Gastric physiology

Question 1

What are invaginations of the parietal cells called which act to increase their surface area?

Answer 1

Canaliculi

Question 2

What are the functions of the stomach?

Answer 2

The functions of the stomach:

Store & mix food
Regulate emptying into the duodenum
Secrete and activate proteases
Mucosal protection
Lubrication
Kill microbes
Secrete intrinsic factor for Vitamin B12 absorption in the ilium

Question 3

Which of the following is not secreted by the body of the stomach:

a) HCl
b) Gastrin
c) Pepsinogen
d) Mucus

Answer 3

The body of the stomach secretes mucus, HCl and pepsinogen (not gastrin)

Question 4

Which of the following is not secreted by the antrum of the stomach:

a) HCl
b) Gastrin
c) Pepsinogen
d) Mucus

Answer 4

The antrum of the stomach secretes mucus, HCl and gastrin (not pepsinogen)

Antrum = gAstrin

Question 5

Which cells secrete somatostatin?

a) Enteroendocrine
b) D cells
c) Chief cells
d) Enterochromaffin like cells

Answer 5

Somatostatin is secreted by D Cells

Question 6

Which cells secrete pepsinogen?

a) Enteroendocrine
b) D cells
c) Chief cells
d) Enterochromaffin like cells

Answer 6

Pepsinogen is secreted chief cells

Question 7

Which cells secrete Histamine?

a) Enteroendocrine
b) D cells
c) Chief cells
d) Enterochromaffin like cells

Answer 7

Histamine is secreted by enterochromaffin cells

Question 8

Which cells secrete gastrin?

a) Enteroendocrine
b) D cells
c) Chief cells
d) Enterochromaffin like cells

Answer 8

Gastrin is secreted by enteroendocrine cells

Question 9

Which nervous system and neurotransmitter are involved in the cephalic phase of turning on gastrin acid secretion?

Answer 9

Turning it on (Cephalic phase):

Parasympathetic nervous system

• Stimulated by sight, smell, taste of food, and chewing

Acetylcholine release

ACh acts directly on parietal cells

ACh triggers release of gastrin and histamine (both act to increase HCl)

Net effect = increased acid production

Question 10

What triggers the gastric phase when turning on gastric acid production?

Answer 10

Turning it on (Gastric phase):

Gastric distension, presence of peptides and amino acids

Stimulates: Gastrin release

Gastrin acts directly on parietal cells & triggers release of histamine

Histamine acts directly on parietal cells

Net effect = increased acid production

Question 11

Why is histamine important in gastric acid production?

Answer 11

• Histamine is involved in the upkeep of proton pumps
• Histamine is the main way to turn of parietal cells
• Mediates the affects of gastrin and ACh

Question 12

Explain how gastric acid production can be stopped/reduced in the gastric phase?

Answer 12

High [H+]

• inhibits gastrin secretion & indirectly inhibits histamine (as less gastrin)
• Increases somatostatin production

Somatostatin inhibits parietal cells = less HCl

Question 13

In the duodenum, what changes turn off gastric acid secretion?

Answer 13

Turning it off (Intestinal phase) In the duodenum:

• Duodenal distension
• Low luminal pH
• Hypertonic luminal contents
• Presence of amino acids and fatty acids

Question 14

During the intestinal phase of turning of gastric acid production, name the 2 enterogasterones that are produced.

Answer 14

Intestinal phases, triggers release of enterogastrones:

Secretin (inhibits gastrin release, promotes somatostatin release)

Cholecystokinin (CCK)

Note: short and long neural pathways reduce the ACh released

Question 15

What is an ulcer?

Answer 15

A breach in a mucosal surface

Question 16

What are the 4 main causes of peptic ulcers?

Answer 16

Helicobacter pylori infection

Drugs – NSAIDS

Chemical irritants – alcohol, bile salts, ? Dietary factors

Gastrinoma

Question 17

How does the gastric mucosa defend itself?

Answer 17

Alkaline mucus

Tight junctions between epithelial cells

Replacement of damaged cells

-ve feedback loops to turn off acid production

Question 18

Which of the following causes peptic ulcers by: Secreting urease, which increases the amount of ammonium in the gastic lumen. Ammonium damaged the epithelium and causes an inflammatory response.

a) Helicobacter pylori
b) NASIDS
c) Cemical irritants

Answer 18

Helicobacter pylori

NSAIDs inhibit cyclo-oxygenase 1. Cyclo-oxygenase 1 needed for prostaglandin synthesis. Mucus secretion is stimulated by prostaglandins = Reduced mucosal defence

Question 19

Which of the following is responsible for converting pepsinogen to the active form pepsin?

a) Somatostatin
b) Histamine
c) Gastrin
d) HCl

Answer 19

HCl

Pepsinogen secretion = HCl secretion

Most efficient at pH 2

+ve feedback - pepsin also catalyses the reaction

Question 20

What percentage of protein digestion is pepsin responsible for?

a) 10%
b) 20%
c) 60%
d) 80%

Answer 20

Normally accounts for ~20% of total protein digestion

Breaks down collagen in meat – helps shred meat into smaller pieces with greater surface area for digestion

Accelerates digestion

Question 21

What is receptive relaxation and what is it mediated by?

Answer 21

Receptive relaxation = relaxation of smooth muscle in body and fundus of stomach to increase volume without increasing pressure.

Mediated by: Parasympathetic nerves in enteric plexus (release NO & serotonin), coordination by afferent via vagus and swallowing centres in the brain.

Question 22

Describe Peristalsis

Answer 22

Peristaltic waves begin in gastric body - Weak contraction in body (little mixing)

More powerful contraction in gastric antrum - Pylorus closes as peristaltic wave reaches it

Little chyme enters duodenum - Antral contents forced back towards body (mixing)

Question 23

What increases the strength of peristaltic contraction?

Answer 23

Gastrin

Gastric distension (medicated by mechanoreceptors)

Question 24

What decreased the strength of peristaltic contractions?

Answer 24

Strength of peristaltic contractions decreased by:

Duodenal distension

Increased: Duodenal luminal fat

Duodenal osmolarity

Sympathetic NS action

Decreased: Duodenal luminal pH

Parasympathetic NS action

(turned on by brain and stomach and off by the duodenum)

Question 25

What is a zymogen?

Answer 25

A zymogen (or proenzyme) is an inactive enzyme precursor. A zymogen requires a biochemical change (such as a hydrolysis reaction revealing the active site, or changing the configuration to reveal the active site) for it to become an active enzyme.