Transcript Fat Digestion and Absorption

C. Chace Tydell, DVM
25-1
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Veterinarian/surgeon for more than 15 years
Immunology researcher for 10 years
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
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UCI
Caltech
Scientific publications
Taught Developmental Biology at Caltech
25-2
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Your name
Your career goals
One disease about which you would like to know
more

Does someone in your family have a health problem?
Diabetes?

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Hepatitis?
25-3
Chapter 25
Lecture Outline
Copyright (c) The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
25-4
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-osis = condition or process
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-itis = inflammation
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eg- gastritis, colitis, hepatitis
-ostomy = a surgically created opening for waste
products to move out of the body
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eg- cyanosis, cirrhosis, leukocytosis
colostomy, ileostomy, ostomy
-scopy = to view, to have a look
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colonoscopy, gastroscopy, endoscopy
25-5
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Mechanical digestion
physical breakdown of food into smaller particles
 teeth and churning action of stomach and intestines
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Chemical digestion
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series of hydrolysis reactions that break macromolecules into
their monomers
enzymes from saliva, stomach, pancreas and intestines
results
 polysaccharides into monosaccharides
 proteins into amino acids
 fats into glycerol and fatty acids
25-6
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Ingestion
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Digestion
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breakdown of molecules
Absorption
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intake of food
uptake nutrients into blood/lymph
Defecation
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elimination of undigested material
25-7
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Motility
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Secretion
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muscular contractions that break up food, mix it with
enzymes and move it along
digestive enzymes and hormones
Membrane transport
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absorption of nutrients
25-8
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Digestive tract (GI tract)
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30 foot long tube extending
from
mouth to anus
Accessory organs
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teeth, tongue, liver,
gallbladder,
pancreas, salivary glands
25-9
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Mucosa
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Submucosa
Muscularis externa
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epithelium
lamina propria
muscularis mucosae
inner circular layer
outer longitudinal layer
Adventitia or Serosa
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areolar tissue or mesothelium
25-10
25-11
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Able to function independently of CNS
Composed of two nerve networks
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submucosal plexus
 controls glandular secretion of mucosa
 contractions of muscularis mucosae
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myenteric plexus
 controls peristalsis
 contractions of muscularis externa
Peristalsis is alternate waves of longitudinal and
circular muscular contraction that pushes food along
the alimentary canal.
25-12
25-13
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Serous membrane that lines the peritoneal cavity of the
abdomen and covers the mesenteries and viscera
Of the GI tract, only duodenum, pancreas and parts of
large intestine are retroperitoneal
Dorsal mesentery suspends GI tract and forms serosa
(visceral peritoneum) of stomach and intestines
Ventral mesentery forms lesser and greater omentum

lacy layer of connective tissue that contains lymph nodes,
lymphatic vessels, blood vessels
25-14
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Lesser - attaches stomach to liver
Greater - covers small intestines like an apron
25-15
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Mesentery of small intestines holds many blood vessels
Mesocolon anchors colon to posterior body wall
25-16
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Neural control
short myenteric reflexes (swallowing)
 long vagovagal reflexes (parasympathetic
stimulation of digestive motility and secretion)
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Hormones
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messengers diffuse into bloodstream, distant
targets
Paracrine secretions
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messengers diffuse to nearby target cells
25-17
25-18
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Cheeks and lips
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Tongue is sensitive, muscular manipulator of food
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keep food between teeth for chewing; essential for speech
and suckling in infants
vestibule - space between teeth and cheeks
lips: cutaneous area versus red area (vermilion)
papillae and taste buds on dorsal surface
lingual glands secrete saliva, tonsils in root
Hard and soft palate
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allow breathing and chewing at same time
palatoglossal and palatopharyngeal arches
25-19
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Baby teeth (20) by 2 years; Adult
(32) between 6 and 25
Occlusal surfaces and cusp
numbers differ
25-20
25-21
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Periodontal ligament is
modified periosteum
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Cementum and dentin
are living tissue
Enamel is noncellular
secretion formed during
development
Root canal leads into
pulp cavity
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anchors into alveolus
nerves and blood vessels
Gingiva or gums
25-22
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Breaks food into smaller pieces to be swallowed
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 surface area exposed to digestive enzymes
Contact of food with sensory receptors triggers
chewing reflex
tongue, buccinator and orbicularis oris manipulate
food
 masseter and temporalis elevate the teeth to crush
food
 medial and lateral pterygoids swing teeth in side-toside grinding action of molars
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25-23
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Functions of saliva
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moisten
begin starch and fat digestion
cleanse teeth
inhibit bacteria
bind food together into bolus
25-24
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Functions of saliva
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moisten, begin starch and fat digestion, cleanse teeth, inhibit
bacteria, bind food together into bolus
Hypotonic solution of 99.5% water and solutes
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salivary amylase, begins starch digestion
lingual lipase, digests fat activated by stomach acid
mucus, aids in swallowing
lysozyme, enzyme kills bacteria
 immunoglobulin A, inhibits bacterial growth
 electrolytes = Na+, K+, Cl-, phosphate and bicarbonate
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pH of 6.8 to 7.0
25-25
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Small intrinsic glands
found under mucous
membrane of mouth,
lips, cheeks and
tongue - secrete at
constant rate
3 pairs extrinsic
glands connected to
oral cavity by ducts
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parotid
submandibular
sublingual
25-26
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Compound tubuloacinar
glands (see pg 177)
Mucous cells secrete
mucus
Serous cells secrete thin
fluid rich in amylase
Mixed acinus has both
25-27
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Total of 1 to 1.5 L of saliva per day
Cells filter water from blood and add other substances
Food stimulates receptors that signal salivatory
nuclei in medulla and pons
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parasympathetic stimulation  salivary glands produce thin
saliva, rich in enzymes
sympathetic stimulation  produce less abundant, thicker
saliva, with more mucus
Higher brain centers stimulate salivatory nuclei so sight,
smell and thought of food cause salivation
25-28
25-29
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Skeletal muscle
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deep layer –
longitudinal
orientation
superficial layer –
circular
orientation
 superior, middle
and inferior
pharyngeal
constrictors
25-30
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Straight muscular tube 25-30 cm long
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Extends from pharynx to cardiac stomach passing
through esophageal hiatus in diaphragm
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nonkeratinized stratified squamous epithelium
esophageal glands in submucosa
skeletal muscle in upper part and smooth in bottom
inferior pharyngeal constrictor excludes air from it
Lower esophageal sphincter closes orifice to reflux
25-31
25-32
25-33
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Series of muscular contractions coordinated by
centers in the brain
Buccal phase
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tongue collects food and pushes it back into oropharynx
Pharyngeal-esophageal phase
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soft palate rises and blocks nasopharynx
infrahyoid muscles lift larynx; epiglottis folded back
pharyngeal constrictors push bolus down esophagus
 liquids in 2 seconds -- food bolus may take 8 seconds
 lower esophageal sphincter relaxes
25-34
25-35
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Mechanically breaks up food, liquifies food and begins
chemical digestion of protein and fat
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resulting soupy mixture is called chyme
Does not absorb significant amount of nutrients
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absorbs aspirin and some lipid-soluble drugs
25-36
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Muscular sac (internal volume from 50ml to 4L)
J - shaped organ with lesser and greater curvatures
 regional differences
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cardiac region just inside cardiac orifice
fundus - domed portion superior to esophageal opening
body - main portion of organ
pyloric region - narrow inferior end
 antrum and pyloric canal
Pylorus - opening to duodenum
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thick ring of smooth muscle forms a sphincter
25-37
Cardia
Fundus
Body
Pylorus
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Notice: bulge of fundus, narrowing of pyloric region, thickness
of pyloric sphincter and greater and lesser curvatures
25-38
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Innervation by
parasympathetic fibers from vagus
 sympathetic fibers from celiac plexus
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All blood from stomach enters hepatic portal
circulation and is filtered through liver before
returning to heart
25-39
25-40
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Mucosa
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simple columnar glandular epithelium
lamina propria is filled with tubular glands (gastric pits)
Muscularis externa has 3 layers
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outer longitudinal, middle circular and inner oblique layers
25-41
25-42
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Mucous cells
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Regenerative cells
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secrete HCl acid and intrinsic factor
Chief cells
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divide rapidly to produce new cells
that migrate to surface
Parietal cells
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secrete mucus
secrete pepsinogen
chymosin and lipase in infancy
Enteroendocrine cells
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G cells
 Make gastrin
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Others secrete hormones and
paracrine messengers
25-43
25-44
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Parietal cells contain carbonic anhydrase (CAH)
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CO2 + H2O  H2CO3  HCO3- + H+
H+ is pumped into stomach lumen by H+K+ATPase
 HCO3- in blood causes alkaline tide (blood pH )
2 to 3 L of gastric juice/day
(H2O, HCl and pepsin)
25-45
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Activates pepsin and lingual lipase
Breaks up connective tissues and plant cell walls
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Converts ingested ferric ions (Fe3+) to ferrous ions (Fe2+)
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liquefies food to form chyme
absorbed and used for hemoglobin synthesis
Destroys ingested bacteria and pathogens
25-46
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Intrinsic factor
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Pepsin - protein digestion
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essential for B12 absorption by small intestine
RBC production (lack causes pernicious anemia)
secreted as pepsinogen (inactive)
HCl converts it to pepsin (active)
Gastric lipase and chymosin
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lipase digests butterfat of milk in infant
chymosin curdles milk by coagulating proteins
25-47
25-48
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Many produced by enteroendocrine cells
hormones enter blood  distant cells
 paracrine secretions  neighboring cells
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Gut-brain peptides
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signaling molecules produced in digestive tract and
CNS
25-49
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Swallowing center signals stomach to relax
Food stretches stomach activating a receptive-relaxation
response
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resists stretching briefly, but relaxes to hold more food
Rhythm of peristalsis controlled by pacemaker cells in
longitudinal muscle layer
gentle ripple of contraction every 20 seconds churns and mixes
food with gastric juice
 stronger contraction at pyloric region; ejects 3 ml
 typical meal emptied from stomach in 4 hours
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25-50
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Induced by
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excessive stretching of stomach, psychological
stimuli or chemical irritants (bacterial toxins)
Emetic center in medulla causes
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retching
 lower esophageal sphincter to relax
 stomach and duodenum to contract spasmodically
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vomiting
 when abdominal contraction forces upper esophageal
sphincter to open
25-51
25-52
25-53
25-54
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Cephalic phase
sight, smell, taste or thought of food
 vagus nerve stimulates gastric secretion and motility
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Gastric phase
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activated by presence of food or semidigested protein
 by stretch or  in pH
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secretion stimulated by ACh (from parasympathetic fibers),
histamine (from gastric enteroendocrine cells) and gastrin (from
pyloric G cells)
25-55
25-56
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Intestinal phase - duodenum regulates gastric
activity through hormones and nervous reflexes
at first gastric activity increases (if duodenum is stretched
or amino acids in chyme cause gastrin release)
 enterogastric reflex - duodenum inhibits stomach

 caused by acid and semi-digested fats in duodenum
 chyme stimulates duodenal cells to release secretin,
cholecystokinin (CCK) and gastric inhibitory peptide
 all 3 suppress gastric secretion and motility
25-57
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All release important secretions into small intestine
to continue digestion
25-58
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3 lb. organ located inferior to the diaphragm
4 lobes - right, left, quadrate and caudate
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falciform ligament separates left and right
round ligament, remnant of umbilical vein
Gallbladder adheres to ventral surface between right
and quadrate lobes
25-59
25-60
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Tiny cylinders called hepatic lobules (2mm by 1mm)
Central vein surrounded by sheets of hepatocyte cells separated
by sinusoids lined with fenestrated epithelium
Blood filtered by hepatocytes on way to central vein
25-61
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3 structures found in corner between lobules
hepatic portal vein and hepatic artery bring blood to liver
 bile duct collects bile from bile canaliculi between sheets of
hepatocytes to be secreted from liver in hepatic ducts
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25-62
25-63
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Bile passes from bile canaliculi between cells to bile
ductules to right and left hepatic ducts
Right and left ducts join outside liver to form
common hepatic duct
Cystic duct from gallbladder joins common hepatic
duct to form bile duct
Duct of pancreas and bile duct combine to form
hepatopancreatic ampulla emptying into
duodenum at major duodenal papilla

sphincter of Oddi (hepatopancreatic sphincter) regulates
release of bile and pancreatic juice
25-64
25-65
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Sac on underside of liver -- 10 cm long
500 to 1000 mL bile are secreted daily from liver
Gallbladder stores and concentrates bile
Yellow-green fluid containing minerals, bile acids,
cholesterol, bile pigments and phospholipids

bilirubin pigment from hemoglobin breakdown
 intestinal bacteria convert to urobilinogen = brown color

bile acid (salts) emulsify fats and aid in their digestion
 enterohepatic circulation - recycling of bile acids from ileum
25-66
25-67
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Retroperitoneal gland posterior to stomach
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Endocrine and exocrine gland
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head, body and tail
secretes insulin and glucagon into the blood
secretes 1500 mL pancreatic juice into duodenum
Pancreatic duct runs length of gland to open at
sphincter of Oddi
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accessory duct opens independently on duodenum
25-68
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Zymogens = proteases
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trypsinogen
chymotrypsinogen
procarboxypeptidase
Other enzymes
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amylase
lipase
ribonuclease and
deoxyribonuclease
25-69
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Trypsinogen converted to trypsin by intestinal epithelium
Trypsin converts other 2 (also digests dietary protein)
25-70
Can you suggest
one cause?
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Activated enzymes accumulate in the pancreas,
overwhelm the inhibitors, and begin to digest the cells
of the pancreas, causing severe inflammation
25-71
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Cholecystokinin (CCK) released from duodenum in
response to arrival of acid and fat
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Secretin released from duodenum in response to
acidic chyme
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causes contraction of gallbladder, secretion of pancreatic
enzymes, relaxation of hepatopancreatic sphincter
stimulates all ducts to secrete more bicarbonate
Gastrin from stomach and duodenum weakly
stimulates gallbladder contraction and pancreatic
enzyme secretion
25-72
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Nearly all chemical
digestion and
nutrient
absorption occurs
in small intestine
25-73
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Duodenum curves around head of pancreas (10 in.)
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Jejunum - next 8 ft. (in upper abdomen)
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retroperitoneal along with pancreas
receives stomach contents, pancreatic juice and bile
neutralizes stomach acids, emulsifies fats, pepsin inactivated by
pH increase, pancreatic enzymes
has large tall circular folds; walls are thick, muscular
most digestion and nutrient absorption occur here
Ileum - last 12 ft. (in lower abdomen)
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
has Peyer’s patches – clusters of lymphatic nodules
ends at ileocecal junction with large intestine
25-74
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Circular folds (plicae circularis) up to 10 mm tall
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Villi are fingerlike projections
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involve only mucosa and submucosa
chyme flows in spiral path causing more contact
1 mm tall
contain blood vessels and
lymphatics (lacteals)
Microvilli 1 micron tall
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brush border on cells
brush border enzymes for
final stages of digestion
25-75
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Pores opening between villi
lead to intestinal crypts
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absorptive cells
goblet cells
rapidly dividing cells
life span of 3-6 days
Paneth cells – antibacterial
secretions
25-76
25-77
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Mixes chyme with enzymes and bile
Churns chyme to increase contact with mucosa for
absorption and digestion
Moves residue towards large intestine
25-78
Segmentation
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random ringlike constrictions mix and churn contents
12 times per minute in duodenum
Peristaltic waves begin in duodenum but each one moves
further down
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
push chyme along for 2 hours
suppressed by refilling of stomach
Food in stomach causes gastroileal reflex (relaxing of
valve and filling of cecum)
25-79
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Purpose of segmentation is to mix and churn not to move
material along as in peristalsis
25-80
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Gradual movement of
contents towards colon
Begins after absorption
occurs
Migrating motor
complex controls waves
of contraction

second wave begins distal
to where first wave began
25-81
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Salivary amylase stops working in stomach (pH < 4.5)

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50% of dietary starch digested before it reaches small intestine
Pancreatic amylase completes first step in 10 minutes
Brush border enzymes act upon oligosaccharides, maltose,
sucrose, lactose and fructose

lactose indigestible after age 4 in most humans (lactase declines)
25-82
Glucose may also be
transported by
“solvent drag”.
Water leaving the
stomach or intestine
‘drags’ solutes with it.
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Sodium-glucose transport proteins (SGLT) in membrane help
absorb glucose and galactose
Fructose absorbed by facilitated diffusion then converted to
glucose inside the cell
25-83

Pepsin has optimal pH of 1.5 to 3.5 -- inactivated when
passes into duodenum and mixes with alkaline
pancreatic juice (pH 8)
25-84

Pancreatic enzymes take over protein digestion by hydrolyzing
polypeptides into shorter oligopeptides
25-85

Brush border enzymes finish task, producing amino acids that are
absorbed into intestinal epithelial cells


amino acid cotransporters move into epithelial cells and facilitated diffusion
moves amino acids out into blood stream
Infants absorb proteins by pinocytosis (maternal IgA)
25-86
Bile acids are the
same as bile salts.
25-87
25-88
Chylomicrons are
produced in the
epithelial cells of
the small intestine
as lipids are
absorbed. The
triglycerides (lipids)
are taken up by the
lacteals NOT the
capillaries.
25-89

Nucleases hydrolyze DNA and RNA to nucleotides


Vitamins are absorbed unchanged


nucleosidases and phosphatases of brush border split them
into phosphate ions, ribose or deoxyribose sugar and
nitrogenous bases
A, D, E and K with other lipids -- B complex and C by simple
diffusion and B12 if bound to intrinsic factor
Minerals are absorbed all along small intestine
Na+ cotransported with sugars and amino acids
 Cl- exchanged for bicarbonate by stomach
 Iron and calcium absorbed as needed

25-90

Digestive tract receives about 9 L of water/day
.7 L in food, 1.6 L in drink, 6.7 L in secretions
 8 L is absorbed by small intestine and 0.8 L by large intestine



Water is absorbed by osmosis following the absorption
of salts and organic nutrients
Diarrhea occurs when too little water is absorbed
25-91
25-92
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
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
5 feet long and 2.5 inches in diameter in cadaver
Begins as cecum and appendix in lower right corner
Ascending, transverse and descending colon frame
the small intestine
Sigmoid colon is S-shaped portion leading down
into pelvis
Rectum - straight portion ending at anal canal
25-93

Mucosa - simple columnar epithelium




No circular folds or villi to increase surface area
Intestinal crypts (glands sunken into lamina propria)
produce mucus only
Muscularis externa


anal canal has stratified squamous epithelium
muscle tone in longitudinal muscle fibers (concentrated in
taeniae coli) form pouches (haustra)
Transverse and sigmoid have a serosa, rest
retroperitoneal

epiploic appendages are suspended fatty sacs
25-94

Bacterial flora populate large intestine




Make up about 30% of the volume of feces
ferment cellulose and other undigested carbohydrates;
we absorb resulting sugars
synthesize vitamins B and K
Flatus (gas)


average person produces 500 mL per day
most is swallowed air but hydrogen sulfide, indole and
skatole produce odor
25-95

Transit time is 12 to 24 hours




reabsorbs water and electrolytes
Feces consist of water and solids (bacteria, mucus,
undigested fiber, fat and sloughed epithelial cells)
Haustral contractions occur every 30 minutes
Mass movements occur 1 to 3 times a day

triggered by gastrocolic and duodenocolic reflexes
25-96
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

Anal canal is 3 cm
total length
Anal columns are
longitudinal ridges
separated by mucus
secreting anal
sinuses
Hemorrhoids are
permanently
distended veins
25-97
1. Filling of the rectum stimulates
stretch receptors
2. A spinal reflex stimulates
contraction of rectum
3. The reflex also relaxes the
internal anal sphincter
3. The brain controls voluntary
relaxation of external
sphincter
25-98
25-99