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ESA 3 - Gastrointestinal System > Stomach > Flashcards

Stomach Flashcards

1
Q

What are the functions of the stomach?

A
  • Stores food
  • Disinfects food
  • Breaks food down in chyme
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2
Q

How does the stomach break food down into chyme?

A
  • Chemical disruption
    • Acid
    • Enzymes
  • Physical disruption
    • Motility
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3
Q

Where do stomach secretions come from?

A

Gastric pits

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4
Q

What are gastric pits?

A

Indentations in the stomach mucosa that are openings to gastric glands

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5
Q

What do gastric pits contain?

A

Neck cells

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6
Q

What do gastric glands contain?

A

Parietal, Chief, and G-cells, along with smooth muscle cells

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7
Q

Label this diagram

A
  • A - Gastric pit (opening to gastric gland)
  • B - Mucous epithelium
  • C - Lymphatic vessel
  • D - Lamina propria
  • E - Muscularis mucosae
  • F - Submucosa
  • G - Oblique muscle
  • H - Circular muscle
  • I - Longitudinal muscle
  • J - Serosa
  • K - Artery and vein
  • L - Myenteric plexus
  • M - Gastric pit
  • N - Gastric gland
  • O - Mucous cells
  • P - Neck
  • Q - Parietal cells
  • R - Chief cells
  • S - Smooth muscle cells
  • T - G cell
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8
Q

What substances are secreted by the stomach?

A
  • Hydrochloric acid
  • Proteolytic enzymes
  • Mucus
  • HCO3-
  • Gastrin
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9
Q

What proteolytic enzyme is secreted by the stomach?

A

Pepsin

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10
Q

What is the cellular origin of hydrochloric acid in the stomach?

A

Parietal (oxyntic) cells

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11
Q

What is the purpose of HCl secreted by the stomach?

A

Acid keeps luminal pH < 2

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12
Q

What is the cellular origin of proteolytic enzymes in the stomach?

A

Chief cells

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13
Q

What is the function of proteolytic enzymes in the stomach?

A

Non-specifically breaks down proteins to peptides

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14
Q

What is the cellular origin of mucus in the stomach?

A

Neck cells (surface cells)

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15
Q

What are the characteristics of stomach mucus?

A
  • Sticky
  • Basic
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16
Q

Why is stomach mucus sticky?

A

So it is not removed easily from the stomach lining

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17
Q

Why is stomach mucus basic?

A

Due to amine groups on proteins

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18
Q

What is the cellular origin of HCO3- in the stomach?

A

Neck cells (surface cells)

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19
Q

What is the purpose of HCO3- in the stomach?

A

Provides a buffer for H+ ions

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20
Q

Where is HCO3- secreted by neck cells into?

A

The mucus

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21
Q

What is the cellular origin of gastrin?

A

G-Cells

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22
Q

What is the function of gastrin in the stomach?

A

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

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23
Q

Are body fluids acid or alkaline?

A

Most are slightly alkaline

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24
Q

What must be true for a slightly alkaline body fluid to secrete H+ ions?

A

It must be created in large quantities

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25
Q

Where does the production of a large amount of body fluid with the intent of secreting H+ occur?

A

In the mitochondria of parietal cells

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26
Q

How is H+ produced in parietal cells?

A

By splitting water into H+ and OH- ions

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27
Q

What happens to OH- ions produced from the splitting of water in parietal cells?

A

They combine with CO2 from metabolsim to from HCO3-, which is exported to the blood

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28
Q

How much HCO3- enters the blood for every mole of H+ secreted into the stomach?

A

1 mole

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29
Q

What allows parietal cells to produce H+ at a high rate?

A

They have lots of mitochondria

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30
Q

Can the ions produced in the mitochondria of parietal cells accumulate in the cell?

A

No

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31
Q

How is the problem of the ions produced by parietal cells being unable to accumulate overcome?

A

Parietal cells have invaginations in their cell walls called canaliculi

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32
Q

What do canaliculi possess that are required for their function?

A

Proton pumps

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33
Q

What do the proton pumps in canaliculi do?

A

Expel H+ from parietal cells up a high concentration gradient

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34
Q

What is the result of the concentration gradient canaliculi are pumping against being very high?

A

This is a very energy intensive process

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35
Q

What are the proton pumps in canaliculi a key target for?

A

Drug action

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36
Q

Why are the proton pumps in canaliculi a key target for drug action?

A

Because if inhibited, they will reduce the amount of acid in the stomach

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37
Q

Draw a diagram illustrating the mechanism of secretion of stomach acid

A
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38
Q

What controls gastric acid secretion?

A

A complex of neural and endocrine systems

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39
Q

What stimulates parietal cells?

A
  • Acetylcholine
  • Gastrin
  • Histamine
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40
Q

How do acetylcholine, gastrin, and histamine act to control parietal cells?

A

They act via seperate receptors to promote acid secretion

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41
Q

What is Ach released from?

A

Postganglionic parasympathetic neurones

42
Q

What stimulates the release of Ach from postganglionic parasympathetic neurones in the stomach?

A

Gastric distention as food arrives

43
Q

What receptors does Ach act on in parietal cells?

A

Muscarinic

44
Q

What releases gastrin?

A

Endocrine cells in the stomach, G-cells

45
Q

What kind of molecule is gastrin?

A

A 17-aa polypeptide

46
Q

What does gastrin bind to?

A

Surface receptors in parietal cells

47
Q

What stimulates gastrin secretion?

A

The presence of peptides and Ach from intrinsic neurones

48
Q

What inhibits gastrin secretion?

A

Low pH in the stomach

49
Q

What does the inhibition of gastrin by low pH in the stomach act as?

A

Feedback control

50
Q

What releases histamine?

A

Mast cells

51
Q

What happens to histamine once released?

A

It diffuses locally to bind to H2 receptors on parietal cells

52
Q

How does histamine stimulate acid secretion?

53
Q

What does histamine work as?

A

An amplifier

54
Q

Why does histamine work as an amplifier?

A

Gastrin and Ach stimulate mast cells

55
Q

What are the phases of gastric secretion?

A
  1. Cephalic phase
  2. Gastric phase
  3. Intestinal phase
56
Q

What is the cephalic phase led by?

57
Q

What happens in the cephalic phase?

A

The sight and smell of food, and the act of swallowing, activates the parasympathetic nervous system, which stimulates the release of Ach

58
Q

What is the effect of the release of Ach in the cephalic phase?

A

It stimulates parietal cells directly and via histamine, increasing stomach acid

59
Q

What happens in the gastric phase?

A
  • Once food reaches the stomach, it causes distension, further stimulating Ach release, and subsequently parietal cells, increasing acid
  • The arrival of food will also buffer the small amount of stomach acid in the stomach in between meals, causing luminal pH to rise. This disinhibits gastrin, increasing acid
  • Acid and enzymes will then act on proteins to produce peptides, further stimulating gastrin release as the pH falls and the initial disinhibition is removed, increasing acid
60
Q

What happens in the intestinal phase?

A
  • Once chyme leaves the stomach in significant quantities, it stimulates the release of the hormones cholecystokinin and gastric inhibitory polypeptide from the intestines that antagonise gastrin, decreasing acid
  • Coupled with this, the small amount of acid left in the stomach is no longer being buffered by food, and the low pH inhibits gastrin, decreasing acid
61
Q

What is the clinical relevance of the low pH of the stomach between meals?

A

It can aggravate ulcers

62
Q

What is the result of the low pH of the stomach between meals aggrevating ulcers?

A

Pain from ulcers is particuarly bad at night

63
Q

What drugs may inhibit acid secretion?

A
  • H2 receptor inhibitors
  • Proton Pump Inhibitors (PPIs)
64
Q

Give an example of a H2 receptor inhibitor

A

Cimetidine

65
Q

How does a H2 receptor inhibitor exert its effects?

A

Removes the amplification of gastrin / Ach signal

66
Q

Give an example of a proton pump inhibitor?

A

Omeprazole

67
Q

How does a proton pump exert its effects?

A

Prevents H+ ions being pumped into parietal cell canaliculi

68
Q

Why do neck cells secrete mucus?

A

To protect the mucosa

69
Q

Why does the stomach mucosa need protection?

A

Because the luminal pH of the stomach is usually below 2, and without any protection, this would dissolve mucosa

70
Q

What does the mucus form in the stomach?

A

An ‘unstirred layer’ that ions cannot move through easily

71
Q

What happens to H+ ions in the unstirred layer?

A

They slowly diffuse in and react with the basic groups on mucus, and with HCO3- that is secreted by surface epithelial cells

72
Q

What is the result of the unstirred layer on HCO3-?

A

It means it stays close to the surface cells

73
Q

What is the result of HCO3- staying close to the surface cells?

A

The pH at the surface cells is well above 6

74
Q

What stimulates mucus and HCO3- secretion?

A

Prostagladins

75
Q

What are prostaglandins promoted by?

A

Most factors that stimulate acid secretion

76
Q

What agents can breach the stomachs defences?

A
  • Alcohol
  • H. Pylori
  • NSAIDS
77
Q

How does alcohol breach the stomachs defences?

A

It dissolves the mucus, allowing the acid to attack the stomach

78
Q

How does H. Pylori breach the stomachs defences?

A

Surface cells become infected, inhibiting mucus/HCO3- production

79
Q

How do NSAIDS breach the stomach defences?

A

They inhibit prostaglandins, therefore reducing defences

80
Q

Why are some NSAIDS not harmful to the stomachs defences?

A

Some are convereted to a non-ionised form by stomach acid, allowing them to pass through the mucus layer into cells before they re-ionise

81
Q

Give an example of an NSAID that is converted to a non-ionised form by stomach acid?

82
Q

What does a breach in the stomachs defences result in?

A

Peptic ulcers

83
Q

What does the treatment of peptic ulcers involve?

A
  • Reducing acid secretion
  • If present, eliminating H. Pylori with antibiotics
84
Q

What happens in receptive relaxation?

A

As food travels down the oesophagus, a neural reflex carried out by the vagus nerve triggers the relaxation of the muscle in the stomach’s wall

85
Q

What is the purpose of receptive relaxation?

A

Prevents the pressure from increasing, so pressure in the stomach doesn’t increase as it fills

86
Q

Why is it important that the pressure in the stomach doesn’t increase as it fills?

A
  • Limits reflex
  • Allows us to consume large meals
87
Q

When can receptive relaxation not occur?

A

If there is damage to the vagus nerve

88
Q

What kind of muscle does the stomach have?

A

Longitudinal and circular

89
Q

What drives the muscle in the stomach?

A

A pacemaker

90
Q

Where is the pacemaker that drives the stomach located?

A

In the cardiac region

91
Q

How often does the stomach pacemaker fire?

A

~3 times a minute

92
Q

What does the firing of the stomachs pacemaker cause?

A

Regular, accelerating peristalic contractions from the cardia to the pylorus

93
Q

What is the action of the stomachs pacemaker combined with to exert its effects?

A

The stomach’s funnel shape

94
Q

What is the result of the stomachs funnel shape and the firing of its pacemaker?

A

It mixes the contents of the stomach and moves liquid chyme into the pyloric region

95
Q

When does the movement of liquid chyme into the pyloric region occur?

A

As the accelerating peristaltic wave overtakes larger lumps

96
Q

What happens to larger lumps when the accelerating peristaltic wave overtakes them?

A

They are driven back into the fundus

97
Q

What happens to chyme as the peristaltic wave overtakes large lumps?

A

It is decanted into the pyloric region

98
Q

What is the oveall effect of the accelerating, rhymtic, peristaltic contraction of the stomach?

A

It moves solid lumps backwards into the fundus of the stomach, whilst letting liquid chyme move forwards

99
Q

What happens as chyme enters the pyloric region?

A

A small squirt is ejected before the peristaltic wave reaches the pylorus and shuts it, so the rest of the chyme returns to the stomach

100
Q

How many squirts of chyme are ejected a minute?

A

3, the same as the number of peristaltic waves

101
Q

What is the volume of chyme squirted out affected by?

A
  • The rate of acceleration of peristaltic wave
  • Hormones from the intestine
102
Q

What slows gastric emptying?

A
  • Fat
  • Low pH
  • Hypertonicity in the duodenum

ESA 3 - Gastrointestinal System (12 decks)

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