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MD3002 Physiology > GI tract secretion > Flashcards

GI tract secretion Flashcards

1
Q

What is the GI system generally co-ordinated by?

A

The integration of neural and hormonal signals

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2
Q

What can the signals be genearlly in the GI system?

A
  • Physical (stretch receptors)
  • Chemical (sight and presence of food)
  • Additionally, there is considerable input from higher centres
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3
Q

What are the three phases the signals in the GI tract can be split into?

A
  • Cephalic
  • Gastric
  • Intestinal
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4
Q

What is the cephalic phase ?

A

HEaring or smelling food, control is largely parasympathetic/vagal stimulation

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5
Q

What is the gastric phase controlled by?

A
  • Parasympathetic

- Feedback control from small intestine

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6
Q

What can act as a feedback control mechanism?

A
  • Secretin
  • CCK
  • GIP
  • Distention
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7
Q

What substance can act as a feed-forward mechanism from gastric to intestinal phase?

A

Gastrin

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8
Q

What can change in the mouth in the cephalic phase in response to food/thinking of food?

A

Increased salivary secretion

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9
Q

What can change in the stomach in the cephalic phase?

A
  • Increased:
  • Secretion in stomach
  • HCL secretion
  • Mucus secretion
  • Pepsin secretion
  • Gastrin secretion
  • Blood flow to stomach
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10
Q

What is the gastric phase initiated by?

A

Food entering the stomach causing distention and release of gastrin - espec caffeine, alcohol, calcium

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11
Q

What stimuli cause the G cells to produce gastrin?

A
  • Parasympathetic pathways
  • Decreased acidity in stomach due to buffering of food
  • Distention of antrum
  • Proteins, peptides and amino acids
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12
Q

What substances works to inhibit the action of G cells?

A

HCl

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13
Q

What does gastrin target?

A
  • Increases gastric motility
  • Parietal cells
  • Trophic maintenance of GI epithelium
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14
Q

What is the intestinal phase initiated by?

A

Presence of food in the duodenum

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15
Q

What happens if the pH is >3 in the duodenum?

A

Duodenal peptides / amino acids cause release of gastrin

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16
Q

What happens if the pH is <2 in the duodenum?

A

Gaastric inhibition and intestinal stimulation

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17
Q

What substances do the duodenal fats and breakdown products cause the release of

A
  • GIP

- CCK

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18
Q

What substance is released in response to acid entering the intestine?

A

Secretin

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19
Q

What is secretion?

A

The addition of substances (fluids, enzymes, ions etc.) into the lumen of the GI tract

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20
Q

What places secrete substances into the GI system?

A
  • Salivary glands
  • Cells of the gastric mucosa (gastric secretion)
  • Exocrine cells of the pancreas (pancreatic secretion)
  • Liver (bile)
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21
Q

What factors decrease secretion of saliva?

A
  • Sleep
  • Dehydration
  • Atropine
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22
Q

How much saliva is secreted per day roughly?

A

1 litre

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23
Q

What are the functions of saliva?

A
  • Initial digestion of starches and lipids (very little)
  • Dilution and buffering of ingested foods
  • Protection of teeth and gums
  • Lubrication of ingested foods with mucous
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24
Q

What cells does the sublingual gland mostly contain?

A

Mucous cells

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25
Q

What cells does the submandibular gland contain?

A

Mixed glands containing serous and mucous cells

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26
Q

What cells does the parotid gland gland contain?

A

Serous cells secreting an aqueous fluid composed of water, ions and enzymes

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27
Q

What are the 3 stages of salivary secretion?

A
  • Acini cells secrete primary secretion (isotonic)
  • Myoepithelial cells stimulated by neural input to eject saliva
  • Duct cells - secondary modification
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28
Q

What is found in the primary secretion?

A
  • Na+, Cl-, K+, HCO3-

- Amylase and mucin production

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29
Q

What happens in secondary modification?

A
  • Duct cells
  • Reabsorb Na+ and Cl- and add K+
  • HCO3- conc is altered depending on flow rate
  • High flow rate, saliva has increased HCO3-
  • Low flow rate, more HCO3- is extracted
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30
Q

What stimulates the salivary centre in the medulla?

A
  • Simple - Chemo/presssure receptors in mouth , impulses sent via afferent nerves
  • Acquired (conditioned), thinking about food, impulses sent from cerebral cortex triggers salivary centre
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31
Q

What kind of saliva is produced as a result of parasympathetic stimulation and how is it produced?

A
  • IP3, Ca2+ produced in acinar or ductal cells

- Large volume of watery enzyme rich saliva produced

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32
Q

What kind of saliva is produced as a result of sympathetic stimulation and how?

A
  • cAMP production increased

- Small volume of thick mucousy saliva produced

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33
Q

What is the main body of the oesophagus lined with and why?

A
  • Simple mucous glands

- To protect against mechanical damage

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34
Q

What glands at the gastric end of the oesophagus protect it against chemical damage?

A
  • Compound mucous glands
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35
Q

What are parietal cells?

A

Located in the body of the stomach and produce HCl and intrinsic factor

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36
Q

What are cheif cells?

A

Located in the body of the stomach and produce pepsinogen

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37
Q

Where are G cells located?

A

The antrum

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38
Q

Where are mucous cells located?

A

Antrum

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39
Q

How does HCl help with protein digestion?

A

Starts to denature them and activates pepsinogen to form pepsin

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40
Q

What can be absorbed as a result of the action of intrinsic factor?

A

Vitamin B12 (absorbed in ileum)

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41
Q

What does mucus do?

A

Protection and lubrication

42
Q

What cells make up the oxyntic (gastric) gland “gastric pit”?

A
  • Surface epithelium
  • Mucous neck cells
  • Oxyntic (or parietal) cells
  • Peptic (or chief) cells
43
Q

What cells secrete mucous?

A

Mucous neck cells

44
Q

What cells secrete HCl and intrinsic factor?

A

Oxyntic (or parietal) cells

45
Q

What cells secrete pepsinogen?

A

Peptic (or chief) cells

46
Q

How is HCl made?

A
  • Secreted seperately in parietal cells
  • H+ is secreted into the lumen via the H+ K+ ATPase
  • Cl- follows by diffusion through an apical channel
47
Q

What happens at the basolateral membrane?

A
  • HCO-3 is exchanged for Cl- via the chloride - bicarbonate exchanger (alkaline tide)
  • Eventually HCO-3 is secreted back into the GI tract in pancreatic secretions
48
Q

What is the ‘alkaline tide’?

A
  • During digestion HCO-3 is exchanged for Cl- via the chloride bicarbonate exchanger
  • Results in increased HCO-3 in blood therefore increased pH
49
Q

What percentage of secretion is in the cephalic phase?

A

30%

- Smell, taste, chewing, swallowing, conditioned reflex in anticipated food

50
Q

What stimulates secretion in the cephalic phase?

A
  • Direct stimulation of the parietal cells by the vagus
  • Indirect stimulation by parietal cells by gastrin
    Vagal gastrin releasing peptide (GRP) stimulates gastrin release from G cells. Gastrin hormone enters the circulation and stimulates parietal cells to release HCl
51
Q

What percentage of secretion is in the gastric phase?

A

60%

52
Q

What does distention cause?

A
  • Direct vagal stimulation of parietal cells
  • Indirect stimulation via gastrin
  • Local reflexes in the antrum that stimulate gastrin release
53
Q

Waht percentage of secretions are in the intestinal phase?

A

10%

54
Q

When is HCl secretion inhibited?

A
  • When HCl is no longer needed to convert pepsinogen to pepsin
  • Occurs after the chyme moves into the SI and the H+ buffering capacity of the food is no longer a factor
55
Q

What peptide hormone inhibits HCl secretion?

A

Somatostatin

56
Q

Explain the 2 ways in which somatostatin can inhibit HCl secretion?

A
  • Direct pathway
    Binds to receptors on parietal cells (and inhibits adenylate cyclase via Gi protein so inhibits HCl release)
  • Indirect pathway - inhibits histamine release from stomach and gastrin release from G cells so HCl inhibited
57
Q

What cells secrete pepsinogen?

A

Chief and mucous cells in the oxyntic glands in response to vagal stimulation

58
Q

What triggers chief cells to secrete pepsinogen?

A

H+

59
Q

How much fluid does the exocrine pancreas secrete?

A

~ 1L into duodenum

60
Q

What does the exocrine solution produced from the pancreas contain?

A

Aqueous solution containing enzymes and high HCO-3

61
Q

What is the function of the bicarbonate from the exocrine pancreas?

A

Neutralises stomach H+

62
Q

What does the exocrine panceatic secretion contain?

A
  • High HCO-3 (isotonic)

- Pancreatic lipase, amylase and proteases

63
Q

What factors increase exocrine pancreatic secretion?

A
  • Secretin
  • CCK (potentiates secretin)
  • Parasympathetic supply
64
Q

What innervates the pancreas?

A
  • Vagus stimulates secretion

- Sympathetic innervation inhibits secretion

65
Q

What are some pancreatic enzymes stored in until release?

A

Condensed zymogen granules

66
Q

What cells cause a modification of the ion composition which results in a fluid secretion rich in HCO-3?

A

Ductal cells

67
Q

What ions make the pancreatic fluid ‘isotonic’?

A

Na+, K+, Cl- and HCO-3

68
Q

What reactionn creates the bicarbonate ions which are then secreted into the duodenum?

A

HCO3- H2CO3 (carbonic anhydrase) CO2 + H2O

69
Q

What cells produce the enzymes for pancreatic secretion?

A

Acinar cells

70
Q

What enzymes are secreted in an inactive form and are activated in the duodenum?

A

Proteases

71
Q

What enzyme breaks down starch?

A

Amylase

72
Q

What are the 3 types of pancreatic enzymes?

A
  • Pancreatic amylase
  • Pancreatic lipase
  • Proteolytic
73
Q

What does pancreatic amylase breakdown?

A

Polysaccharides to disaccharides

74
Q

What do pancreatic lipases breakdown?

A

Triglycerides -> monoglycerides + FA

75
Q

What do proteolytic enzymes do?

A

Cleave proteins at different sites -> AA and small peptides

76
Q

What are the 3 major proteolytic enzymes?

A
  • Trypsinogen
  • Chymotrypsinogen
  • Procarboxypeptidase
    All inactive (zymogens) to prevent self digestion
77
Q

What enzyme activates trypsinogen into trypsin?

A

Enteropeptidase (aka) enterokinase

78
Q

What does trypsin activate?

A
  • Chymotrypsinogen into chymotrypsin
  • Procarboxypeptidase into carboxypeptidase
  • Positive feedback mechanism which activates more trypsinogen to be produced
79
Q

The intestinal phase accounts for how much pancreatic secretion?

A

80%

80
Q

What cells secrete CCK?

A

Duodenal I cells

81
Q

What is CCK produced in response to?

A
  • Presense of AA
  • Small peptides
  • Fatty acids
    in intestinal lumen
  • Vagal release of ACh potentiates CCK action
82
Q

What does CCK trigger?

A

Acinar cells to produce enzymes

83
Q

What cells release secretin?

A

S cells of the duodenum

84
Q

What does Secretin stimulate the secretion of?

A

Aqueous rich HCO-3

85
Q

What is secretin released in response to?

A
  • Arrival of acidic chyme in duodenum

- Ach and CCk potentiate secretin action

86
Q

What does Secretin trigger the releae of?

A

Ductal cells to produce Na+, K+, Cl- and HCO-3

87
Q

What is the function of the gall bladder?

A
  • Stores bile
  • Concentrates bile
  • Ejects bile (CCK triggers release, begins ~30 mins after meal)
88
Q

How is bile concentrated in the gall bladder?

A

Epithelial cells lining the gllbladder absorb ions and water iso-osmotically

89
Q

What cells produce bile?

A

Hepatocytes, flows to gallbladder through bile ducts

90
Q

What is bile?

A
  • Essential for digestion and absorption of lipids
  • Bile salts emulsify lipids
  • Mixture of bile salts, pigments and cholesterol
  • Not enzymic
91
Q

What system recirculates bile salts?

A

Recirculated via enterohepatic system

92
Q

What does CCK do to release bile?

A
  • Gall bladder contracion

- Sphincter of Oddi relaxation

93
Q

What does secretin do to bile?

A

Triggers bile secretion, espec NaHCO3-

94
Q

What makes up bile?

A
  • Bile salts (50%)
  • Phospholipids (40%)
  • Cholesterol (4%)
  • Bile pigments (e.g bilirubin) (2%)
  • HCO3-
95
Q

What does secreton stimulate (in relation to bile)?

A

Production of bile

96
Q

What are crypts of LIeberkuhn?

A
  • Secretions in SI
97
Q

What 2 cells covevr crypts and villi in SI?

A
  • Goblet cells - secrete mucus
  • Enterocytes - in crypts secrete water and electrolytes (1800ml/day)
  • In villi absorb water and electrolytes (along with products of digestion)
98
Q

What are secretions of the SI largely regulated by?

A

Distention and tactile or irritative stimuli from chyme

99
Q

What is present in the alkaline mucus of LI?

A
  • High K+ and bicarbonate

- No digestive enzymes

100
Q

What are the functions of the alkaline mucus of the colon?

A

Potection and lubrication

101
Q

What neutralises alkaline mucus in the colon?

A

H+ produced by gut bacteria

102
Q

What triggers the release of alkaline mucus in LI?

A

Greatest trigger is distention / mechanical stimulation of walls
Increased secretion triggered by Ach and VIP
Decresed secretion triggered by adrenaline and somatostatin

MD3002 Physiology (24 decks)

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