Transcript Functions of the GI Tract - McGraw

Chapter 18
The Digestive System
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Functions of the GI Tract
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Motility:
Movement of of food through the GI tract.
 Ingestion:
 Taking food into the mouth.
 Mastication:
 Chewing the food and mixing it with saliva.
 Deglutition:
 Swallowing the food.
 Peristalsis:
 Rhythmic wave-like contractions that move food
through GI tract.
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Functions of the GI Tract
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Secretion:
Includes both exocrine and endocrine secretions.
 Exocrine:
 HCl, H20, HC03 , bile, lipase, pepsin, amylase, trypsin,
elastase,and histamine, etc. are secreted into the lumen
of the GI tract.
 Endocrine:
 Stomach and small intestine secrete hormones to help
regulate the GI system.
 Gastrin, secretin, CCK, GIP, GLP-1, guanylin, VIP, and
somatostatin, etc.
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Functions of the GI Tract
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Digestion:
Breakdown of food particles into subunits
(chemical structure change).
Absorption:
Process of the passage of digestion (chemical
subunits) into the blood or lymph.
Storage and elimination:
Temporary storage and elimination of
indigestible food.
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Digestive System
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GI tract divided into:
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Alimentary canal.
Accessory digestive
organs.
GI tract is 30 ft long
and extends from
mouth to anus.
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Layers of GI Tract
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Mucosa
Submucosa
Muscularis
Serosa
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Mucosa
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Lines the lumen of GI tract.
Consists of simple columnar epithelium.
Lamina propria: Thin layer of connective
tissue containing lymph nodes.
Muscularis mucosae:
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Thin layer of smooth muscle responsible for the
folds.
Increase surface area.
Goblet cells:

Secrete mucus.
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Submucosa
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Thick, highly vascular layer of
connective tissue.
Absorbed molecules enter the blood and
lymph vessels in this layer.
Submucosal plexuses:

Provide autonomic nerve supply to the
muscularis mucosae.
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Muscularis
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Responsible for segmental contractions and
peristaltic movement through the GI tract.
Inner circular smooth muscle.
Outer longitudinal smooth muscle.
Contractions of these layers move food
through the tract and pulverize the food.
Myenteric plexus located between 2 muscle
layers.
 Major nerve supply to GI tract.
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Serosa
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Binding and
protective
outer layer.
Consists of
connective
tissue.
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Regulation of the GI Tract
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Extrinsic innervation:
 Parasympathetic Nervous System:
 Stimulate motility.
 Sympathetic Nervous System:
 Reduce peristalsis and secretory activity.
Enteric Nervous System:
 Submucosal plexus and myenteric plexus:
 Local regulation of the GI tract.
Paracrine secretion:
 Molecules acting locally.
Hormonal secretion:
 Secreted by the mucosa.
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Esophagus
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Deglutition begins as a voluntary
activity.
Larynx is raised.
Epiglottis covers the entrance to the
respiratory tract.
Involuntary muscular contractions and
relaxations in the esophagus follow.
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Esophagus
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Connects pharynx to the stomach.
Upper third contain skeletal muscle.
Middle third contains a mixture of
skeletal and smooth muscle.
Terminal portion contains only smooth
muscle.
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Esophagus
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Peristalsis:
Produced by a series of
localized reflexes in
response to distention of
wall by bolus.
Wave-like contractions:
 Circular smooth muscle
contract behind, relaxes
in front of the bolus.
 Followed by longitudinal
contraction (shortening)
of smooth muscle.
 Rate of 2-4 cm/sec.
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Stomach
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Most distensible part of GI tract.
Empties into the duodenum.
Functions of the stomach:
 Store food.
 Initiate digestion of proteins.
 Kill bacteria.
 Move food (chyme) into intestine.
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Stomach
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Contractions of
the stomach churn
chyme.
Mix chyme with
gastric secretions.
Push food into
intestine.
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Stomach
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Gastric mucosa
has gastric pits in
the folds.
Cells that line the
folds deeper in
the mucosa are
gastric glands.
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Gastric Glands
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Gastric Juice:
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Goblet cells: mucus.
Parietal cells: HCl and intrinsic factor.
Chief cells: pepsinogen.
Enterochramaffin-like cells (ECL): histamine
and serotonin.
G cells: gastrin.
D cells: somatostatin.
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HCl Production
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Parietal cells secrete
H+ into gastric lumen
by primary active
transport trough H+ /
K+ ATPase pump.
Parietal cell’s
basolateral membrane
couples Cl- against its
electrochemical
gradient by coupling
its transport with
HC03-.
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HCl Production
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HCL production is stimulated:
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Indirectly by gastrin.
Indirectly by ACh.
ACh and gastrin stimulate release of
histamine.
Histamine:

Stimulates parietal cells to secrete HCL.
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HCL Functions
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Makes gastric juice
very acidic.
 Optimum pH of of
stomach is 2.0.
Denatures ingested
proteins (alter tertiary
structure) so become
more digestible.
Activates pepsinogen to
pepsin.
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Digestion and Absorption in
the Stomach
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Proteins partially digested by pepsin.
Carbohydrate digestion by salivary
amylase is soon inactivated by acidity.
Alcohol and aspirin are the only
commonly ingested substances
absorbed.
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Protective Mechanisms of
Stomach
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Parietal and chief cells impermeable to
HCL.
Alkaline mucus contains HC03-.
Tight junctions between adjacent cells.
Rapid rate of cell division (3 days).
Prostaglandins which inhibit gastric
secretions.
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Small Intestine
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Each villus is a fold in the
plasma membrane.
Covered with columnar
epithelial cells and goblet
cells.
Epithelial cells at the tips
are exfoliated and
replaced by replaced by
mitosis in crypt of
Lieberkuhn.
Lamina propria contain
lymphocytes, capillaries,
and central lacteal.
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Absorption in Small
Intestine
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Duodenum and jejunum:
 Carbohydrates, amino acids, lipids,
Ca++, and Fe++.
Ileum:
 Bile salts, vitamin B12, electrolytes,
and H20.
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Intestinal Enzymes
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Microvilli contain brush border enzymes that
are not secreted into the lumen.
Brush border enzymes remain attached to
the cell membrane with their active sites
exposed to the chyme.
Absorption requires both brush border
enzymes and pancreatic enzymes.
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Intestinal Contractions and
Motility
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2 major types of contractions
occur in the small intestine:
Peristalsis:
 Slow movement.
 Pressure at the pyloric
end of small intestine is
greater than at the distal
end.
Segmentation:
 Major contractile activity
of the small intestine.
 Contraction of circular
smooth muscle.
 Mix chyme.
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Contractions of Intestinal
Smooth Muscles
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Occur automatically in response to
endogenous pacemaker activity.
Rhythm of contractions is paced by
graded depolarizations.
Slow waves produced by interstitial cells
of Cajal.
Slow waves spread from 1 smooth
muscle cell to another through nexuses.
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Contractions of Intestinal
Smooth Muscles
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Slow waves decrease in amplitude as
they are conducted.
May stimulate contraction in proportion
to the magnitude of depolarization.
When slow waves above threshold, it
triggers APs by opening of VG Ca++
channels.
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Smooth Muscle Action
Potentials
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At threshold, VG Ca++ channels open.
Repolarization: VG K+ channels open.
Stimulates contraction.
Parasympathetic NS, stretch and gastrin
increase the amplitude of slow waves.
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Stimulate APs.
SNS decrease APs.
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Large Intestine
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Aid absorption of H20, electrolytes, vitamin B
complex vitamins, vitamin K, and folic acid.
Secrete H20, via active transport of NaCl.
Guanylin stimulates secretion of Cl- and H20
and inhibits absorption of Na+.
Contains lymphocytes, lymph nodes, goblet
cells.
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Fluid and Electrolyte
Absorption in the Intestine
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Small intestine:
 Most of the fluid and electrolytes are absorbed by
small intestine.
 Absorbs about 90% of the remaining volume.
 Absorption of H20 occurs passively as a result of the
osmotic gradient created by active transport.
 Aldosterone stimulates NaCl and H20 in the ileum.
Large intestine:
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
Absorbs about 90% of the remaining volume.
Absorption of H20 occurs passively as a result of the osmotic
gradient created by active transport of Na+ and Cl-.
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Defecation
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Waste material passes to the rectum.
Defecation reflex:
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Longitudinal rectal muscle contract to
increase rectal pressure.
Relaxation of of internal anal sphincter.
Excretion is aided by contractions of
abdominal and pelvic skeletal muscles.
Push feces from the rectum.
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Structure of Liver
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Liver largest internal
organ.
Hepatocytes form
hepatic plates that
are 1 – 2 cells thick.
Plates separated by
sinusoids.
More permeable
than other
capillaries.
Contain phagocytic
Kupffer cells.
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Hepatic Portal System
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Products of digestion that are absorbed
are delivered to the liver.
Drain into the hepatic portal vein.
¾ blood is deoxygenated.
Hepatic vein drains liver.
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Enterohepatic Circulation
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Variety of exogenous
compounds are
secreted by the liver
into the bile ducts.
Can excrete these
compounds into the
intestine with the bile.
Eliminated in the
feces.
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Table 18.4
Major Categories of Liver Function
Functional Category
Actions
Detoxication of Blood
Phagocytosis by Kupffer cells
Chemical alteration of biologically active molecules (hormones and drugs)
Production of urea, uric acid, and other molecules that are less toxic than parent
compounds
Excretion of molecules in bile
Carbohydrate Metabolism
Conversion of blood glucose to glycogen and fat
Production of glucose from liver glycogen and from other molecules (amino acids, lactic
acid) by gluconeogenesis
Secretion of glucose into the blood
Lipid Metabolism
Synthesis of triglyceride and cholesterol
Excretion of cholesterol in bile
Production of ketone bodies from fatty acids
Protein Synthesis
Production of albumin
Production of plasma transport proteins
Production of clotting factors (fibrinogen, prothrombin, and others)
Secretion of Bile
Synthesis of bile salts
Conjugation and excretion of bile pigment (bilirubin)
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Bile Production and
Secretion
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The liver produces and secreted 250 – 1500
ml of bile/day.
Bile pigment (bilirubin) produced in spleen,
bone marrow and liver.

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Derivative of the heme groups (without iron) from
hemoglobin.
Free bilirubin combines with glucuronic acid
and forms conjugated bilirubin.

Secreted into bile.
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Bile Production and
Secretion
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Converted by bacteria in intestine to
urobilinogen.
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Urobilogen is absorbed by intestine and
enters the hepatic vein.
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Recycled in bile or general circulation.
Filtered by kidneys into the urine.

Produces amber color.
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Bile Production and
Secretion
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Bile acids are derivatives of
cholesterol.
 Major pathway of
cholesterol breakdown in
the body.
Principal bile acids are:
 Cholic acid.
 Chenodeoxycholic acid.
 Combine with glycine or
taurine to form bile salts.
 Bile salts aggregate as
micelles.
95% of bile acids are
absorbed by ileum.
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Detoxification of the Blood
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Liver can remove hormones, drugs, and
other biologically active molecules from
the blood by:
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Excretion into the bile.
Phagocytosis by Kupffer cells.
Chemical alteration of the molecules.
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Ammonia is produced by deamination of amino
acids in the liver.
Liver converts it into urea.
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Secretion of Glucose,
Triglycerides and Ketones
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Liver helps regulate blood glucose
concentration by:
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Glycogenesis and lipogenesis.
Glycogenolysis and gluconeogenesis.
Contains enzymes required to convert
free fatty acids into ketones.
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Production of Plasma
Proteins
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Albumin and most of the plasma globulins
(except immunoglobulins) are produced by
the liver.
Albumin constitutes 70% of the total
plasma protein.
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Contributes most to the colloid osmotic
pressure in the blood.
Globulins:
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Transport cholesterol and hormones.
Inhibit trypsin and blood clotting.
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Gallbladder
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Sac-like organ attached to the inferior
surface of the liver.
Stores and concentrates bile.
When gallbladder fills with bile it
expands.
Contraction of the muscularis layer of
the gallbladder, ejects bile into the
common bile duct.
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Pancreas
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Exocrine:
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Acini: secrete
pancreatic juice.
Endocrine:

Secrete insulin
and glucagon.
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Pancreatic Juice
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Contain H20 , HC03- and digestive enzymes.
Enzyme
Zymogen
Activator
Action
Trypsin
Trypsinogen
Enterokinase
Cleaves internal peptide bonds
Chymotrypsin
Chymotrypsinogen
Trypsin
Cleaves internal peptide bonds
Elastase
Proelastase
Trypsin
Cleaves internal peptide bonds
Carboxypeptidase
Procarboxypeptidase
Trypsin
Cleaves last amino acid from carboxylterminal end of polypeptide
Phospholipase
Prophospholipase
Trypsin
Cleaves fatty acids from phospholipids such
as lecithin
Lipase
None
None
Cleaves fatty acids from glycerol
Amylase
None
None
Digests starch to maltose and short chains
of glucose molecules
Cholesterolesterase
None
None
Releases cholesterol from its bonds with
other molecules
Ribonuclease
None
None
Cleaves RNA to form short chains
Deoxyribonuclease
None
None
Cleaves DNA to form short chains
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Pancreatic Juice
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Complete digestion of
food requires action of
both pancreatic and
brush border enzymes.
Most pancreatic
enzymes are produced
as zymogens.
Trypsin (when activated
by enterokinase)
triggers the activation
of other pancreatic
enzymes.
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Neural and Endocrine
Regulation
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Neural and endocrine mechanisms
modify the activity of the GI system.
GI tract is both an endocrine gland and
a target for the action of hormones.
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Regulation of Gastric
Function
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Gastric motility and secretion are automatic.
Waves of contraction are initiated
spontaneously by pacesetter cells.
Extrinsic control of gastric function is divided
into 3 phases:
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Cephalic phase.
Gastric phase.
Intestinal phase.
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Cephalic Phase
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Sight, smell, and taste of food.
Activation of vagus:
 Stimulate chief cells to secrete pepsinogen.
 Directly stimulate G cells to secrete gastrin.
 Directly stimulate ECL cells to secrete
histamine.
 Indirectly stimulate parietal cells to secrete
HCl.
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Gastric Phase
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Arrival of food in stomach stimulates the gastric
phase.
Gastric secretion stimulated by:
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Distension.
Chemical nature of chyme (amino acids and peptides).
Activation of vagus:
 Stimulate G cells to secrete gastrin.
 Stimulate chief cells to secrete pepsinogen.
 Stimulates ECL cells to secrete histamine.
 Histamine stimulates secretin of HCl.
Positive feedback effect.
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Gastric Phase

Secretion of HCl is also
regulated by a negative
feedback effect:
 HCl secretion decreases if
pH < 2.5.
 At pH of 1.0, gastrin
secretion ceases.
 D cells stimulate
secretion of
somatostatin.
 Inhibits secretion
of gastrin.
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Intestinal Phase
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Inhibits gastric activity when chyme
enters the small intestine.
Arrival of chyme increases osmolality
and distension.
Activate sensory neurons of vagus and
produce an inhibitory neural reflex.
In the presence of fat, enterogasterone
inhibits gastric motility and secretion.
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Intestinal Phase
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Hormone secretion:
Inhibit gastric activity:
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Somatostatin.
CCK.
GLP-1.
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Enteric Nervous System
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Submucosal and myenteric plexuses
contain 100 million neurons.
Include preganglionic parasympathetic
axons, ganglion cell bodies,
postganglionic sympathetic axons, and
sensory neurons.
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Enteric Nervous System
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Peristalsis:
ACh and substance
P stimulate smooth
muscle contraction.
NO, VIP, and ATP
stimulate smooth
muscle relaxation.
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Paracrine Regulators of the
Intestine
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Serotonin:
 Stimulates intrinsic afferents which send impulses
into intrinsic nervous system and activate motor
neurons.
Motilin:
 Stimulates contraction of the duodenum and
stomach antrum.
Guanylin:
 Activates cGMP which stimulates the intestinal cells
to secrete Cl- and H20.
+
 Inhibits the absorption of Na .
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Intestinal Reflexes
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Gastroileal reflex:
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Ileogastric reflex:
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Increased gastric activity causes increased motility
of ileum and movement of chyme.
Movement through ileocecal sphincter.
Distension of ileum causes decreased gastric
motility.
Intestino-intestinal reflex:

Overdistension in 1 segment, causes relaxation of
rest of intestine.
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Secretion of Bile
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Secretion of pancreatic juice and bile is stimulated by:
Secretin:
 Occurs in response to duodenal pH < 4.5.
 Stimulates the liver to secrete HC03 into the bile.
CCK:
 Occurs in response to fat and protein.
 Enhances secretin.
 Stimulates contraction of the sphincter of Oddi.
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Digestion and Absorption of
Carbohydrates
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Salivary amylase:
Begins starch
digestion.
Pancreatic amylase:
Digest starch to
oligosaccharides
Oligosaccharides
hydrolyzed by brush
border enzymes.
Transported by
secondary active
transport with Na+.
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Digestion and Absorption of
Protein
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
Digestion begins in the stomach when pepsin
digestion to form polypeptides.
In the duodenum and jejunum:


Endopeptidases cleave peptide bonds in the interior of the
polypeptide:
 Trypsin.
 Chymotrypsin.
 Elastase.
Exopeptidases cleave peptide bonds from the ends of the
polypeptide:
 Carboxypeptidase.
 Aminopeptidase.
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Digestion and Absorption of
Protein

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Free amino acids
absorbed by
cotransport with Na+.
Dipeptides and
tripeptides
transported by
secondary active
transport using a H+
gradient to transport
them into the
cytoplasm.
Hydrolyzed into free
amino acids and then
secreted into the
blood.
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Digestion and Absorption of
Lipids
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Arrival of lipids in the duodenum serves as a stimulus
for secretion of bile.
Emulsification:
 Bile salts micelles are secreted into duodenum to
break up fat droplets.
Pancreatic lipase and colipase hydrolyze triglycerides
to free fatty acids and monglycerides.
 Colipase coats the emulsification droplets and
anchors the lipase enzyme to them.
 Form micelles and move to brush border.
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Digestion and Absorption of
Lipids
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Free fatty acids, monoglycerides, and
lysolecithin leave micelles and enter into
epithelial cells.
Resynthesize triglycerides and
phospholipids within cell.


Combine with a protein to form
chylomicrons.
Secreted into central lacteals.
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Transport of Lipids
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In blood, lipoprotein lipase hydrolyzes
triglycerides to free fatty acids and glycerol
for use in cells.
Remnants taken to liver.
Form VLDL which take triglycerides to cells,
and converted to LDLs.
LDLs transport cholesterol to organs and
blood vessels.
HDLs transport excess cholesterol back to
liver.
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