Digestive System Part 3
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Transcript Digestive System Part 3
Digestive System Part 3
Honors Anatomy & Physiology Chapter 23
Regulation of Gastric Secretion
Neural and hormonal mechanisms
Gastric mucosa up to 3 L gastric juice/day
Vagus nerve stimulation secretion
Sympathetic stimulation secretion
Hormonal control largely gastrin
Enzyme and HCl secretion
Most small intestine secretions - gastrin antagonists
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Regulation of Gastric Secretion
Three phases of gastric secretion
Cephalic (reflex) phase – conditioned reflex
triggered by aroma, taste, sight, thought
Gastric phase – lasts 3–4 hours; ⅔ gastric juice
released
Stimulated by distension, peptides, low acidity,
gastrin (major stimulus)
Enteroendocrine G cells stimulated by caffeine,
peptides, rising pH gastrin
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Stimuli of Gastric Phase
Gastrin enzyme and HCl release
Low pH inhibits gastrin secretion (as between
meals)
Buffering action of ingested proteins
rising pH gastrin secretion
Three chemicals - ACh, histamine, and
gastrin - stimulate parietal cells through
second-messenger systems
All three are necessary for maximum HCl
secretion
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HCl Formation
Parietal cells pump H+ (from carbonic acid
breakdown) into stomach lumen
K+ goes into cells to balance charge
HCO3– from carbonic acid breakdown
blood (via Cl– and HCO3– antiporter)
blood leaving stomach more alkaline
Alkaline tide
Cl– (from blood plasma via antiporter) follows H+
HCl
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Figure 23.18 Mechanism of HCl secretion by parietal cells.
Gastric gland
Blood
capillary
Chief cell
CO2
CO2 + H2O
H2CO3
Stomach lumen
Carbonic
anhydrase
H+
K+
HCO3−
Alkaline
tide
Parietal cell
H+-K+
ATPase
H+
K+
HCI
HCO3−
Cl−
HCO3−- Cl−
Interstitial antiporter
fluid
Cl−
Cl−
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Regulation of Gastric Secretion
Intestinal phase
Stimulatory component
Partially digested food enters small intestine
brief intestinal gastrin release
Inhibitory effects (enterogastric reflex and
enterogastrones)
Chyme with H+, fats, peptides, irritating
substances inhibition
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Enterogastric Reflex
Three reflexes act to
Inhibit vagal nuclei in medulla
Inhibit local reflexes
Activate sympathetic fibers tightening of pyloric
sphincter no more food entry to small intestine
Decreased gastric activity protects small
intestine from excessive acidity
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Intestinal Phase
Enterogastrones released
Secretin, cholecystokinin (CCK), vasoactive
intestinal peptide (VIP)
All inhibit gastric secretion
If small intestine pushed to accept more chyme
dumping syndrome
Nausea and vomiting
Common in gastric reduction for weight loss
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Figure 23.17 Neural and hormonal mechanisms that regulate release of gastric juice.
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
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Gastric Contractile Activity
Peristaltic waves move toward pylorus at rate of 3
per minute
Basic electrical rhythm (BER) set by enteric
pacemaker cells (formerly interstitial cells of
Cajal)
Pacemaker cells linked by gap junctions entire
muscularis contracts
Distension and gastrin increase force of contraction
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Gastric Contractile Activity
Most vigorous near pylorus
Chyme is either
Delivered in ~3 ml spurts to duodenum, or
Forced backward into stomach
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Figure 23.19 Deglutition (swallowing).
Pyloric
valve
closed
1 Propulsion: Peristaltic
waves move from the fundus
toward the pylorus
Pyloric
valve
slightly
opened
2 Grinding: The most
vigorous peristalsis and
mixing action occur close
to the pylorus. The pyloric
end of the stomach acts as a
pump that delivers small
amounts of chyme into the
duodenum.
Slide 1
Pyloric
valve
closed
3 Retropulsion: The peristaltic
wave closes the pyloric valve,
forcing most of the contents of
the pylorus backward into the
stomach.
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Regulation of Gastric Emptying
As chyme enters duodenum
Receptors respond to stretch and chemical
signals
Enterogastric reflex and enterogastrones inhibit
gastric secretion and duodenal filling
Carbohydrate-rich chyme moves quickly through
duodenum
Fatty chyme remains in duodenum 6 hours or more
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Figure 23.20 Neural and hormonal factors that inhibit gastric emptying.
Presence of fatty, hypertonic,
acidic chyme in duodenum
Duodenal enteroendocrine cells
Chemoreceptors and
stretch receptors
Secrete
Enterogastrones
(secretin,
cholecystokinin,
vasoactive intestinal
peptide)
Target
Via short
reflexes
Enteric
neurons
Duodenal
stimuli
decline
Via long
reflexes
CNS centers
sympathetic
activity;
parasympathetic
activity
Contractile force and
rate of stomach
emptying decline
Initial stimulus
Stimulate
Physiological response
Inhibit
Result
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Homeostatic Imbalance
Vomiting (emesis)
Extreme stretching
Intestinal irritants, e.g., bacterial toxins,
excessive alcohol, spicy food, certain drugs
Chemicals/sensory impulses emetic center of
medulla
Excessive vomiting dehydration, electrolyte and
acid-base imbalances (alkalosis)
caused by
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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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Large Intestine
“colon”
larger in diameter but shorter than small
intestine (which it “frames”)
functions:
1. dry out indigestible food residue by absorbing
water
2. elimination
Parts of the Large Intestine
Cecum
Appendix
Colon
Ascending
Transvere
Descending
Rectum
Anal Canal
Cecum
sack-like
1st part of large intetine
site of ileoceal valve: empties chyme into large
intestine from small intestine
Appendix
Vermiforme appendix
vestigial organ
easily twists small amounts food & bacteria
get trapped appendicitis
Signs of Appendicitis
Appendicitis
Appendectomy
Colon
Colonoscopy
Colon
Descending colon enters pelvis in LLQ where it
becomes S-shaped = sigmoid colon
Rectum
Wall of the Colon
muscularis externa’s longitudinal layer in 3
bands called teniae coli
“ribbons of the colon”
are normally partially contracted colon wall
puckers forming haustra
Histology of Colon
no villi (absorption
mostly in small
intestine)
many Goblet cells
mucus (aids
movement of feces)
Rectum
storage area for feces
Anal Canal
2 sphincters:
1. external sphincter
skeletal muscle (voluntary)
internal sphincter
2.
smooth muscle (involuntary)