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ESA3 > GI S4 & S5 - The Stomach > Flashcards

GI S4 & S5 - The Stomach Flashcards

0
Q

Why does food need to be stored in the stomach?

A

Food is ingested faster than it can be digested so chyme must be released into the duodenum at a controlled rate

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1
Q

What are the purposes of the stomach?

A

To store food
Physical disruption
Chemical disruption
Disinfection

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2
Q

What is the pH of the stomach?

A

Below 2.0

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3
Q

How is the acidic environment of the stomach maintained?

A

Hydrochloride acid concentration of up to 100mmol/L

Secreted by parietal cells located in gastric pits

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4
Q

How is acid secretion controlled?

A

ACh, Gastrin and histamine all increase acid secretion from parietL cells

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5
Q

How does ACh increase acid secretion from parietal cells

A 
Food arrives in the stomach
This causes gastric distension
This stimulates post ganglionic parasympathetic nerves to secrete ACh
Binds to M3 muscarinic receptors
Increases H+ secretion
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6
Q

How does Gastrin increase acid secretion from parietal cells

A 
Amino acid content in the stomach stimulates endocrine (G) cells
Low pH inhibits
G cells release Gastrin
Gastrin binds to the CCK receptor
Causes increased H+ secretion
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7
Q

How does histamine increase acid secretion from parietal cells

A

Mast cells are stimulated by ACh and Gastrin
This causes histamine release
Histamine diffuses locally to parietal cells
Binds to H2 receptors
Causes increased H+ secretion

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8
Q

What are drug targets for increasing stomach pH

A

H2 antagonists

Proton pump inhibitors

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9
Q

How do H2 antagonists work?

A

H2 receptor antagonists inhibit histamine binding to H2 receptors on parietal cells
Reduced stimulation of acid production
Reduced acid secretion

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10
Q

How do PPIs work

A

Proton pump inhibitors act on proton channels on the apical membrane of parietal cells so less H+ is secreted into the lumen of the stomach

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11
Q

How is H+ produced in parietal cells?

A

The mitochondria split water: H2O -> OH- + H+
The H+ is secreted into the stomach lumen via a proton pump on the apical membrane
CO2 binds to OH-: CO2 + OH- -> HCO3-
Bicarbonate diffuses into the blood via the basolateral membrane

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12
Q

Where exactly are proton pumps located

A 
Stomach
Gastric pits
Parietal cells
Apical membrane
Canaliculi
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13
Q

What do chief cells secrete

A

Enzymes

Mainly pepsinogen, the inactive precursor of pepsin which is cleaved by acid in the lumen of the stomach

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14
Q

What are the phases of gastric secretion?

A

Cephalic
Gastric
Intestinal

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15
Q

Describe the cephalic phase of gastric secretion

A

The sight and smell of food and the act of swallowing cause parasympathetic stimulation
Therefore ACh is released

16
Q

Describe the gastric phase of gastric secretion

A

Food reaches the stomach, causing distension and pH buffering
This causes ACh stimulation and Gastrin disinhibition
Acid secretion is stimulated
Gastric peptide concentration increases due to breakdown by acid and enzymes
This stimulates Gastrin, but low pH inhibits it

17
Q

Describe the intestinal phase of gastric secretion

A

Chyme leaves the stomach
Low duodenal pH stimulates cholecystokinin (CCK) and gastric inhibitory peptide release
These reduce gastric acid secretion

18
Q

How is the stomach protected from low pH?

A

Surface stomach mucosal cells secrete a thick alkaline mucus

This provides mechanical protection and chemical protection because H+ ions become trapped by HCO3 in the mucus

19
Q

How are stomach defences controlled?

A

Prostoglandins stimulate mucus secretion

20
Q

How does poetic ulceration occur?

A

When stomach defences are damaged or overwhelmed then acid reaches the stomach mucosal cells
This causes gastritis then ulceration

21
Q

What can disrupt stomach defences?

A

Infection by Helicobacter pylori
NSAIDs
Excess alcohol

22
Q

Briefly describe gastric motility

A

Swallowing induces a vagal reflex called receptive relaxation to reduce resting tension in the walls of the stomach so pressure remains low so oesophageal reflux is less likely
Peristalsis is induced by a pacemaker in the cardiac region. It moves from cardia to pylorus and both mixes contents and causes a small squirt of chyme to be deposited in the duodenum

23
Q

What controls the rate of gastric emptying?

A

Feedback from the duodenum, induced by the presence of:

  • Fats
  • Low pH
  • Hypotonicity
24
Q

What is a gastric ulcer?

A

Mucosal inflammation and breakdown

25
Q

How does the area of the stomach that is colonised by H pylori affect the patients symptoms and pathology?

A

If Antrum-predominant, most likely pathology is duodenal ulcer
If body-predominant, most likely pathology is gastric ulcer and cancer
If both Antrum and body predominant, most likely patient will be asymptomatic

ESA3 (17 decks)

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