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Physiology Exam 5 > 60 - Gastric Secretions > Flashcards

60 - Gastric Secretions Flashcards

1
Q

Cells located in the cardia of the stomach

A

Cardia has NO PARIETAL CELLS – reason for this is because the cardia is located just below the esophagus. Having acid producing cells right outside of the esophagus makes damage more likely due to acid reflux.

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2
Q

Cells located in the body of the stomach

A

Stomach body has parietal, chief, neck and Enterochromaffin-like cells

These are located within the gastric pits.

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3
Q

Secretions of parietal cells

A

Parietal cells secrete HCl (protein digestion, sterilization, and nutrient absorption) as well as intrinsic factor (B12 absorption – complexing occurs mainly in duodenum)

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4
Q

Secretions of chief cells

A

Chief cells produce pepsinogen (protein digestion once converted).

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5
Q

Secretions of neck cells

A
  • Pepsinogen

- Also produce mucous and bicarb - a balance between aggressive factors and protective factors

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6
Q

Secretions of enterochromaffin-like cells

A

ECL cells produce histamine – stimulates HCl secretion

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7
Q

Cells located in the antrum of the stomach

A

Antrum has chief cells, G cells and D cells

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8
Q

Secretions of G cells

A

G cells produce gastrin (promote HCl secretion)

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9
Q

Secretions of D cells

A

D cells produce somatostatin (remember delta cells of the pancreas? Function is to suppress HCl secretion

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10
Q

Are G and D cells found exclusively in the antrum of the stomach?

A

No - C and D cells are found elsewhere as well but in the highest concentration in the antrum

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11
Q

What lines the entire stomach?

A

The entire stomach is lined by superficial epithelial cells, which secrete mucous and bicarb (function in gastroprotection from acid)

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12
Q

Gastric glands produce ___ L of fluid per day

A

2 L

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13
Q

What two ions are secreted from gastric glands?

A

Na+ rich secretions (non-parietal cells)
H+ rich secretions (parietal cells)

Also note that as secretory rate increases, H+ concentration does as well

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14
Q

Which two ways is the secretion of acid by the stomach regulated?

A

Directly and indirectly

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15
Q

What three pathways contribute to this regulation?

A

Neuronal
Paracrine
Endocrine

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16
Q

What does the neuronal pathway use to regulate acid secretion?

A

ACh

Promotes HCl secretion

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17
Q

What does the paracrine pathway use to regulate acid secretion?

A

Histamine

Promotes HCl secretion

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18
Q

What does the endocrine pathway use to regulate acid secretion?

A

Gastrin

Promotes HCl secretion

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19
Q

How does gastrin function both directly ad indirectly to stimulate HCl secretion?

A

Directly
- Gastrin is secreted into the blood by the endocrine G cells of the stomach and can directly stimulate the parietal cells to increase H+/K+ ATPase pump activity (this is a H+ pump that transports H+ ions into the stomach)

Indirectly
- After gastrin is secreted into the blood it can also act indirectly by inducing the release of histamine from ECL cells

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20
Q

How does acetylcholine function both directly and indirectly to stimulate HCl secretion?

A

Directly
- Stimulation of the parietal cell

Indirectly
- Stimulation of ECL cells resulting in the release of histamine (same as gastrin indirect)

21
Q

How does histamine function directly to stimulate HCl secretion?

A

Directly
- Histamine is released from ECL cells onto neighboring parietal cells in order to stimulate the H+/K+ ATPase pump to increase the amount of H+ being secreted into stomach lumen

22
Q

How does the paracrine release of prostaglandins (PGE2) regulate the HCl secretion in the stomach?

A

Prostaglandins (PGE2) decrease or negatively regulate the H+/K+ ATPase pump on the apical surface of parietal cells, which decreases the amount of HCl secreted into the stomach and decreases the acidity of the stomach

23
Q

How does the release of somatostatin into the blood regulate the HCl secretion in the stomach?

A

The release of somatostatin into the blood by gastric D cells decreases acid secretion by inhibiting the release of gastrin from G cells, inhibiting the release of histamine from ECL cells and inhibiting H+/K+ ATPase activity

Overall, the acidity of the stomach will decrease

24
Q

Refresher: neuronal, paracrine, endocrine

A

Neuronal
- Hormones from the nervous system (released from axons for signalling)

Paracrine
- Paracrine signaling is a form of cell-cell communication in which a cell produces a signal to induce changes in nearby cells, altering the behavior or differentiation of those cells

Endocrine
- Secrete hormones from endocrine organs via ducts into the circulation to carry a signal to a distant location

25
Q

What are the five functions of gastric acid (HCl)?

A

1 - Protein digestion (conversion of pepsinogen to pepsin)
2 - Provide a sterile environement
3 - Prevent bacterial and fungal growth
4 - Facilitate absorption
5 - Promote bile and pancreatic enzyme flow

26
Q

How is pepsinogen converted into the active protease pepsin in the lumen of the stomach?

A

Pepsinogen is a zymogen, meaning it’s converted into its active form by HCl

Pepsinogen can also be auto-activated by other pepsin molecules

This process functions optimally at pH 1.8-3.5 and is inhibited beyond pH of 3.5

27
Q

How is the pH restriction a protective measure?

A

The pH difference serves a protective role by preventing not only damage from the acid but also prevents the protease from cleaving proteins expressed on the surface of duodenal mucosa

28
Q

What two secretions in the stomach breakdown proteins?

A
  • HCl and Gastrin
  • HCl by pepsinogen -> pepsin -> protein breakdown
  • Gastrin directly
29
Q

What is the site of intrinsic factor secretion?

A

Parietal cells secrete intrinsic factor

30
Q

What is the function of intrinsic factor?

A

Intrinsic factor

  • Forms a complex with vitamin B12 which has been released from food being broken down in stomach
  • Complexing occurs in the duodenum
  • IF-B12 is absorbed in the ileum
31
Q

Damage to parietal cells will result in…

A

What is important is that intrinsic factor is coming from the parietal cells, so if you have damage to parietal cells, you will not only have decrease in acid secretion, but also a decreased absorption of vitamin B12 due to the lack of intrinsic factor being released

32
Q

What are the components of mucous?

A
  • Mucin
  • Phospholipids
  • Electrolytes
  • Water

All of these are protective factors

33
Q

What else is secreted and has a protective function?

A

Bicarb

34
Q

Describe the multiple levels of protection

A
  • Mucous acts as a physical barrier
  • Mucous has a high viscosity, allowing for slow diffusion
  • Chief and parietal cell membranes are highly resistant
  • High pH of bicarb inactivates pepsin and acts as a bufffer
35
Q

What is the overall result of these protective measures?

A

Physical barrier, in addition to high pH and buffering capacity prevents easy access of pepsin to underlying epithelium – even if they get there, they’re close to the point of irreversible inhibition (pH 7.2)

36
Q

What are the two key stimuli for mucous secretion by epithelial and neck cells of the stomach?

A

Neurocrine regulation - ACh

Paracrine regulation - prostaglandin

37
Q

What do prostaglandins do?

A
  • Promote mucous and bicarb secretion
  • Suppress HCl
  • Increase gastric blood flow
38
Q

How to NSAIDS work?

A

NSAIDS inhibit cyclooxygenase, which is a rate-limiting enzyme in the production of prostaglandins. This means that use of NSAIDS can inhibit mucous and bicarb secretion, increase HCl secretion, and reduce gastric blood flow –> increase the chances of damage to the stomach

39
Q

What are the four phases of gastric acid secretion?

A

1 - Interdigestive phase
2 - Cephalic phase
3 - Gastric phase
4 - Intestinal phase

40
Q

Interdigestive phase

A

lowest in early morning rises throughout the day. pH varies from 3-7 depending on parietal cell number, body weight and presence of food in the stomach (this is why eating breakfast is important).

41
Q

Cephalic phase

A

Accounts for 30% of acid secreted – activated by stimuli like sight or smell or food – trigger vagal response –> acid secretion.

42
Q

Gastric phase

A

Similar to cephalic – vagal stimulation mediates secretion but stimulus is from stomach distention. Protein products in the stomach increase acid secretion – inhibited by D cells (most secretion – 50-60% during this phase)

43
Q

Intestinal phase

A

10% of acid secretion – mediated by presence of peptides in the duodenum. These induce release of gastrin to increase activity of parietal cells – may be also regulated by amino acids absorbed in the duodenum (p5 handout). Remember – the majority of your gross protein digestion occurs in the stomach, if some inappropriately digested product comes out into the duodenum, it will send out the signal to slow things down and get a more thorough digestion to occur.

44
Q

Describe the role of secretin in the stomach and its target cells

A

Secretin

  • Secretin is produced in the S cells of the duodenum, which are located in the crypts of Lieberkühn
  • Secretin also helps regulate the pH of the duodenum by inhibiting the secretion of gastric acid from the parietal cells of the stomach
  • Also regulates pH by stimulating the production of bicarbonate from the centroacinar cells and intercalated ducts of the pancreas
45
Q

Pathophysiology of gastritis

A
  • Gastritis is characterized by inflammation of the gastric mucosa caused by damage to the protective mucosal barrier
  • The inflammation does not necessarily cause a break in the mucosal lining but the inflammation has the potential to result in ulcer formation
46
Q

Pathophysiology of H. pylori infection

A
  • Helicobacter pylori are spiral shaped bacteria that colonize the luminal surface of the gastric epithelium and can cause inflammation and ulceration of the stomach
  • 70% to 90% of patients with gastric or duodenal ulcers are positive for H. pylori
  • The bacterium is a causative agent of ulcer formation, but its precise role in initiating ulcer formation is unclear at this time
  • In addition to playing a role in ulcer formation, H. pylori infections have been linked to gastric lymphomas and adenocarcinomas.
  • Transmission of H. pylori is generally through the fecal-oral route by ingestion of waste tainted food or water
  • H. pylori can also be transmitted from person to person by passing bacteria from the stomach to the mouth by reflux or belching.
47
Q

Pathophysiology of peptic ulcers

A
  • Impaired regulation of gastric acidity or the loss of mucosal protection, for example by excessive secretion of gastrin or impaired secretion of mucus, can lead to a break down in the epithelial lining or mucosal layer of the gastrointestinal tract (ulcer)
  • There are multiple types of ulcers (see handout if necessary)
48
Q

Pathophysiology of Zollinger-Ellison syndrome

A
  • One or more tumors form in your pancreas or the upper part of your small intestine (duodenum)
  • These tumors, called gastrinomas, secrete large amounts of the hormone gastrin, which causes your stomach to produce too much acid
  • The excess acid, in turn, leads to peptic ulcers

Physiology Exam 5 (10 decks)

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