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GI (Semester 4) > 4.1 The Stomach > Flashcards

4.1 The Stomach Flashcards

1
Q

What are the 3 main process involved in the stomach?

A

1) Stores food (2-3 litres)
2) Disinfects it (also has bacteriocidal function)
3) Breaks it down to Chyme

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2
Q

How is chemical and physical disruption of food in the stomach achieved?

What is the end product?

A

Chemical disruption: acid and enzymes
Physical disruption: motility

Final product is chyme

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3
Q

What are the 2 main ways in which the stomach protects itself from self-digestion?

A

Via protective secretions:

1) Thick mucus lining: protection of gastric wall
2) Alkali to neutralise acid and prevent digestion of stomach wall by pepsin and HCl

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4
Q

What is the stomach’s 2 main mechanical functions?

A

1) Storage (stores food 3-4 hours) with aid of rugae
2) Aids in the formation of chyme by peristaltic movement that mixes the bolus with gastric juice

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5
Q

What is the stomach’s hematopoietic function?

A

Secretes intrinsic factor which is necessary for Vit B12 absorption

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6
Q

What can occur as a result of B12 defiency?

A

Pernicious anemia

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7
Q

What two things are secreted that aid the stomach’s digestive function?

A

1) gastric juice: HCl and electrolytes that activate digestive enzymes, together these break down proteins into amino acids
2) proteolytic enzymes like pepsin and renin

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8
Q

What is the stomachs excretory function?

A

toxic substances like toxins, metals are excreted from gastric juice

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9
Q

What is the purpose of HCl secreted by the stomach (enzymes)

A

HCl activates pepsinogen in pepsin, this provides acidic environment for action of hormones

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10
Q

When the stomach is empty what can be seen in its histological appearance?

A

when empty, the gastric mucosa is thrown into longitudinal folds, called RUGAE. These expand to increase SA following ingestion of food

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11
Q

What are the openings found within the gastric mucosa called?

A

gastric pits

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12
Q

What is the name of the glands in the stomach and what are the 3 types?

What do these glands empty into?

A

Gastric glands, 3 types:

  • Cardia
  • Fundus & Body,
  • Pyloric

gastric glands empty into the bottom of gastric pits

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13
Q

Describe the varying composition of the 3 gastric glands between different regions of the stomach

A

Cardia:

  • mostly mucus-secreting glands (no enzymes)

Fundus and Body:

  • Neck cells that secret mucus and HCO3
  • Parietal/Oxyntic cells: secrete acid (HCl)
  • Chief cells: secrete pepsinogen
  • Other local hormone producing cells

Pyloric region:

  • Mucus and G cells (releases Gastrin)
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14
Q

Where do gastric ulcers usually occur?

A

In the antrum and along the lesser curvature of the stomach

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15
Q

What two things do neck cells secrete?

What is the stimulus and the function of this secretion?

A

1) Mucous

  • Stimulus: tonic secretion (irritation of the mucosa may increase stimulation)
  • Function: provides a physical barrier between the lumen and epithelia

2) Bicarbonate

  • Stimulus: secreted with mucus
  • Function: buffers gastric acid to prevent damage to the epithelia
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16
Q

What two things do Parietal cells secrete?

What is the stimulus and function of this secretion?

A

1) HCl:

  • Stimulation: ACh, histamine and gastrin
  • Function: activates pesinogen to pepsin to kill bacteria

2) Intrinsic factor

  • Stimulation: ACh, histamine and gastrin
  • Fucntion: complexes with Vit B12 to permit absorption
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17
Q

What do enterochromaffin-like cell secrete?

What is the stimulus and the function of this secretion?

A

Histamine

  • Stimulation: gastrin and ACh
  • Function: stimulates gastric acid secretion
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18
Q

What two things do chief cells secrete?

What is the stimulus and the function of this secretion?

A

1) Pepsinogen (gets converted to pepsin)

  • Stimulation: ACh, acid and secretin
  • Function: digests proteins

2) Gastric lipase

  • Stimulation: ACh, acid and secretin
  • Function: digests fat
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19
Q

What do D cells secrete?

What is the stimulus and the function?

A

Somatostatin

  • Stimulation: acid in the stomach
  • Function: inhibits gastric acid secretion
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20
Q

What do G-cells secrete?

What is the stimulus and the function

A

G-cells secrete Gastrin

  • Stimulation: Ach, peptides and amino acids
  • Function: stimulates gastric acid secretion
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21
Q

What cells are responsible for the “attack” and “defence” balance between the stomach acid secretions?

What controls these?

A

“Attack” = Parietal cells secrete HCl causing a luminal pH below 2 and Chief cells secrete proteolytic enzymes (pepsin)

“Defence” = mucus cells secrete mucus and HCO3-

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22
Q

What 3 signals act on parietal cells to promote acid secretion?

How do each of these work?

list one other hormone that can indirectly stimulate HCl production

A

Gastrin (G-cells), Histamine (ECL-cells) and ACh (NTs)

  • Gastrin from G cells and ACh bind to parietal cells and increase [Ca+] to activate Protein Kinases
  • Histamine works via a H2 receptor and cAMP mechanism

All 3 work synergistically

Ghrelin is also a hormone that is released in response to hunger. It stimulates appetite and food ingestion which indirectly leads to HCl production through above mechanisms

23
Q

What cell/secretion inhibits HCl production from parietal cells and what stimulates it to do so?

A

Parietal cell secretion of HCl negativly feeds back to D cells and stimulates them to produce Somatostatin

This inhibits action of G-cell, ECL cell and directly acts to inhibit the parietal cell which overall reduces HCl secretion

24
Q

What does HCl seceretion from the parietal cells stimulate and how?

What does this lead too?

A

HCl secretion creates an acidic environment in the gastric lumen to acitivate other enzymes and break down proteins

This acidity converts the protease precursor pepsinogen (secreted by cheif cell) to pepsin

Many subsequent conversions of other inactive enzymes occur quickly as a result of pepsin’s protease activity

25
Q

Explain the process of how acid is made in parietal cells

A

Source for making acid is the ECF

1) In mitochondria of parietal cells H20 is slip into ➞ H+ (want these) and OH- (very dangerous)
3) Both are exported into cytoplasm of the cell
4) OH- combines immediately with CO2 to form HCO3- ➞ enters blood
5) H+ rapidly transported into the stomach via canaliculi

Must produce 1 mol alkali for every mol of acid (acid into stomach, alkali into blood)

26
Q

Explain the process of how H+ ions are exported out of parietal cells and why this must happen “immediatly”

A

H+ production is occuring in many mitochondria within the parietal cell at a very high rate. H+ can NOT accumulate within the cell (high acidity) so it must very rapidly be exported into the stomach

Process takes place in the invaginations of the membrane of the parietal cell wall across channels known as “Canaliculi”

Membranes of the canaliculi are packed with many proton pumps which activly export H+ out of cell and into the stomach (against a very high concentration gradient so lots of ATP needed )

27
Q

What is the characteristic shape of a parietal cell?

A

characteristic chopped triangular shape, short side facing stomach

28
Q

Acid production in the parietal cell depends on what molecule and what enzyme generates this?

A

Carbonic acid, carbonic anhydrase

29
Q

What is Gastrin and what cell secretes it?

What does it act on and what does this stimulate?

What stimulates and inhibits its secretion?

A

Hormone secreted by G-cells in stomach (polypeptide)

Endocrine hormone, hence a chemical produced indirectly travels in bloodstream and then binds to surface receptors on parietal cell

Binding ➞ activates 2nd messenger cascade ➞ causes cell to secrete acid (HCl) and intrinsic factor

Stimulated by: peptides and ACh from nerves within G-cells which are activated by stomach distension

Inhibited by: Low pH detected by G-cells (- feedback)

30
Q

What is ACh and what secretes it?

What does it act on and what does this stimulate?

What stimulates its secretion?

A

Neurotransmitter released from post ganglionic parasympathetic neurones

Acts on muscarinic receptors on parietal cell, stimulating the
release of acid

Stimulated by: distension and pre-ganglionic neurones from the vagus nerve (CNS)

31
Q

What is Histamine and what secretes it?

What does it act on and what does this stimulate?

What stimulates its secretion?

A

Released from Mast cells (found all over body)

In stomach it diffuses to parietal cells and binds to H2 surface receptor, stimulating acid secretion by via cAMP

Mast cells are stimulated by: Gastrin and ACh

Hitamine acts as an “amplifier” of Gastrin and ACh

32
Q

What is a good drug target to reduce acid secretion in the stomach?

A

H2 receptors on parietal cells are uncommon, so antagonists are good at reducing acid secretion

Hitamine also acts as an “amplifier” of Gastrin and ACh, hence blocking H2 receptors would decrease effect of other two substances

33
Q

Describe how ACh, directly and indirectly, stimulates acid production

A

Released from post-ganglionic parasympathetic neurones (VAGUS)

Directly: stimulates parietal cells by acting on muscarinic receptors

Indirectly: via 2nd messengers (stimulates ECL cells to release histamine and G cells to release gastrin)

34
Q

List the three phases of digestion

A

1) Cephalic phase
2) Gastric phase
3) Intestinal phase

35
Q

What happens in the cephalic phase and under what control is it?

A

Occurs BEFORE a meal is ingested, under CNS control

1) Initiated by sensing of food by sight or smell
2) Stimulates salivation AND sends autonomic signals via the vagus nerve to release ACh in the stomach. This stimulates parietal cells to secrete acid

* Hence stomach is already producing acid in preparation for ingestion of a meal

36
Q

During the cephalic phase what are the two ways in which the ANS can stimulate the parietal cell?

A

The Cephalic phase involves detecting & ingesting food

ANS: Ach stimulates parietal cells directly & via histamine

37
Q

What is the pH of the stomach when it is empty (high/low)?

Why is this the case?

A

When stomach is empty it resides at a very low pH (very acidic)

This is because parietal cells secrete acid until pH drops low enough to switch of Gastrin secretion (prevents over-acid production)

38
Q

What happens in the gastric phase? (include what is stimulated as a result of it)

A

Characterised by short reflexes via the stomach wall (ENS)

1) Food reaches stomach (very good at absorbing H+ ions) so buffers stomach acid causing pH to rise
3) increase in pH DISINHIBITS gastrin (switched back on)
4) Stomach distends and stimulates intrinsic nerves to release Ach
6) Initial digestion releases peptides which furhur stimulate gastrin release
7) Histamine release is stimulated by gastrin & ACh and amplifies processes leading to large acid production

39
Q

What happens in the intestinal phase of digestion?

A

1) stomach begins to empty into duodenum
2) chyme stimulates release of hormones from intestine which antagonize gastrin
3) accumulation of acid in empty stomach inhibits gastrin secretion
4) stomach pH low between meals

40
Q

What is the effect of the resting stomach pH on ulcers?

What common symptom is experienced and why?

A

Resting stomach pH is very low which can aggravate ulcers

Common symptom is: Night pain (stomach empty = pH low)

41
Q

Why do we have alkaline mucus as a stomach defence and what does this allow?

A

Luminal pH is usually below 2 which would dissolve unprotected mucosa

Therefore, we have a sticky, basic mucus layer that forms an ‘unstirred layer’ that allows the pH at the surface to be well above 6

42
Q

Explain the mechanism that allows the pH at the surface lining of the stomach to be well over 6

A

1) mucosa covered by mucus to form unstirred layer which allows molecules to diffuse into it
2) as H+ enters via luminal side it binds amine groups and H2 receptors within the mucus to neutralises it. BUT capacity is limited so…
3) cells on surface of stomach (outside) are constantly secreting alkali (HCO3-) which diffuses into mucus to neutralise H+ diffussing in from lumen
4) these react to produce CO2 + H20 ➞ increases pH on luminal side

43
Q

What can be said about the pH on the luminal side in comparison to the pH on the surface of the cells on the other side?

What reaction causes this?

A

At the luminal side we have a pH of 2 and on the surface of cells on the other side we have a pH of around 6

It is the reaction between the HCO3- and H+ within the “unstirred layer” of mucus that help protect the stomach layer

44
Q

What stimulates the production of mucus and alkaline that aid in stomach defences?

A

Prostaglandins which are stimulated by most factors that stimulated acid secretion

Hence ➞ hence formation of acid = formation of defences (defences match attack)

45
Q

List 3 things that can breach stomach defences and how they do this

A

1) Alcohol: dissolves the mucus
2) H. Pylori: very suited to stomach conditions

  • flagllum and chemotaxis to maintain its preferred position within mucus layer (avoids the more acidic lumen)
  • produces the enzyme urease which converts urea in stomach to Ammonia and CO2. The Ammonia then converts to Ammonium which neutralises acidity surrounding bacteria.

3) NSAIDs: Inhibit prostaglandins ➞ reduces defences
* some (eg.aspirin) gets converted into non-ionised form by stomach acid, passes into cells, then re-ionise ➞ peptic ulcers

46
Q

List 2 drugs that can be used to reduce acid secretion and how

How would you treat H.Pylori infection and what would this eliminate?

A

1) Histamine receptor antagonists eg. H2 – cimetidine

  • H2 receptors on parietal cells are uncommon, so antagonists are good at reducing acid secretion
  • Hitamine also acts as an “amplifier” of Gastrin and ACh, hence blocking this would decrease effect of other two substances

2) Proton pump inhibitors eg. Omeprazole
* would block the PPs found on canaliculi of cells which would reduce expulsion of H+ into stomach lumen

** Use antibiotics to eliminate H.Pylori which would treat ulcers

47
Q

What is receptive relaxation and why is it important?

A

Neural reflex triggered by swallowing. Stimulates
efferent pathway via vagus to cause tension in resting stomach wall to ‘actively’ relax

Important because:

  • It ensures pressure in stomach does not increase as it fills ➞ limits reflux
  • It ensures large meals can be consumed

Damage to vagus has a significant effect on the above

48
Q

If a patient has damage to the Vagus nerve what symptom (regarding eating) may they present with?

A

Patients find they are unable to eat large meals, because as
pressure increases they begin to feel very uncomfortable

49
Q

What muscles and cells generate rhythmic contractions within the stomach?

A

Stomach has longitudinal and circular muscle that generate rhymic contractions driven by pacemaker cells in cardia region

50
Q

Explain how rhymic contractions allow gastric emptying following a meal

A

1) pacemaker cells generate a wave of contractions that spreads through nerves + muscles layers towards antrum, accelerating as it goes
2) Initially wave is slow (less muscle in cardia) which generates a squeeze in upper stomach which drives all stomach contents ahead of it
3) as wave moves lower it accelerates due to the J shape (more muscle) which generates a rapid squeeze towards pylorus
5) chyme reaches pyloric sphincter and enters immediately as muscle here is NOT contracted (open)
6) As wave passes sphincter it contracts and quickly closes. Hence only a single squirt of chyme is ejected into the duodenum
7) Process repeats until stomach is empty

51
Q

Explain how rhymic contractions allow gastric mixing following a meal

A

When a peristaltic contraction reaches the pyloric sphincter, the sphincter is tightly closed and no furthur emptying takes place until end of next wave

So… when the chyme that was being propelled forwards hits this closed sphincter, it is tossed back into the antrum

Chyme is continuously propelled forward and tossed back into the antrum with each peristalic contraction with achieves mixing of chyme

52
Q

What two things affect the volume of each squirt?

A

1) Rate of acceleration of peristaltic waves

  • more acceleration = squirt is short at low volume (slower gastric emptying)
  • less acceleration = squirt is long and at high volume (faster gastric emptying)

2) Hormones from the intestine

53
Q

What 3 things slow down the gastric emptying?

A

1) Fat
2) low pH
3) hypertonicity

All in the duodenum. These all slow down gastric emptying because they indicated there is still material that needs to be digested

GI (Semester 4) (19 decks)

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