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GI System > Gastric Secretion > Flashcards

Gastric Secretion Flashcards

1
Q

What are the important parts of the stomach?

A

Fundus
Body
Antrum
Cellular composition (mucous neck cells, chief cells and parietal cells)

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2
Q

What is the function of the fundus?

A

Storage: walls are thin and stretchy to allow for expansion to balance pressure

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3
Q

What is the function of the body of the stomach?

A

Storage

Release of mucus, HCl, pepsinogen and intrinsic factor

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4
Q

What is the function of the antrum?

A

Mixing/grinding

Release of gastrin (produced by G cells and stimulates HCl release)

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5
Q

What is the function of mucous neck cells?

A

Mucus secreting and precursors for mature surface mucous cells

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6
Q

What is the function of chief cells?

A

Secretes pepsinogen (to digest proteins) and gastric lipase (to digest fat)

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7
Q

What is the function of parietal cells?

A

Releases HCl and intrinsic factor

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8
Q

What are the effects of the release of HCl into the stomach from parietal cells?

A

Releases of pepsin (digest protein)
HCl denatures proteins by breaking disulphide and hydrogen bonds to allow pepsin to get more peptide bonds
HCl kills bacteria and other ingested organisms
Inactivates salivary amylase,(stops carb digestion that began in the mouth)

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9
Q

Other than secretion, what is the function of parietal cells?

A

Specialised epithelial cells with tight junctions important to prevent gastric acid from diffusing though and damaging outside environment

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10
Q

How is HCl formed in parietal cells?

A
  1. CO2 diffuses across membrane from the blood and combine with water in the cytoplasm
  2. The conversion of CO2 and H2O into unstable carbonic acid is aided by carbonic anhydrase
  3. Acid immediately dissociates
  4. The H+ ions are actively pumped out of cell (with ATP) whilst K ions are pumped in – proton pump H-K-ATPase
  5. Bicarbonate diffuses out of the basolateral membrane into the blood, in exchange for Cl, making the blood that leaves the stomach less acidic
  6. Cl- immediately exits the cell via Cl channel in apical membrane
  7. Water is drawn from the blood and into the stomach lumen as HCl is formed in stomach lumen
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11
Q

What four hormones that stimulate generation of ATP in the stomach?

A

Gastrin
Histamine
Prostglandin
Acetylcholine

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12
Q

How does Histamine stimulate acid production?

A

It is a paracrine released from enterochromaffin (ECL) cells and its adhesion to molecule on the basolateral membranes, stimulates protein Gs to activate andenylyl cyclase, which converts ATP –> cAMP

cAMP stimulates protein kinase to allow the H/K-ATPase pump

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13
Q

What is the key protein for acid secretion in parietal cells?

A

H/K-ATPase (proton pump) - expressed at the apical membrane which uses energy derived from ATP hydrolysis to pump H+ ions into the lumen in exchange for potassium ions

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14
Q

Name a regulatory molecule that inhibits acid secretion in parietal cells

A

Somatostatin

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15
Q

How does gastrin activate H/K-ATPase?

A

Binding releases Ca to act on protein kinase

Also stimulates ECL to release Histamine

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16
Q

How does Prostaglandins activate H/K-ATPase?

A

Stimulate Gi protein which is positively coupled to adenylyl cyclase to convert ATP to cAMP

17
Q

How does Acetylcholine activate H/K-ATPase?

A

Acts on muscarinic cholinergic receptors and binding increases levels of Ca which activates protein kinase C

ACh also stimulates ECL cells to release histamine

18
Q

What three mechanisms control gastric acid secretion?

A

Neurocrine (vagus/local reflexes)
Endocrine (gastrin)
Paracrine (histamine)

19
Q

How does the parasympathetic system effect gastric secretion?

A

Sight, smell, taste –> triggers vagus nerve which stimulates release of acetyl choline to act on parietal cells

Also innervates G cells which release gastrin which stimulates HCl release and histamine from ECL cells

20
Q

What are the three phases of mechanisms involved in inhibiting gastric acid secretion?

A

Cephalic phase
Gastric phase
Intestinal phase

21
Q

What occurs in the cephalic phase during inhibition of gastric acid secretion?

A

Stopping eating –> decrease in vagal activity

22
Q

What occurs in the Gastric phase during inhibition of gastric acid secretion?

A

Decrease in pH (due to increase in HCl) –> decrease in gastrin (less release of HCl)

23
Q

What occurs during the Intestinal phase of inhibition of gastric acid secretion?

A

Acid in duodenum –> Secretin release AND enterogastric (splanchnic) reflex –> Decrease gastrin secretion and gastrin stimulation of parietal cells

24
Q

What is GIP?

A

Gastric inhibitory polypeptide - inhibits gastric acid secretion

25
Q

What is the effect of the presence of Fat/CHO in the duodenum?

A

GIP release –> decrease in gastrin secretion and parietal HCl secretion (as no more need for it if food now exited the stomach)

26
Q

What is the role of secretin?

A

Stimulates secretion of bicarbonate from pancreas to neutralise the acid in the duodenum, so gastrin is inhibited to stop more acid being released

27
Q

What are enterogastrones and give 3 examples?

A

Hormones released from gland cell is duodenal mucosa

Secretin
Cholecystokinin (CCK)
GIP

28
Q

What stimulates release on enterogastrones?

A

Acid, hypertonic solutions, fatty acids or monoglycerides in the duodenum

They act to prevent further build up in duodenum

29
Q

What are the mechanisms of enterogastrones?

A

Inhibit gastric acid secretion

Reduce gastric emptying (inhibit motility/contract pyloric sphincter)

30
Q

What is the inactive precursor of pepsinogen?

A

Zymogen

31
Q

Why is pepsinogen stored as zymogen?

A

To prevent cellular digestion and is inactive at neutral pH

32
Q

What cells secrete pepsinogen?

A

Chief cells

33
Q

What is the role of pepsinogen?

A

At low pHs (< 3 when acid present), pepsinogen converted to pepsin through acid hydrolysis - i.e. when food is present, and acid released to break down food, pepsin is required to digest proteins

34
Q

What cells of the stomach produce gastric mucus?

A

Epithelial cells and mucus neck cells

35
Q

What is the function of gastric mucus?

A 
Cytoprotective role:
Protect mucosal surface from mechanical injury 
Neutral pH (HCO3) --> protects against gastric acid corrosion and pepsin digestion
36
Q

What is the function of intrinsic factors?

A

Required to form a complex with vitamin B12 to allow Vitamins B12 to be absorbed by ileum

37
Q

What can deficiency in Vitamins B12 cause?

A

Pernicious anaemia

GI System (28 decks)

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