Gastric Secretions Flashcards Preview

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Flashcards in Gastric Secretions Deck (28)
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1
Q

Oxyntic (gastric) glands

A
  • mucous neck cells that secrete mucus and some pepsinogen
  • peptic (chief) cells that secrete pepsinogen
  • parietal (oxyntic) cells: HCl and intrinsic factor
2
Q

Pyloric glands

A
  • primarily mucus
  • small amounts of pepsinogen and large amounts of thin mucus
  • gastrin
3
Q

*Actions of gastrin

A

Secreted by G cells in response to proteins in the gastric lumen

  • stimulates secretion of histamine by enterochromaffin-like cells
  • histamine then acts on parietal cells stimulating acid secretion
  • gastrin also has trophic effects on enterochromaffin and parietal cells
4
Q

*Increased gastrin =

A

Increased gastric acid

5
Q

Pepsinogen secretion

A

Secreted pepsinogen is an inactive precursor

  • activated by HCl to form pepsin
  • catalyzed by presence of pepsin
  • HCl is required for proteolytic activity of pepsin
6
Q

Regulation of pepsinogen secretion

A

Release from peptic cells is stimulated by Ach release or presence of acid
- rate of release strongly influenced by amount of acid in the stomach

7
Q

Acid secretion

A

Stimulated parietal cells secrete acid solution

  • isotonic to body fluids
  • pH: 0.8
  • requires 1500 calories per liter of gastric juice
8
Q

Mechanisms of acid secretion: process 1

A

Chloride is actively transported from cytoplasm to lumen, while Na is actively transported in the opposite direction
- causes negative potential in lumen = K diffuses out of cell into lumen

9
Q

Mechanisms of acid secretion: process 2

A

H2O dissociated via carbonic anhydrase

  • H ions actively secreted into lumen in exchange for potassium
  • catalyzed by H K ATPase pump
  • remaining Na in lumen is actively absorbed, allowing H to take its place
10
Q

Mechanisms of acid secretions: process 3

A

CO2 combines with hydroxyl ions in cytoplasm to form bicarb

- bicarb diffuses out of cell into ECF in exchange for chloride ions

11
Q

Mechanisms of acid secretions: process 4

A

Water passes into lumen by osmosis following ions

12
Q

Final secretion contains

A
  • HCl
  • KCl
  • small amount of NaCl
13
Q

*Hydrogen-potassium ATPase

A

Unique to parietal cell

  • pumps H against an enormous concentration gradient (3 million to one)
  • exists, preformed in cytoplasm of cell until needed
  • transported to cell membrane upon stimulation of the parietal cell
14
Q

Neural and hormonal control

A
  • Ach: acts on all cell types in gastric glands
  • gastrin: secreted by G cells in pyloric glands and acts on parietal cells in gastric glands
  • histamine: acts on parietal cells, secreted by enterochromaffin-like cells
15
Q

Control of acid secretion

A

Parietal cells are only source of HCl, closely associated with enterochromaffin-like cells which lie adjacent to parietal cells in gastric glands
- rate of acid secretion is related to the rate of histamine secretion

16
Q

Enterochromaffin-like cells are stimulated by ____

A
  • gastrin secreted by pyloric glands in response to proteins

- acetylcholine released from vagus nerve

17
Q

Activation of what receptors leads to production of gastric acid?

A
  • acetylcholine (M3)
  • histamine (H2)
  • gastrin (G)
18
Q

What contributes to complete blockade of acid secretion?

A

H-K pump (proton pump) blockade

- drug of choice for reduction of acid secretion

19
Q

Delayed effect

A

Complete pump inhibition may take up to 4 days

- include histamine blockers for first few days of therapy

20
Q

When pH drops below 3.0

A
  • gastrin secreted by G cells is stopped
  • gastrin mediated HCl release is reduced
  • reduction in acid (raising pH) slows pepsinogen release
21
Q

If acid production is blocked?

A
  • feedback inhibition is lost
  • gastrin release continues
  • may lead to hypertrophy of parietal and enterochromaffin-like cells
22
Q

*Cephalic phase (neural stimulation)

A
  • release of HCl by parietal cells
  • release of gastrin by G cells
  • gastrin subsequently acts on parietal cells increasing acid secretion
23
Q

*Cephalic phase is the ______ response to food

A

Brain

  • sight, smell, thought, taste, intensity increases with appetite
  • mediated by neural signals: transmitted via vagus nerve, responsible for 20% of gastric secretion
24
Q

Gastric phase

A

Presence of food in the stomach

  • stimulates vagovagal and enteric reflexes
  • stimulates gastrin mechanism
  • accounts for 70% of gastric secretion
25
Q

Intestinal phase

A

Presence of food in proximal SI

  • releases small amounts of gastrin from duodenum
  • leads to a weak stimulus for secretion
26
Q

*Inhibition of gastric secretion intestinal factors

A

Following the weak intestinal phase the intestines often inhibit gastric secretion
- presence of chyme may lead to an inhibitory enterogastric reflex

27
Q

*Enterogastric reflex

A
  • distention of small bowel
  • presence of acid in duodenum
  • presence of protein breakdown products
  • mucosal irritation
28
Q

Presence of acid, fat, protein breakdown products, hyper/hypo osomolar fluids, or any irritation leads to release of

A

Secretin: opposes gastric secretion

  • gastric inhibitory peptide
  • vasoactive intestinal polypeptide
  • somatostatin