the few waste products
excreted in bile. Macrophages in the liver remove worn out red blood
cells from the blood. Bilirubin results from the breakdown of the
hemoglobin is excreted into bile by hepatocytes.
• Storing certain vitamins, minerals, and sugars The liver stores
enough glucose in the form of glycogen to provide about a day's worth
of energy.
• The liver also stores fats, iron, copper, and many vitamins including
vitamins A, D, K, and B12.
• Processing nutrients absorbed from digestive tract
• The liver converts glucose into glycogen, its storage form.
• The fatty acids produced by the digestion of lipids are used to
synthesize cholesterol and other substances.
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Liver: bile secretion
• Bile leaves the liver via:
• Bile ducts, which fuse into the common hepatic
duct
• The common hepatic duct, which fuses with the
cystic duct
• These two ducts form the bile duct
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Composition of Bile
• A yellow-green, alkaline solution containing bile salts, bile
pigments, cholesterol, neutral fats, phospholipids, and
electrolytes
• Bile salts emulsify fat to facilitate fat and cholesterol
absorption
• The main bile pigment is bilirubin, a waste product of
heme
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The Gallbladder
• Thin-walled, green muscular sac on the ventral surface of
the liver
• Stores and concentrates bile by absorbing its water and
ions
• Releases bile via the cystic duct, which flows into the bile
duct
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Pancreas
• Location
• Lies deep to the greater curvature of the stomach
• The head is encircled by the duodenum and the tail
is close to the spleen
• Exocrine function
• Secretes pancreatic juice which breaks down all
food
• The pancreas also has an endocrine function – release of
insulin and glucagon
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Figure 24.21a, b
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Composition and Function of Pancreatic Juice
• Water solution of enzymes and electrolytes (primarily
HCO3–)
• Neutralizes acid chyme
• Provides optimal environment for pancreatic enzymes
• Enzymes
• Amylase, lipases, nucleases are secreted in active form
but require ions or bile for optimal activity
• Proteases secreted in inactive form
• Protease activation in duodenum
• The pancreas starts to synthesize enzymes before food
even arrive to the stomach
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Anatomy of the large intestine
• The large intestine extends from the ileum to the anus
• The opening from the ileum to the large intestine is guarded by the
ileocecal sphincter
• 4 major regions are:
• Cecum - expended pouch close to the ileum entrance to the large
intestine; Collects and stores materials from the ileum.
• The appendix is attached to the cecum. The mucosa and the
submucosa are dominate by lymphoid nodules and it
primary function is as an organ of the lymphatic system
• Colon – divided into ascending, transverse, descending and sigmoid
portions
• Rectum – last part of GI tract
• Anal canal – last portion of rectum; anus is the opening of the anal
canal
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Large Intestine
• Has unique features:
• Teniae coli – three bands of longitudinal smooth
muscle in its muscularis
• Haustra – pocketlike sacs caused by the tone of the
teniae coli
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Large intestine histology
• The ET of the mucosa contain mainly absorptive and
goblet cells.
• both cell types are found in long, straight intestinal glands
– crypts of Lieberkuhn – that extend the full thickness of
the mucosa.
• Colon mucosa is simple columnar epithelium except in
the anal canal
• Anal canal mucosa is stratified squamous epithelium
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Functions of the Large Intestine
• Digestion of enteric bacteria
• Reabsorption Vitamins, water, and electrolytes
• Propulsion of fecal material toward the anus
• Though essential for comfort, the colon is not essential for
life
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Bacterial Flora
• The bacterial flora of the large intestine consist of:
• Bacteria surviving the small intestine that enter the
cecum
• Those entering via the anus
• These bacteria:
• Colonize the colon
• Ferment indigestible carbohydrates
• Synthesize B complex vitamins and vitamin K
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Valves and Sphincters of the Rectum and Anus
• The anus has two sphincters:
• Internal anal sphincter composed of smooth muscle
• External anal sphincter composed of skeletal
muscle
• These sphincters are closed except during defecation
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Defecation
• Distension of rectal walls caused by feces:
• Stimulates contraction of the rectal walls
• Relaxes the internal anal sphincter
• Voluntary signals stimulate relaxation of the external anal
sphincter and defecation occurs
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