Transcript Digestive system B

Chapter 23B
Digestive System
Slides by Barbara Heard and W. Rose.
figures from Marieb & Hoehn 9th ed.
Portions copyright Pearson Education
Digestive System
Introduction/Overview
Functional Anatomy
Mouth, Pharynx, Esophagus
Digestive Processes in the Mouth
Stomach
Small Intestine & Related Organs
Large Intestine
Physiology of Digestion & Absorption
Mouth
Oral (buccal) cavity
– Bounded by lips, cheeks, palate, and tongue
– Lined with stratified squamous epithelium
Functions
– Ingestion
– Mechanical digestion
– Chemical digestion
– Propulsion
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Figure 23.7a Anatomy of the oral cavity (mouth).
Soft
palate
Palatoglossal
arch
Uvula
Hard
palate
Oral cavity
Palatine
tonsil
Tongue
Oropharynx
Lingual tonsil
Epiglottis
Hyoid bone
Laryngopharynx
Esophagus
Trachea
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Sagittal section of the oral cavity and pharynx
Figure 23.7b Anatomy of the oral cavity (mouth).
Upper lip
Gingivae
(gums)
Palatine
raphe
Hard
palate
Soft
palate
Uvula
Palatine
tonsil
Superior
labial
frenulum
Palatoglossal
arch
Palatopharyngeal
arch
Posterior wall
of oropharynx
Tongue
Sublingual
fold with
openings of
sublingual
ducts
Lingual frenulum
Opening of
Submandibular
duct
Gingivae (gums)
Oral vestibule
Lower lip
Anterior view
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Inferior labial
frenulum
Tongue
Skeletal muscle
Functions include
– Repositioning and mixing food during chewing
– Formation of bolus
– Initiation of swallowing, speech, and taste
Intrinsic muscles change shape of tongue
Extrinsic muscles alter tongue's position
Lingual frenulum: attachment to floor of mouth
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Tongue
Surface bears papillae
– Some contain taste buds
– All help give traction on food
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Figure 23.8 Dorsal surface of the tongue, and the tonsils.
Epiglottis
Palatopharyngeal
arch
Palatine tonsil
Lingual tonsil
Palatoglossal
arch
Terminal sulcus
Foliate papillae
Vallate papilla
Medial sulcus
of the tongue
Dorsum of tongue
Fungiform papilla
Filiform papilla
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Salivary Glands
•
•
•
•
Lie outside oral cavity
Parotid
Submandibular
Sublingual
Functions of saliva
•
•
•
•
Cleanses mouth
Dissolves food chemicals for taste
Moistens food; compacts into bolus
Begins breakdown of starch with enzymes
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Figure 23.9 The salivary glands.
Tongue
Teeth
Ducts of
sublingual
gland
Frenulum
of tongue
Sublingual
gland
Parotid
gland
Parotid duct
Masseter muscle
Body of mandible
(cut)
Posterior belly of
digastric muscle
Mylohyoid
muscle (cut)
Submandibular
duct
Anterior belly of
digastric muscle
Submandibular
gland
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Mucous cells Serous cells
forming demilunes
Salivary Glands
• Two types of secretory cells
– Serous cells
• Watery, enzymes, ions, bit of mucin
– Mucous cells
• Mucus
• Parotid, submandibular glands mostly
serous; sublingual mostly mucous
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Composition of Saliva
• 97–99.5% water, slightly acidic
– Electrolytes—Na+, K+, Cl–, PO4 2–, HCO3–
– Salivary amylase and lingual lipase
– Mucin
– Metabolic wastes—urea and uric acid
– Lysozyme, IgA, defensins, and a cyanide
compound protect against microorganisms
PLAY
Animation: Rotating head
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Control of Salivation
• 1500 ml/day
• Intrinsic glands continuously keep mouth moist
• Major salivary glands activated by
parasympathetic nervous system when
– Ingested food stimulates chemoreceptors and
mechanoreceptors in mouth 
– Salivatory nuclei in brain stem send impulses along
parasympathetic fibers in cranial nerves VII and IX
• Strong sympathetic stimulation inhibits salivation
and results in dry mouth (xerostomia)
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Teeth
Mechanical digestion
• Incisors cut
• Canines tear or pierce
• Premolars (bicuspids) grind & crush
• Molars grind & crush
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Figure 23.10 Human dentition.
Incisors
Central (6–8 mo)
Lateral (8–10 mo)
Canine (eyetooth)
(16–20 mo)
Molars
First molar
(10–15 mo)
Second molar
(about 2 yr)
Deciduous
(milk) teeth
Incisors
Central (7 yr)
Lateral (8 yr)
Canine (eyetooth)
(11 yr)
Premolars
(bicuspids)
First premolar
(11 yr)
Second premolar
(12–13 yr)
Molars
First molar (6–7 yr)
Second molar
(12–13 yr)
Third molar
(wisdom tooth)
(17–25 yr)
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Permanent
teeth
Number and types of Teeth
Primary:
Permanent:
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Figure 23.11 Longitudinal section of a canine tooth within its bony socket (alveolus).
Enamel
Dentin
Crown
Neck
Dentinal
tubules
Pulp cavity
(contains
blood vessels
and nerves)
Gingival
sulcus
Gingiva
(gum)
Cement
Root
Root canal
Periodontal
ligament
Apical
foramen
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Bone
Tooth & gum disease
Cavities (dental caries) due to destruction of
enamel & dentin by lactic acid made by
bacteria digesting sugar in a film on tooth
surface.
Periodontal (gum) disease: affects up to 95%
over age 35, accounts for 80-90% of adult
tooth loss. Due to bacterial infection btn tooth
& gum, preventable by flossing & brushing.
College or Department name here
Pharynx = throat, passes air and food via
sequential contraction of muscles
College or Department name here
Esophagus
• Carries food from throat to stomach,
collapsed when empty
• Diaphragm and esophageal sphincter (and
gravity) keep food in stomach
• Heartburn: often due to "gastroesophageal
(GE) reflux" (stomach contents spurting up
into esophagus), which can be caused by
hiatal hernia (top of stomach protrudes
above diaphragm)
College or Department name here
Deglutition=swallowing
• Involves coordinated contraction of
muscles in tongue, soft palate, pharynx,
esophagus
• Buccal phase
– Voluntary contraction of tongue
• Pharyngeal-esophageal phase
– Involuntary – primarily vagus nerve
– Control center in the medulla and lower pons
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Figure 23.13
Deglutition (swallowing)
Bolus of food
Tongue
Uvula
Pharynx
Bolus
Epiglottis
Epiglottis
Glottis
Upper
esophageal
sphincter
Trachea
Esophagus
1
During the buccal phase, the upper
esophageal sphincter is contracted.
The tongue presses against the hard
palate, forcing the food bolus into the
oropharynx.
2
The pharyngeal-esophageal phase
begins as the uvula and larynx rise to prevent
food from entering respiratory passageways.
The tongue blocks off the mouth. The upper
esophageal sphincter relaxes, allowing food
to enter the esophagus.
3 The constrictor muscles of the
pharynx contract, forcing food into
the esophagus inferiorly. The upper
esophageal sphincter contracts
(closes) after food enters.
5
Relaxed muscles
Circular muscles
contract
Bolus of food
4
Peristalsis moves
food through the
esophagus to the
stomach.
Longitudinal muscles
contract
Relaxed
muscles
The gastroesophageal
sphincter surrounding the
cardial oriface opens, and
food enters the stomach.
Circular muscles contract
Gastroesophageal
sphincter closed
Gastroesophageal
sphincter opens
Stomach
Bolus
Stomach: Gross Anatomy
Cardia
Fundus
Body
Pyloric region
Pyloric sphincter (valve)
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Figure 23.14a Anatomy of the stomach.
Cardia
Fundus
Esophagus
Muscularis
externa
• Longitudinal layer
• Circular layer
• Oblique layer
Serosa
Body
Lumen
Lesser
curvature
Rugae of
mucosa
Greater
curvature
Duodenum
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Pyloric sphincter
(valve) at pylorus
Pyloric
canal
Pyloric
antrum
Figure 23.30b Mesenteries of the abdominal digestive organs.
Liver
Gallbladder
Lesser omentum
Stomach
Duodenum
Transverse colon
Small intestine
Cecum
Urinary bladder
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Figure 23.30c Mesenteries of the abdominal digestive organs.
Greater omentum
Transverse colon
Transverse
mesocolon
Descending colon
Jejunum
Mesentery
Sigmoid
mesocolon
Sigmoid colon
Ileum
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Figure 23.15a Microscopic anatomy of the stomach.
Surface
epithelium
Mucosa
Lamina
propria
Muscularis
mucosae
Submucosa
(contains
submucosal
Oblique
plexus)
layer
Muscularis
Circular
externa
layer
(contains
Longitudinal
myenteric
layer
plexus)
Stomach wall
Serosa
Layers of the stomach wall
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Figure 23.15b Microscopic anatomy of the stomach.
Gastric pits
Surface epithelium
(mucous cells)
Gastric
pit
Mucous neck cells
Parietal cell
Gastric
gland
Chief cell
Enteroendocrine cell
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Enlarged view of gastric pits and
gastric glands
Figure 23.15c Microscopic anatomy of the stomach.
Pepsinogen
Pepsin
HCI
Mitochondria
Parietal cell
Chief cell
Enteroendocrine
cell
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Location of the HCl-producing parietal cells
and pepsin-secreting chief cells in a gastric
gland
Gastric Gland Secretions
• Parietal cells
• Hydrochloric acid (HCl): pH 1.5–3.5 denatures
protein, activates pepsin, breaks down plant cell
walls, kills bacteria
• Intrinsic factor
• Glycoprotein required for vitamin B12 absorption
• Chief cells
• Pepsinogen: Inactive protease; activated to pepsin by
HCl and by pepsin itself (positive feedback)
• Lipases
• Enteroendocrine cells
• Serotonin1, histamine1, somatostatin1,2, gastrin2
1. Paracrine (acts locally); 2. Hormone
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Mucosal Barrier
• Harsh digestive conditions in stomach
• Has mucosal barrier to protect
– Thick layer of bicarbonate-rich mucus
– Tight junctions between epithelial cells
• Prevent juice seeping underneath tissue
– Damaged epithelial cells quickly replaced by
division of stem cells
• Surface cells replaced every 3–6 days
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Homeostatic Imbalance
• Gastritis
– Inflammation caused by anything that
breaches mucosal barrier
• Peptic or gastric ulcers
– Erosions of stomach wall
• Can perforate  peritonitis; hemorrhage
– Most caused by Helicobacter pylori bacteria
– Some by NSAIDs
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Figure 23.16 Photographs of a gastric ulcer and the H. pylori bacteria that most commonly cause it.
Bacteria
Mucosa
layer of
stomach
A gastric ulcer lesion
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H. pylori bacteria
Digestive Processes in the Stomach
• Mechanical breakdown
• Chemical digestion
– Denaturation of proteins by HCl
– Enzymatic digestion of proteins by pepsin
• Propulsion: delivers chyme to small
intestine
• Absorption: alcohol, aspirin, secretion of
intrinsic factor for B12 absorption in S.I.
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Neural & Hormonal Regulation
of Gastric Secretion
• Neural
– Vagus nerve activity  secretion 
– Sympathetic activity  secretion 
• Hormonal
– Gastrin
• Gastrin  ⇒ enzyme and HCl secretion 
• Most small intestine secretions are gastrin
antagonists
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Phases of Gastric Secretion
• Cephalic (reflex) phase: conditioned reflex
triggered by aroma, taste, sight, thought
• Gastric phase: 3–4 hours
– Stimulated by gastrin (from G cells),
distension, peptides, low acidity
• Intestinal phase
– Chyme entering S.I. is initially and briefly
stimulatory to stomach
– Later, chyme in SI inhibits stomach
secretion: enterogastric reflex
– Too much chyme entering SI ⇒ dumping
syndrome (nausea, vomiting), common
after gastric reduction surgery
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Figure 23.17
Neural and hormonal regulation of gastric secretion
Inhibitory events
Stimulatory events
Cephalic
phase
Gastric
phase
1 Sight and thought
of food
Cerebral cortex
Conditioned reflex
2 Stimulation of
taste and smell
receptors
Hypothalamus
and medulla
oblongata
1 Stomach
distension
activates
stretch
receptors
Vagovagal
reflexes
Intestinal
phase
Stimulate
Inhibit
Vagus
nerve
Local
reflexes
2 Food chemicals
G cells
(especially peptides and
caffeine) and rising pH
activate chemoreceptors
1 Presence of
partially digested
foods in duodenum
or distension of the
duodenum when
stomach begins to
empty
Medulla
Vagus
nerve
Lack of
stimulatory
impulses to
parasympathetic
center
Cerebral
cortex
Gastrin
secretion
declines
G cells
Overrides
parasympathetic
controls
Sympathetic
nervous
system
activation
1 Loss of
appetite,
depression
1 Excessive
acidity
(pH < 2)
in stomach
2 Emotional
stress
Gastrin
release
to blood
Intestinal
(enteric)
gastrin
release
to blood
Stomach
secretory
activity
Enterogastric
reflex
Brief
effect
Local
reflexes
Vagal
nuclei
in medulla
Pyloric
sphincter
Release of
enterogastrones
(secretin, cholecystokinin,
vasoactive intestinal
peptide)
1 Distension
of duodenum;
presence of
fatty, acidic, or
hypertonic
chyme; and/or
irritants in
the duodenum
2 Distension;
presence of
fatty, acidic,
partially
digested food
in the
duodenum
Response of the Stomach to Filling
• Stretches to accommodate incoming food
– Pressure constant until 1.5 L food ingested
• Reflex-mediated receptive relaxation
– Coordinated by swallowing center of brain stem
– Gastric accommodation
• Plasticity (stress-relaxation response) of smooth
muscle (see Chapter 9)
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Gastric Contractile Activity
• Peristaltic waves move toward pylorus at
rate of 3 per minute
• Distension and gastrin increase force of
contraction
• Most vigorous near pylorus
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Figure 23.19 Deglutition (swallowing).
Pyloric
valve
closed
1 Propulsion: Peristaltic
waves move from the fundus
toward the pylorus.
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Slide 1
Pyloric
valve
closed
2 Grinding: The most
vigorous peristalsis and
mixing action occur close to
the pylorus.
Pyloric
valve
slightly
opened
3 Retropulsion: The pyloric
end of the stomach acts as a
pump that delivers small amounts
of chyme into the duodenum,
simultaneously forcing most of its
contained material backward into
the stomach.
Small Intestine: Gross Anatomy
• Major organ of digestion and absorption
• 2-4 m long; from pyloric sphincter to
ileocecal valve
• Subdivisions
– Duodenum (retroperitoneal)
– Jejunum (attached posteriorly by mesentery)
– Ileum (attached posteriorly by mesentery)
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Figure 23.1 Alimentary canal and related accessory digestive organs.
Mouth (oral cavity)
Tongue*
Parotid gland
Sublingual gland
Submandibular gland
Salivary
glands*
Pharynx
Esophagus
Stomach
Pancreas*
(Spleen)
Liver*
Gallbladder*
Transverse colon
Small
intestine
Anus
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Duodenum
Jejunum
Ileum
Descending colon
Ascending colon
Cecum
Sigmoid colon
Rectum
Appendix
Anal canal
Large
intestine
Figure 23.21 The duodenum of the small intestine, and related organs.
Right and left
hepatic ducts
of liver
Cystic duct
Common hepatic duct
Bile duct and sphincter
Accessory pancreatic duct
Mucosa
with folds
Tail of pancreas
Pancreas
Jejunum
Gallbladder
Major duodenal
papilla
Hepatopancreatic
ampulla and sphincter
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Main pancreatic duct and sphincter
Duodenum
Head of pancreas
Small Intestine
• Duodenum
0.25 m long; receives bile & pancreatic
enzymes; common bile duct, main pancreatic
duct join at hepatopancreatic ampulla, enter
duodenum at major duodenal papilla, controlled
by hepatopancreatic sphincter
• Jejunum
2.5 m long
• Ileum
3.6 m long; joins large intestine at ileocecal
valve
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Small Intestine
• Innervated by vagus nerve
(parasympathetic) and sympathetics from
thoracic splanchnic nerves
• Blood supply: Superior mesenteric artery
• Portal circulation
– Capillaries to portal vein to more capillaries:
– Nutrient-rich blood from small intestine caps
goes by hepatic portal vein to liver & liver
capillaries
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Structural Features to
Increase Surface Area
• Circular folds
Permanent folds (~1 cm deep) force chyme to
slowly spiral through lumen, increases nutrient
absorption
• Villi
Extensions (~1 mm high) of mucosa with
capillary bed and lacteal for absorption
• Microvilli (brush border)
Contain enzymes for carbohydrate and protein
digestion
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Figure 23.22a Structural modifications of the small intestine that increase its surface area for digestion and
absorption.
Vein carrying
blood to
hepatic portal
vessel
Muscle
layers
Circular
folds
Villi
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Lumen
Figure 23.22b Structural modifications of the small intestine that increase its surface area for digestion and
absorption.
Microvilli
(brush border)
Absorptive
cells
Lacteal
Goblet
cell
Blood
capillaries
Mucosaassociated
lymphoid
tissue
Intestinal
crypt
Muscularis
mucosae
Duodenal
gland
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Villus
Enteroendocrine
cells
Venule
Lymphatic vessel
Submucosa
Figure 23.22c Structural modifications of the small intestine that increase its surface area for digestion and
absorption.
Absorptive cells
Goblet
cells
Villi
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Intestinal crypt
Homeostatic Imbalance
• Chemotherapy targets rapidly dividing
cells
– Kills cancer cells
– Kills rapidly dividing GI tract epithelium 
nausea, vomiting, diarrhea
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Mucosa
• Peyer's patches protect especially distal
part against bacteria
– May protrude into submucosa
• B lymphocytes leave intestine, enter
blood, protect intestinal lamina propria with
their IgA
• Duodenal (Brunner's) glands of the
duodenum secrete alkaline mucus to
neutralize acidic chyme
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Intestinal Juice
• 1-2 L secreted daily in response to
distension or irritation of mucosa
• Slightly alkaline; isotonic with blood
plasma
• Largely water; enzyme-poor (enzymes of
small intestine only in brush border);
contains mucus
• Facilitates transport and absorption of
nutrients
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Accessory Organs:
Liver and Gallbladder
• Liver
– Many functions
– Digestive function: produce bile (emulsify fats)
– Largest gland
• Gallbladder
– Chief function: store bile
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Figure 23.24a
Liver
Sternum
Bare area
Liver
Falciform
ligament
Left lobe of
liver
Right lobe of liver
Gallbladder
Round ligament
(ligamentum
teres)
Figure 23.21
Liver, Pancreas, Duodenum
Right and left
hepatic ducts
of liver
Cystic duct
Common hepatic duct
Bile duct and sphincter
Accessory pancreatic duct
Mucosa
with folds
Tail of pancreas
Pancreas
Jejunum
Gallbladder
Major duodenal
papilla
Hepatopancreatic
ampulla and sphincter
Main pancreatic duct and sphincter
Duodenum
Head of pancreas
Figure 23.25a–b Microscopic anatomy of the liver.
Lobule
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Central Connective
vein tissue septum
Liver: Microscopic Anatomy
• Portal triad at each corner of lobule
– Branch of hepatic artery supplies oxygen
– Branch of hepatic portal vein brings nutrient-rich
blood
– Bile duct receives bile from bile canaliculi
• Liver sinusoids - leaky capillaries between
hepatic plates
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Interlobular veins
(to hepatic vein)
Central vein
Microscopic anatomy of the liver
Sinusoids
Bile canaliculi
Plates of
hepatocytes
Bile duct (receives
bile from bile
canaliculi)
Fenestrated
lining (endothelial
cells) of sinusoids
Stellate macrophages
in sinusoid walls
Portal vein
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Bile duct
Portal venule
Hepatic arteriole
Portal triad
Hepatocytes
• Abundant rough &
smooth ER, Golgi,
mitochondria
• Process bloodborne
nutrients
• Store fat-soluble
vitamins
• Detoxification
• Produce ~900 ml bile
per day
• Regenerative capacity
Jean Louis Cesar Lair, 1819
Homeostatic Imbalance
• Hepatitis
– Usually viral infection, drug toxicity, wild
mushroom poisoning
• Cirrhosis
– Progressive, chronic inflammation from
chronic hepatitis or alcoholism
– Liver  fatty, fibrous  portal hypertension
• Liver transplants successful, but livers
scarce
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Bile
• Yellow-green, alkaline solution containing
– Bile salts = compounds derived from
cholesterol. Help emulsify & absorb lipids.
– Bilirubin = pigment formed from heme
• Brown color of feces due to bilirubin, after bacteria
convert it to stercobilin
– Cholesterol
– Triglycerides, phospholipids, electrolytes
– Water
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The Gallbladder
• Thin-walled muscular sac on ventral
surface of liver
• Stores and concentrates bile by absorbing
water and ions
• Muscular contractions release bile via
cystic duct, which flows into bile duct
• “Biliary tree” = gall bladder plus the ducts
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Gallstones
• Cholelithiasis = gallstone formation
• “Stones” are crystals of compounds found in bile
• Most gallstones are cholesterol stones
– Bile salts help keep cholesterol in solution
– Cholesterol stones more likely to form if bile is low in bile salts
• Obstruct flow of bile from gallbladder
• May cause pain, obstructive jaundice, fat
malabsorption, diarrhea, …
• Tx: drugs, lithotripsy, minimally invasive removal
• If recurrent: cholecystectomy = gall bladder removal
– Many patients do OK without a gall bladder (must modify diet)
– Some have pain, diarrhea, bloating, etc
Pancreas
• Mostly retroperitoneal, deep to greater
curvature of stomach; head by duodenum,
tail by spleen
• Endocrine: pancreatic islets secrete insulin
and glucagon
• Exocrine: secretes pancreatic juice
– To duodenum via main pancreatic duct
– Zymogen granules contain proenzymes
– Secreted in inactive form, activated in gut
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Figure 23.26a
Enzyme-producing tissue of pancreas
Small
duct
Acinar cell
Basement
membrane
Zymogen
granules
Rough
endoplasmic
reticulum
Duct cell
One acinus
Pancreatic Juice
• 1200-1500 ml/day, alkaline (pH 8, HCO3-)
neutralizes chyme
• Enzymes
– Amylase, lipases, nucleases secreted in
active form but require ions or bile for optimal
activity
– Proteases secreted in inactive form, activated
in duodenum
– Trypsinogen activated to trypsin by brush
border enzyme enteropeptidase
– Procarboxypeptidase, chymotrypsinogen
activated by trypsin
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Figure 23.27
Activation of Pancreatic Proteases in Small Intestine
Stomach
Pancreas
Epithelial
cells
Membrane-bound
enteropeptidase
Trypsinogen
(inactive)
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Trypsin
Chymotrypsinogen
(inactive)
Chymotrypsin
Procarboxypeptidase
(inactive)
Carboxypeptidase
Regulation of Bile Secretion
• Bile secretion stimulated by secretin from
intestinal cells
• Gallbladder contraction stimulated by
– Cholecystokinin (CCK) from intestinal cells
exposed to acidic, fatty chyme
– Vagal activity
• CCK also causes
– Secretion of pancreatic juice
– Hepatopancreatic sphincter to relax
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Regulation of Pancreatic Secretion
• CCK induces secretion of enzyme-rich
pancreatic juice by acini
• Secretin causes secretion of bicarbonaterich pancreatic juice by duct cells
• Vagal stimulation also causes release of
pancreatic juice (minor stimulus)
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Figure 23.28
Promotion of Bile and Pancreatic Juice
Secretion and Release
1. Chyme entering
duodenum
causes duodenal
enteroendocrine
cells to release
cholecystokinin
(CCK) and
secretin.
2. CCK (red dots)
and secretin
(yellow dots)
enter blood.
3. CCK, secretin
induce pancreatic
juice secretion.
4. Bile salts &
secretin stimulate
liver to produce
more bile.
5. CCK causes
gallbladder to
contract and
hepatopancreatic
sphincter to
relax; bile enters
duodenum.
6. During cephalic
and gastric phases,
vagus nerve
activity causes
weak gallbladder
contractions.
CCK secretion
Secretin secretion
Digestion in the Small Intestine
• Chyme from stomach contains
– Partially digested carbohydrates and proteins
– Undigested fats
• 3–6 hours in small intestine
– Most water absorbed
– ~ All nutrients absorbed
• Small intestine, like stomach, no role in
ingestion or defecation
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Requirements for Digestion and Absorption
in the Small Intestine
• Slow delivery of acidic, hypertonic chyme
• Delivery of bile, enzymes, and bicarbonate
ions from liver and pancreas
• Mixing
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Motility of the Small Intestine
• Segmentation
– Most common motion of small intestine
– Initiated by intrinsic pacemaker cells
– Mixes/moves contents toward ileocecal valve
– Intensity altered by long & short reflexes;
hormones
• Parasympathetic  ; sympathetic 
– Wanes in late intestinal (fasting) phase
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Motility of the Small Intestine
• Peristalsis
– Initiated by rise in hormone motilin in late
intestinal phase; every 90–120 minutes
– Each wave starts distal to previous
• Migrating motor complex
– Meal remnants, bacteria, and debris moved to
large intestine
– From duodenum  ileum ~ 2 hours
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Figure 23.3a Peristalsis and segmentation.
From
mouth
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Peristalsis: Adjacent segments of alimentary
tract organs alternately contract and relax,
moving food along the tract distally.
Motility of the Small Intestine
• Local enteric neurons coordinate intestinal
motility
• Cholinergic sensory neurons may activate
myenteric plexus
– Causes contraction of circular muscle
proximally and of longitudinal muscle distally
– Forces chyme along tract
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Motility of the Small Intestine
• Ileocecal sphincter relaxes, admits
chyme into large intestine when
– Gastroileal reflex enhances force of
segmentation in ileum
– Gastrin increases motility of ileum
• Ileocecal valve flaps close when chyme
exerts backward pressure
– Prevents regurgitation into ileum
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Large Intestine
Cecum: connects to ileum at ileocecal valve
Appendix: part of MALT of immune system
Colon: more below
Rectum
Anal canal
• Opens to body exterior at anus
• Internal anal sphincter—smooth muscle
• External anal sphincter—skeletal muscle
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Unique features of Large Intestine
• Teniae coli
Three bands of longitudinal smooth muscle in
muscularis
• Haustra
Pocketlike sacs created by tone of teniae coli
• Epiploic appendages
Fat-filled pouches of visceral peritoneum
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Figure 23.29a
Large Intestine
Left colic
(splenic) flexure
Right colic
(hepatic) flexure
Transverse
mesocolon
Transverse colon
Epiploic
appendages
Superior
mesenteric artery
Descending colon
Haustrum
Ascending colon
IIeum
Cut edge of
mesentery
IIeocecal valve
Tenia coli
Sigmoid colon
Cecum
Appendix
Rectum
Anal canal
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External anal sphincter
Colon
• Retroperitoneal except for transverse and
sigmoid regions
• Ascending colon (right side – to level of
right kidney)
• Transverse colon
• Descending colon
• Sigmoid colon in pelvis  rectum
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Figure 23.30c
Mesenteries of the Abdomen
Greater omentum
Transverse colon
Descending colon
Jejunum
Mesentery
Sigmoid colon
Ileum
Figure 23.30d
Mesenteries of the Abdomen
Liver
Lesser omentum
Pancreas
Stomach
Duodenum
Transverse mesocolon
Transverse colon
Mesentery
Greater omentum
Jejunum
Ileum
Visceral peritoneum
Parietal peritoneum
Urinary bladder
Rectum
Figure 23.29b
Rectum and Anus
Rectum
Hemorrhoidal
veins
Anal canal
External anal
sphincter
Internal anal
sphincter
Anus
Bacterial Flora
• Enter from small intestine or anus
– Colonize colon
– Synthesize B complex vitamins and vitamin K
– Metabolize some host-derived molecules
(mucin, heparin, hyaluronic acid)
– Ferment indigestible carbohydrates
– Release irritating acids and gases (~500
ml/day)
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Digestive Processes in the Large Intestine
• Residue remains in large intestine 12–24
hours
• No food breakdown except by enteric
bacteria
• Vitamins (made by bacterial flora), water,
and electrolytes (especially Na+ and Cl–)
reclaimed
• Major functions - propulsion of feces to
anus; defecation
• Colon not essential for life
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Large Intestinal Motility
• Most colonic activity is haustral
contractions
– Slow segmenting movements
• Gastrocolic reflex
– Initiated by presence of food in stomach
– Activates three to four slow powerful
peristaltic waves per day in colon (mass
movements)
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Homeostatic Imbalance
• Irritable bowel syndrome
– Functional GI disorder
– Recurring abdominal pain, stool changes,
bloating, flatulence, nausea, depression
– Stress common precipitating factor
• Stress management important in treatment
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Figure 23.31
Defecation Reflex
Impulses from
cerebral cortex
(conscious
control)
Sensory
nerve fibers
Voluntary motor
nerve to external
anal sphincter
Sigmoid
colon
External anal
sphincter
(skeletal muscle)
Rectum
1 Feces move into and
distend the rectum,
stimulating stretch receptors
there. The receptors transmit
signals along afferent fibers
to spinal cord neurons.
Stretch receptors in wall
2 A spinal reflex is initiated in which
parasympathetic motor (efferent) fibers
stimulate contraction of the rectum and
sigmoid colon, and relaxation of the
internal anal sphincter.
Involuntary motor nerve
(parasympathetic division)
Internal anal sphincter (smooth muscle)
3 If it is convenient to defecate,
voluntary motor neurons are inhibited,
allowing the external anal sphincter to
relax so feces may pass.