Chapter 18
The Digestive
System
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Functions of GI Tract
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Secretion
Includes
release of exocrine and endocrine products into GI
tract
Exocrine secretions include: HCl, H2O, HCO3-, bile, lipase,
pepsin, amylase, trypsin, elastase, and histamine
Endocrine includes hormones secreted into stomach and
small intestine to help regulate GI system
E.g. gastrin, secretin, CCK, GIP, GLP-1, guanylin, VIP,
and somatostatin
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Absorption
Is
passage of digested end products into blood or lymph
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Digestive System continued
 Organs
include oral
cavity, pharynx,
esophagus, stomach,
and small and large
intestine
 Accessory organs
include teeth, tongue,
salivary glands, liver,
gallbladder, and
pancreas
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Regulation of GI Tract
Parasympathetic
effects, arising from vagus and spinal
nerves, stimulate motility and secretions of GI tract
Sympathetic activity reduces peristalsis and secretory
activity
GI tract contains an intrinsic system that controls its
movements--the enteric nervous system
GI motility is influenced by paracrine and hormonal signals
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Stomach
Is
most distensible part of GI tract
Empties into the duodenum
Functions in: storage of food; initial digestion of proteins;
killing bacteria with high acidity; moving soupy food
mixture (chyme) into intestine
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Stomach continued
 Gastric
glands contain cells
that secrete different products
that form gastric juice
 Goblet cells secrete mucus
 Parietal cells secrete HCl
and intrinsic factor
(necessary for B12
absorption in intestine)
 Chief cells secrete
pepsinogen (precursor for
pepsin)
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HCl in Stomach continued
Is
secreted in response to the hormone gastrin; and ACh
from vagus
These are indirect effects since both stimulate release of
histamine which causes parietal cells to secrete HCl
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HCl in Stomach continued
 Makes
gastric juice
very acidic which
denatures proteins to
make them more
digestible
 Converts pepsinogen
into pepsin
 Pepsin is more
active at low pHs
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Protection of Stomach Against HCL and Pepsin
Both
HCL and pepsin can damage lining and produce a
peptic ulcer
1st line of defense is the adherent layer of mucus
= a stable gel of mucus coating the gastric epithelium
Contains bicarbonate for neutralizing HCL
Is a barrier to actions of pepsin
Gastric epithelial cells contain tight junctions to prevent
HCL and pepsin from penetrating the surface
Gastric epithelial cells are replaced every 3 days
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Digestion and Absorption in Stomach
Proteins
are partially digested by pepsin
Carbohydrate digestion by salivary amylase is soon
inactivated by acidity
Alcohol and aspirin are the only commonly ingested
substances that are absorbed
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Gastric and Peptic Ulcers
 Peptic
ulcers are erosions of mucous membranes of stomach or
duodenum caused by action of HCl
 In Zollinger-Ellison syndrome, duodenal ulcers result from
excessive gastric acid in response to high levels of gastrin
 Helicobacter pylori infection is associated with ulcers
 Antibiotics are useful in treating ulcers
 And also proton pump inhibitors such as Prilosec
 Acute gastritis is an inflammation that results in acid damage due
to histamine released by inflammation
 Is why histamine receptor blockers such as Tagamet and
Zantac can treat gastritis
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Small Intestine (SI) continued
Absorption
of digested food occurs in SI
Facilitated by long length and tremendous surface area
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Small Intestine (SI) continued
 Surface
area
increased by
foldings and
projections
 Large folds are
plicae circulares
 Microscopic fingerlike projections are
villi
 Apical hair-like
projections are
microvilli
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Small Intestine (SI) continued
 Each
villus is covered with
columnar epithelial cells
interspersed with goblet cells
 Epithelial cells at tips of villi
are exfoliated and replaced
by mitosis in crypts of
Lieberkuhn
 Inside each villus are
lymphocytes, capillaries, and
central lacteal
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Intestinal Enzymes
Attached
to microvilli are brush border enzymes that are not
secreted into lumen
Enzyme active sites are exposed to chyme
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Large Intestine (LI) or Colon
 Has
no digestive function but absorbs H2O, electrolytes, B and K
vitamins, and folic acid
 Internal surface has no villi or crypts and is not very elaborate
 Contains large population of microflora
 = 400 different species of commensal bacteria
which produce folic acid and vitamin K and ferment
indigestible food to produce fatty acids
And reduce ability of pathogenic bacteria to infect LI
antibiotics can negatively affect commensals
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Fluid and Electrolyte Absorption in LI
SI
absorbs most water but LI absorbs 90% of water it
receives
Begins with osmotic gradient set up by Na+/K+ pumps
Water follows by osmosis
Salt and water reabsorption stimulated by aldosterone
LI can also secrete H2O via AT of NaCl into intestinal lumen
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Detoxification of Blood
Liver
can remove hormones, drugs, and other biologically
active molecules from blood by:
Excretion into bile
Phagocytosis by Kupffer cells
Chemical alteration of molecules
E.g. ammonia is produced by deamination of amino
acids in liver
 Liver converts it to urea which is excreted in urine
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Secretion of Glucose, Triglycerides and Ketones
Liver
helps regulate blood glucose by removing it from
blood or releasing it to blood
Removes it via glycogenesis and lipogenesis
Or produces it via glycogenolysis and gluconeogenesis
Can convert free fatty acids into ketone bodies (ketogenesis)
that can be used for energy during fasting
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Neural and Endocrine Regulation
 Neural
and endocrine mechanisms modify activity of GI system
 Vagus nerve is heavily involved in regulating and coordinating
digestive activities
 GI tract is both an endocrine gland and target for action of
hormones
 Hormones include secretin, gastrin, CCK, and GIP
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Enteric Nervous System
Submucosal
and myenteric plexuses contain as many
neurons as spinal cord
Includes preganglionic Parasymp axons, ganglion cell
bodies, postganglionic Symp axons; and afferent intrinsic
and extrinsic sensory neurons; interneurons, and glia
Peristalsis is controlled by enteric NS
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Digestion and Absorption of Protein
Begins
in stomach when pepsin digests proteins to form
polypeptides
In SI, endopeptidases (trypsin, chymotrypsin, elastase)
cleave peptide bonds in interior of polypeptides
SI exopeptidases (carboxypeptidase, aminopeptidase) cleave
peptide bonds from ends of polypeptides
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Digestion and Absorption of Lipids
Occurs
in SI
Arrival of lipids in duodenum causes secretion of bile
Fat is emulsified by bile salt micelles
Forms tiny droplets of fat dissolved in bile salt micelles
Greatly increases surface area for fat digestion
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Digestion and Absorption of Lipids continued
 Pancreatic lipase
hydrolyzes triglycerides
to free fatty acids and
monglycerides
 Phospholipase A breaks
down phospholipids
into fatty acids and
lysolecithin
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Transport of Lipids continued
Cholesterol
and triglycerides from liver form VLDLs which
are secreted and take triglycerides to cells
Once triglycerides are removed, VLDLs become LDLs
LDLs transport cholesterol to organs and blood vessels
HDLs transport excess cholesterol back to liver
High ratio of HDL-cholesterol to total cholesterol is
believed to confer protection against atherosclerosis
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