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Semester 3 - GI > Lecture 4.1 - Functions of the Stomach > Flashcards

Lecture 4.1 - Functions of the Stomach Flashcards

1
Q

Why must food be stored within the stomach temporarily?

A

We eat food faster than we digest it

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2
Q

What would happen if the food remained static within the stomach during storage?

A

Pressure within the stomach would rise, causing reflux

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3
Q

How is reflux avoided during storage of food in the stomach?

A
  • Receptive relaxation: vagally mediated relaxation of the orad region of the stomach
  • Rugae distend, increasing stomach volume
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4
Q

How are the stomach contents mixed?

A

Peristalsis - coordinated movements every 20s or so

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5
Q

Briefly explain how the stomach separates out the larger chunks of food (which need further digestion) from the liquid chyme which is ready to move on to the duodenum

A

Peristalsis propels food from proximal to distal.
This allows the stomach to act as a funnel, as it is larger proximally than distally –> separates out the liquid chyme from the larger chunks

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6
Q

Is the pyloric sphincter open during peristalsis? Explain

A

No - only opes when chunks of food are small enough to progress

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7
Q

What is achieved by the acidity of the stomach’s internal environment?

A
  • Unravels proteins
  • Activates zymogens such as proteases
  • Disinfects stomach contents
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8
Q

What do the parietal cells produce?

A

HCl and intrinsic factor

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9
Q

Where are the parietal cells found?

A

Proximal part of stomach - fundus and body only

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10
Q

Why is intrinsic factor important?

A

Vital for vitamin B12 absorption

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11
Q

Where are the G cells found?

What do they secrete?

A

Found in antrum

Secrete gastrin

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12
Q

What do enterochromaffin-like cells produce?

A

Histamine

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13
Q

What do the chief cells secrete?

A

Pepsinogen

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14
Q

What do the D cells produce?

A

Somatostatin

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15
Q

List the important cell types found in the stomach

A 
Parietal cells
G cells
D cells
Chief cells
Enterochromaffin-like cells
Mucous cells
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16
Q

What substances does the stomach secrete?

A 
HCl
Intrinsic factor
Mucus
HCO3-
Pepsinogen
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17
Q

Where does secretion take place within the stomach walls?

A

Secretion occurs from gastric pits which overly gastric glands.
Gastric pits appear as pin-prick like holes in the stomach wall

18
Q

List the cell types lining the gastric pit and gland, from superficial to deep

A

Mucous cells - most superficial - line pit
Parietal cells
Chief cells
Enteroendocrine cells - deepest

19
Q

Briefly outline the mechanisms of stomach acid secretion

A

1) Water splits into OH- and H+
2) H+ moves into stomach lumen via H+/K+-ATPase –> highly ATP dependent
3) At the same time, Cl- moves into the parietal cell from the ECF in exchange for HCO3-, which is produced from the combination of CO2 and OH-. HCO3- moves into the bloodstream
4) Cl- then moves into the stomach lumen where it combines with H+ to form HCl

20
Q

Explain the phenomenon known as the “alkaline tide”

A

Just after eating, there is a transient increase in blood pH.
This is caused by the influx of HCO3- moving into the blood from parietal cells, in exchange for secretion of HCl into the stomach lumen.

21
Q

What stimulates parietal cells to secrete HCl?

A

Gastrin
Histamine
ACh

22
Q

What does histamine bind to on parietal cells to stimulate HCl secretion?
Does histamine have a major role in stimulation of secretion?

A

H2 receptors

No - fine-tuning only

23
Q

What sort of receptors does ACh bind to on parietal cells to stimulate acid secretion?
What nerve controls this?

A

Muscarinic receptors

Vagus nerve

24
Q

What sort of receptors does gastrin bind to on parietal cells?

A

CCK receptors

25
Q

What sort of molecule is gastrin and hence how does it travel to the parietal cells from its site of synthesis?

A

Hormone - travels in blood

26
Q

Where is gastrin made and by what cell type?

A

The antrum of the stomach, by G cells

27
Q

What stimulates gastrin secretion?

A

Peptides/AAs in stomach lumen

Vagal stimulation - ACh and gastrin-releasing peptide

28
Q

What inhibits gastrin secretion?

What effect does this have in the stomach?

A

Somatostatin

Decreases HCl secretion

29
Q

How is HCl production inhibited once food has left the stomach?

A
  • Lower pH as food no longer acting as buffer - stimulates D cells to produce somatostatin –> inhibits G cells –> less gastrin –> less parietal cell stimulation
  • Reduced distention means reduced vagal activity, which reduces stimulation of G cells and parietal cells
30
Q

List 2 classes of drug which reduce gastric acid secretion and give an example of each.

A

H2 receptor blockers - cimetidine

Proton pump inhibitors (PPIs) - omeprazole

31
Q

Outline the mechanism by which PPIs work

A

Inhibit the H+/K+-ATPase so H+ cannot be pumped out of the parietal cell into the stomach lumen

32
Q

What are the 3 phases of digestion?

What proportion of HCl is produced during each phase?

A

Cephalic - 30%
Gastric - 60%
Intestinal - 10%

33
Q

What is the purpose of the cephalic phase of digestion?

A

Occurs before food reaches for stomach - aim is to prepare for that moment

34
Q

What stimulates the cephalic phase of digestion?

A

Smell/taste/chewing/swallowing -> stimulate vagus nerve which acts on parietal and G cells

35
Q

How is the gastric phase of digestion initiated?

A

Food reaches stomach and causes it to distend, stimulating the vagus nerve

36
Q

What happens during the intestinal stage of digestion?

A

Initially chyme stimulates gastrin secretion, but soon G cells inhibited as food has now left stomach

37
Q

Why is it important for the stomach to have mechanims of defence?

A

It is designed to digest organic material, including itself

38
Q

What are the 3 major mechanisms of defence of the stomach?

A

Mucus/HCO3- secretion
High turnover of epithelial cells
Prostaglandins

39
Q

How does mucous protect the stomach from damage?

A

Forms a thick layer that adheres to epithelium, keeping it at higher pH

40
Q

How do prostaglandins defend the stomach?

A

Vasodilators - maintain mucosal blood flow, hence supplying epithelia with plenty of nutrients for renewal

41
Q

What may breach the stomach defences, causing stomach complaints?
How?

A

Alcohol - dissolves mucus layer
H. pylori
NSAIDs - inhibit prostaglandins

42
Q

What are the 3 basic functions of the stomach?

A

Receiving food from the oesophagus and storing it temporarily
Disrupting food mechanically
Continuing digestion

Semester 3 - GI (6 decks)

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