Download Chapter 15 - main concepts

16.1
I. digestive processes
A. secretion
1. controlled by nervous and endocrine systems
2. general components: water, ions, mucus, enzymes, special materials
3. most water and electrolytes later reabsorbed
B. motility
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smooth muscle cells are connected to each other by gap junctions
when stimulated they contract as a unit
contraction is caused by intrinsic and extrinsic (ANS, hormones) factors
1. tone = constant partial contraction
2. mixing – increases rate of digestion and absorption
3. propulsion – moves contents through gut
peristalsis = wavelike muscular contraction
bolus of food initiates localized muscle responses
contraction occurs behind bolus
relaxation occurs ahead of bolus
C. digestion = breaking ingested food into molecules small enough to be absorbed
1. mechanical – breaks food into smaller pieces
increases surface area for enzymes
2. chemical – uses enzymes to break chemical bonds in nutrient molecules
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(hydrolysis)
D. absorption – molecules are moved from the gut lumen, through the intestinal
epithelium and into vascular or lymphatic capillaries.
II. regulation
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local vs systemic
neural vs endocrine
feed-forward vs feed-back
A. GI hormones
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gastrin
secretin
cholecystokinin (CCK)
glucose-dependent insulinotropic peptide (GIP)
1. release stimulated by:
local chemical signals
local mechanical signals
ANS
2. targets are smooth muscle and exocrine glands
B. digestive reflexes
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receptors: chemo-, mechano-, or osmoeffectors: smooth muscle, exocrine glands, endocrine glands
1. short reflexes - entire reflex arc is located in gut: integrating center is enteric
nervous system
2. long reflexes - autonomic reflexes with integrating centers in the CNS
C. feedback vs feedforward
feedback is a response in reaction to a change
feedforward is a response in anticipation of a change
III. Mouth
A. saliva
1. volume = 1 to 2 liters/day
2. composition
a. enzymes: salivary amylase
substrate = polysaccharides (starch)
end products = disaccharides (maltose)
optimal pH range = 6.6 to 6.8
b. mucus - moistens and lubricates food
c. lysozyme - antibacterial
B. mechanical digestion = chewing/mastication
C. propulsion = swallowing
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reflex initiated by voluntarily moving a bolus of food into the pharynx with the
tongue
1. receptors = pressure receptors in pharynx
2. integrating center = medulla oblongata
a. oropharyngeal phase
soft palate elevated
larynx elevated; epiglottis covers glottis
respiration inhibited
pharyngeal muscles contract
b. esophageal phase
pharyngoesophageal sphincter relaxes
peristalsis in esophagus
gastroesophageal (cardiac) sphincter relaxes
IV. Stomach
A. stomach function = storage up to 1 liter
B. gastric juice
1. volume = 2 liters / day
2. composition
a. mucus - protects mucosa from enzymes and acid
b. pepsinogen = inactive form of protease pepsin
activated by HCl or pepsin
substrate = proteins
end product = peptides
optimal pH range =
c. HCl:
denatures proteins, facilitates hydrolysis, activates pepsin
d. intrinsic factor - binds to Vit. B12 and causes it to be absorbed in the ileum
e. gastrin - increases gastric activity
3. control of gastric secretion
a. cephalic phase - before food reaches stomach
stimuli: food in mouth, chewing, thought of food (act via ANS/vagus nerve)
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effectors: gastric mucosa
response: increased secretion of gastric juice
b. gastric phase - caused by food in stomach
stimuli: peptides, distention
(act via: short and long reflexes, gastrin)
effectors: gastric mucosa
response: increased secretion of gastric juice
c. intestinal phase - caused by food in intestine
stimuli: distention, fats, acid, hypertonicity
(act via: short & long reflexes, hormones)
effectors: gastric mucosa
response: decreased secretion of gastric juice
enterogastrones = hormones produced by the intestine that inhibit stomach activity
enterogastric reflex = reflex inhibition of stomach activity by small intestine
C. mechanical digestion
peristaltic contractions in antrum mix food with gastric juice to make chyme
D. propulsion and emptying
1. peristaltic contractions in antrum propel chyme into duodenum when pyloric sphincter
is open
2. factors that affect rate of emptying:
a. gastric distention - increases emptying by increasing gastric muscle excitability,
stimulating short and long reflexes and increasing gastric secretion
b. fluidity of chyme - increases emptying
c. intestinal distention - decreases emptying by initiating enterogastric reflex and
enterogastrone secretion
d. fat or acid in intestine - decreases emptying by initiating enterogastric reflex and
enterogastrone secretion
V. Pancreas
A. acini (exocrine tissue) produce pancreatic juice, volume = 1 to 2 liters / day
B. pancreatic juice composition
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1. HCO3 - neutralizes stomach acid
2. enzymes: optimal pH range =
a. proteases:
inactive form
trypsinogen
chymotrypsinogen
procarboxypeptidase
substrate = peptides
end products = smaller peptides
active form
trypsin
chymotrypsin
carboxypeptidase
b. amylase
substrate = polysaccharides
end products = disaccharides
c. lipase
substrate = triglycerides
end products = fatty acids, monoglycerides
C. control
1.
acidic chyme in duodenum
release of secretin from intestinal mucosa
increase HCO3 content of pancreatic juice
2.
fat and peptides in chyme in duodenum
release of CCK from intestinal mucosa
increase enzyme content of pancreatic juice
VI. Liver and Gall Bladder
A. functions of liver
1. metabolic processing of nutrients
2. detoxification of wastes and hormones
3. synthesis of plasma proteins
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4.
5.
6.
7.
8.
storage of glycogen, fats, Fe, Cu, vitamins
activation of Vit. D
phagocytosis of bacteria and RBCs
excretion of cholesterol and bilirubin
bile synthesis
B. bile - produced by hepatocytes; 250 ml to 1 liter / day
1. composition
water
HCO3
bile salts
cholesterol
bilirubin
lecithin
2. bile salts
polar and nonpolar ends
derived from cholesterol
recycled via enterohepatic circulation
emulsify fats in chyme to increase surface area for lipase
3. secretion and storage of bile
a. bile is secreted at constant rate into bile ducts
 if hepatopancreatic sphincter is open, bile enters duodenum
 if hepatopancreatic sphincter is closed, bile backs up through cystic duct into
gall bladder
b. gall bladder removes water; concentrates bile
4. control of secretion and release of bile
a. choleretic: bile salts from the enterohepatic circulation increase bile synthesis
and release
b. secretin: increases bile and HCO3 secretion
c. parasympathetic: increases synthesis and release
d. CCK: causes gall bladder contraction and sphincter of Oddi relaxation
VII. Small Intestine
A. function = digestion and absorption
B. intestinal juice - 1.5 liters / day; water and electrolytes, no enzymes
C. mechanical digestion
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segmentation = alternating contraction and relaxation in a segment of the gut
occurs when stimuli bring smooth muscle cells to threshold at the peaks of their
rhythmic membrane potential fluctuations
D. chemical digestion - intestinal enzymes are components of the luminal membrane of
the intestinal cells
1. disaccharidases (maltase, sucrase, lactase)
substrates = disaccharides
end products = monosaccharides
2. aminopeptidases
substrates = peptides
end products = amino acids
E. absorption
1. Na
a. passive - down conc. gradient
b. active - pumped out of cell at basal surface
2. Cl - follows Na passively
3. water - follows Na by osmosis
4. amino acids, glucose, galactose - secondary active transport with Na
5. fructose - facilitated diffusion
6. fats
a. bile salts and lecithin form a shell around fat digestion products, cholesterol, and
fat-soluble vitamins in the intestinal lumen (micelles)
b. when micelles approach mucosal cells, fatty acids, monoglycerides and
cholesterol diffuse into cell
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c. in sER, fatty acids and monoglycerides form triglycerides
d. triglycerides coated with lipoproteins form chylomicrons
e. chylomicrons leave mucosal cell via basal membrane and enter lymphatic
capillaries
7. Fe
a. Fe pumped into mucosal cells from lumen
b. if Fe is needed by the body, transferrin is present in plasma and can pick up Fe
from mucosal cell
c. unneeded Fe sequestered in non-absorbable form called ferritin
d. ferritin lost when mucosal cell sloughed
8. Ca - vitamin D increases uptake
9. vitamin B12
F. propulsion
1. segmentation - propels food during active absorption
following a meal
2. migrating motility complex - moves unabsorbed remnants out of small intestine
between meals
VIII. Large Intestine
A. function: converts remaining chyme into feces by removing water and electrolytes
B. mixing - haustral contractions
increase absorption
controlled by intrinsic plexuses (30 min intervals)
C. propulsion
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1. mass movements
strong peristaltic waves
propel feces towards rectum (3-4 times/day)
2. gastrocolic reflex - mass movements are initiated by gastric distention
D. defecation reflex
1. stretch receptors in rectum
2. integrating center in sacral spinal cord
3. effectors:
a. rectum and sigmoid colon contract (parasympathetic)
b. internal anal sphincter relaxes (parasympathetic)
c. external sphincter voluntarily relaxes (somatic)
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