Download The Digestive System Functions of the Digestive System Histological

The Digestive
System
Functions of the Digestive System
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Ingestion
Mechanical processing
Digestion
Secretion
Absorption
Excretion
Histological organization
•  In lab
•  Know it for both lecture and lab tests
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Movement of Material
•  Visceral smooth muscle
•  Pacesetter cells
•  Peristalsis
–  Longitudinal movement
Second type of movement
•  Segmentation
–  Churning
–  Mixing
•  Nervous tissue of
GI tract
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Control of Digestive Tract
•  Motility & secretion
•  Neural mechanisms
–  Many neurons
–  Short & long reflexes
•  Hormones - 18!
•  Local mechanisms
–  Prostaglandins
–  histamine
Stomach
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Temporary storage of food
Mechanical breakdown of food
Chemical digestion
Intrinsic factor secretion
–  intestinal absorption of vitamin B12
Stomach lining
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Gastric pits
•  Mucous cells
–  Secrete mucus
–  Provides protection
from low pH
Gastric glands
•  Parietal cells
•  Chief cells
Gastric pits
•  Mucous cells
–  Secrete mucus
–  Provides protection
from low pH
Gastric glands
•  Parietal cells
•  Chief cells
Chief cells
•  secrete pepsinogen
•  Pepsin
–  proteolytic enzyme
•  2 enzymes secreted
by infants
–  Rennin
–  Gastric lipase
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Pyloric glands
•  Mucous secretion
•  Enteroendocrine cells G cells
–  secrete Gastrin
•  Increases activity of
–  Parietal cells
–  Chief cells
•  Increases gastric motility
–  D cells FYI only
•  Somatostatin
•  Inhibits gastrin secretion
•  Overcome by neural
stimuli
gastric digestion & absorption
•  carbohydrate and lipid
digestion occurs until gastric
pH hits 4.5
•  proteins digested in stomach to
polypeptides/short peptides by
pepsin
•  no nutrient absorption occurs
•  other absorbed molecules…
Small Intestine
digestion and absorption
•  duodenum 10 in
–  Duodenal (Brunner’s)
glands
•  jejunum 8 ft
–  Prominent plicae and
villi
•  ileum 12 ft
–  Peyer’s patches
–  Terminates at colon
ileocecal valve
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Intestinal characteristics
•  increased
surface area
(2200 sq ft)
–  plicae
–  villi
–  microvilli
•  brush border
•  lacteals
Intestinal secretions
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1.8 liters of intestinal juice
moistens chyme
buffers gastric acid
intestinal juice secreted by
–  osmosis from mucosa
–  secretion by intestinal glands
•  Stimulated by touch and stretch receptors
Intestinal peristalsis
1 - Weak peristalsis toward jejunum; continuous
–  Via local myenteric reflexes and pacesetter cells
–  pacesetter cells, p.317 or 328, spontaneously
depolarize…
–  Gap junctions allow spread of signal analward…“law of
the gut”
2 - Strong peristalsis, result of
• parasympathetic stimulation
• reflexes stimulated by gastric stretch
–  Gastroenteric reflex
stimulates motility along small intestine
–  Gastroilieal reflex
relaxes ileocecal valve
–  hormones enhance or suppress these reflexes
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Pancreas
•  organization
–  islets
–  acini (s. is acinus)- acinar cells secrete enzymes
–  ducts - cuboidal epithelium secretes water & ions
•  pancreatic juice
–  alkaline mixture of above secretions
–  secreted into small intestine
pancreatic physiology
•  pancreas: controlled by
–  duodenal hormones secretin and CCK
–  vagus nerve
•  secretin
–  secreted as chyme enters duodenum
–  targets duct cells
–  stimulates secretion of a watery buffer solution containing
•  bicarbonate & phosphate ions (buffers)
•  CCK
–  secreted as chyme enters duodenum
–  targets acinar cells
–  stimulates secretion of pancreatic enzymes
pancreatic enzymes
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alpha-amylase
pancreatic lipase
nuclease
proteolytic enzymes
–  proteases & peptidases
–  trypsinogen, chymotrypsinogen,
procarboxypeptidase, proelastase
–  trypsinogen activated by enteropeptidase, trypsin
activates the others…
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Liver Functions;
~ 200 functions
•  metabolic regulation
–  carbohydrate/lipid/amino acid metabolism
–  waste removal
–  vitamin storage
–  mineral storage
–  drug inactivation
more liver functions:
hematological regulation
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phagocytosis and antigen presentation
plasma protein production
removal of hormones, worn RBCs
vitamin D synthesis
removal of antibodies
removal/storage of toxins
synthesis of bile
secretion of bile
–  Emulsification
Biliary ducts
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Gallbladder
•  stores/modifies bile
•  bile ejection stimulated by CCK
–  stimulates gall bladder contractions, ejects bile into duct
–  relaxes hepatopancreatic sphincter (sphincter of Oddi)
•  Bile used for lipid breakup, more on this to follow
Emulsification
Mechanical breakup of lipid droplets
Bile acid Regulation of gastric activity
•  controlled by NS
•  Regulated by
–  short reflexes of ENS
–  GI tract hormones
•  Three phases
–  Cephalic
–  Gastric
–  Intestinal
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Regulation of gastric activity
1 Cephalic phase
•  Initiated by
–  sight, smell, taste, thought of food
•  Parasympathetic vagal efferent
impulses stimulate stomach
•  Prepares stomach for food
–  Gastric juice secreted
•  Mucus
•  Pepsinogen
•  HCl
•  Emotions may exacerbate or
inhibit the cephalic phase
2 Gastric phase: 3-4 hrs
•  Initiated by: Arrival of food; stretch
–  Distension, increase in pH , protein,
peptides, caffeine, low levels of
alcohol
•  Responses to arrival of food:
–  Local: stretch causes histamine
release
•  histamine stimulates parietal cells
–  Neural: stretch and elevated pH
•  Myenteric and submucosal plexus
activated
–  Hormonal: neural impulses and
peptides stimulate
•  gastrin secretion by G cells and
duodenum
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2 Gastric phase
functions: 3-4 hrs
•  Submucosal plexus
–  Enhances gastric juice
secretion
•  Myenteric plexus
–  Increased motility
•  Result: homogenize & acidify
chyme via motility 5
•  Initiate protein digestion
•  Resolves as pH drops, [protein]
drops…less gastrin…
3 Intestinal phase
•  Controls rate of gastric emptying
•  Initiated by: chyme arrival in
duodenum
•  Chyme presence causes 2 stimuli
–  stretch
–  decreased pH
•  Response to stimuli
–  Duodenal mucus production
•  From Brunner’s glands
–  Enterogastric reflex
•  Inhibits gastrin production
•  Constricts pyloric sphincter
Intestinal phase
Hormone reflexes
•  CCK and GIP secretion
–  Stimulated by presence of
Lipids and carbs
•  Secretin secretion
–  If pH is less than 4.5
–  Inhibits parietal & chief cells
–  Stimulates pancreatic buffer
secretion
–  Stimulates hepatic bile
secretion
•  Intestinal Gastrin secreted
–  Proteins present in chyme
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The Large Intestine
•  Cecum
–  Appendix
•  Colon
•  Rectum
–  Anus
The Large Intestine
•  Physiology
–  absorption
•  Water, ions, vitamins, bile salts
–  vitamin synthesis by coliform bacteria
•  K, biotin, B5
–  excretion of organic wastes
•  Urobilin, stercobilin,
•  indole, skatole, ammonium ions, hydrogen sulfide
from peptide breakdown….
•  mass movement
–  stimulated by gastric/duodenal distension
Defecation reflex
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Time frame for digestion
•  swallowing: 4-8 sec food, 1-2 sec for liquid.
•  gastric digestion: ~4 hors for an average meal,
up to 6 hours if high in fat.
•  small intestine: About 2 hrs.
•  large intestine: 12 to 24 hours.
Digestive
secretion and
absorption of
water
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