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Semester 3 - Gastrointestinal > Session 4 - The Stomach > Flashcards

Session 4 - The Stomach Flashcards

1
Q

Describe the functions of the stomach

A

• Stores food
• Disinfects food
• Breaks food down into chyme
○ Chemical disruption (acid and enzymes)

Physical disruption (motility)

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2
Q

What is the structural component of the stomach through which acid is secreted?

A

Gastric pits, indentations in the stomach mucosa

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3
Q

What are the five cell types found in the gastric pits?

A
  • Neck cells
    • Parietal cells
    • Chief cells
    • G-cells
    • Smooth muscle cells
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4
Q

Give five secretions of the gastric pits

A
  • Hydrochloric acid
    • Proteolytic enzymes
    • Mucus
    • HCO3-

Gastrin

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5
Q

What is the purpose of HCL and from where is it secreted?

A

• Keeps luminal pH <2

Parietal cells

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6
Q

What is the purpose of proteolytic enzymes and from where are they secreted?

A
  • Non-specifically break down protein into peptides

* Chief cells

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7
Q

What is the purpose of mucus and from where is it secreted?

A
  • Sticky (not easily removed from stomach lining)
    • Basic, due to amine groups on proteins

Neck cells (surface cells)

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8
Q

What is the purpose of HCO3- and from where is it secreted?

A
  • Secreted by surface cells into the mucus
    • Provides a buffer for H+ ions

Neck cells (surface cells)

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9
Q

What is the purpose of gastrin and from where is it secreted?

A

• Binds to surface receptor on parietal cell, stimulating acid and intrinsic factor

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10
Q

What is a key issue when considering the difficulties surrounding the secretion of stomach acid?

A

• Most body fluids are slightly alkaline, so to secrete H+ ions they must be created in large quantities

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11
Q

How are H+ ions produced in gastric pits?

A

• Formed in the mitochondria of parietal cells by splitting water into H+ and OH-

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12
Q

What does the splitting of water into H+ and OH- do?

A

• OH- ions removed by combining with CO2 from metabolism to form HCO3-, which is exported to blood

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13
Q

What is the functions of canaliculus?

A
  • Invaginations in cell walls into which protons are pumped by proton pumps
    • Avoids accumulation of protons within the cell
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14
Q

What is useful about the proton pumps which line the canaliculi?

A

If inhibited they will reduce the amount of acid in the stomach

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15
Q

What three hormones stimulate parietal cells to release acid?

A
  • Acetycholine
    • Gastrin
    • Histamine
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16
Q

Where is acetycholine released from and why?

A
  • Postganglionic parasympathetic neurones

* Released as a result of distension of the stomach as food arrives and CNS stimulation after food stimuli sensed

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17
Q

What is Gastrin released from in the stomach and why?

A
  • G-cells

* The presence of peptides and Ach stimulates its release

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18
Q

Outline the structure of gastrin

A
  • 17 amino acid

* Binds to surface receptors on parietal cells

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19
Q

What inhibits gastin release from the stomach?

A

• Low pH

20
Q

Where is histamine released from?

A
  • Mast cells

* Diffuses locally to bind H2 surface receptors on parietal cells

21
Q

How does histamine secretion stimulate acid secretion?

A

• Acid secretion stimulated by c-amp

22
Q

What stimulates mast cells to release histamine?

A

Gastrin and Ach

23
Q

What are the three phase of gastric secretion?

A
  • Cephalic phase
    • Gastric Phase
    • Intestinal phase
24
Q

Outline what occurs in the cephalic phase

A
  • The sight, small and act of swallowing food activates the parasympathetic nervous system
    • Ach released, which directly stimulates parietal cells to release acid, and sitmulates release of histamine
25
Q

Outline what occurs in the gastric phase

A
  • Food reaches stomach, causing distension, the release of Ach and thus parietal cells to release acid
    • Arrival of food buffers small amount of stomach acid, causing luminal pH to rise
    • Disinhibits Gastrin (increase in acid)
    • Acid and enzymes then act on proteins to produce peptides, further stimulating gastrin
26
Q

Outline what happens in the intestinal phase

A
  • Chyme leaves stomach, stimulating release of cholecystokinin and gastric inhibitory polypeptide from the intestines
    • These antagonise gastrin (decrease stomach acid)
    • Small amount of acid in stomach no longer buffered by food, low pH inhibits gastrin (decreased acid)
27
Q

Outline two ways in which acid secretion can be reduced?

A
  • Inhibition of histamine at H2 receptors

* Proton pump inhibitors

28
Q

What does inhibitions of histamine at H2 receptors do?

A

• Removes the amplification of gastrin/Ach signal

29
Q

What do proton pump inhibitors do?

A

Prevent H+ ions being pumped into parietal cell canaliculi

30
Q

Why does the stomach need defences?

A
  • Because the luminal pH is usually below 2

* Without any protection this would dissolve mucosa

31
Q

Where is mucus secreted from in the stomach?

A

• Neck cells

32
Q

What two properties of mucus make it good for defense?

A

• Sticky so not easily removed from the stomach lining

Basic, due to amine groups on the proteins

33
Q

What does the mucus form which prevents acid reaching the stomach wall?

A
  • An ‘unstirred layer’ that ions cannot move through easily

* H+ ions slowly diffuse in and react with the basic groups on mucus and HCO3- by surface epithelial cells

34
Q

What is the pH of the surface cells?

A

6

35
Q

What causes the secretion of mucus and HCO3- from neck and surface cells?

A

• Prostaglandins, which are promoted by factors that stimulate acid secretion

36
Q

Give three factors which break down the stomach’s defences

A
  • Alcohol
    • H pylori
    • NSADIs
37
Q

What does alcohol do to the stomachs defences?

A

• Dissolves the mucus, allowing the acid to attack the stomach

38
Q

What does H pylori do to stomach’s defences?

A

• Surface cells become infected, inhibiting mucus/HCO3- production

39
Q

What do NSAIDs do?

A

• Inhibit prostaglandin, reducing defences

ASPIRIN

40
Q

What occurs if the stomachs defences are breached?

A

Peptic ulcers

41
Q

What is receptive relaxation?

A
  • As food travels down the oesophagus, a neural reflex carried out by the vagus nerve triggers the relaxation of muscle in the stomach’s wall, so pressure does not increase
    • This prevents reflux and allows us to consume large meals7
42
Q

What causes rhythmic contractions of the stomach?

A
  • Has longitudinal and circular muscle driven by a pacemaker in the cardiac regions
    • Pacemaker fires 3 times a minute, causing regular, accelerating peristaltic contractions from the Cardia -> Pylorus
    • This, combined with the stomach’s funnel shape mixes the contents of the ttoamch and moves liquid chyme into the pyloric regions
43
Q

What keeps larger lumps of food in the stomach in the fundus of the stomach?

A
  • Accelerating peristaltic wave overtakes larger lumps, driving them back into fundus
    • Chyme decanted into pyloric region
44
Q

Outline gastric emptying

A

• Three peristaltic waves -> Three ejectd squirts of chyme occur a minute

45
Q

What is squirt volume affected by?

A

• The rate of acceleration of peristaltic waves and hormones from the instestine

46
Q

What is gastric emptying slowed by?

A
  • Fat
    • Low pH
    • Hypertonicity in the duodenum

Semester 3 - Gastrointestinal (28 decks)

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