011 Physiology of the GI tract: Gastric secretion

Question 1

What are the 3 parts of the stomach?

Answer 1

Fundus, Body, Pyloric antrum

Question 2

How much volume does the stomach hold up to during expansion and during fasting?

Answer 2

50ml during fasting, 2L during expansion

Question 3

Where is the major area of exocrine secretion in the stomach?

Answer 3

Body of the stomach

Question 4

Where is the major area of endocrine secretion in the stomach?

Answer 4

Pyloric antrum

Question 5

How many gastric glands per mm2 of surface are there?

Answer 5

100 gastric glands per mm2 (approximately 50% of the surface)

Question 6

How much gastric secretion is produced per day?

Answer 6

2L

Question 7

What does gastric secretion contain?

Answer 7

HCl, Pepsin, intrinsic factor

Question 8

Which glands are found in the body of the stomach? and what cells do they contain?

Answer 8

Gastric (oxyntic) glands - containing enterochromaffin cells, parietal cells, mucus neck cells, surface mucus cells, D cells, Chief cells and

Question 9

Which glands are found in the pyloric antrum?

Answer 9

Pyloric glands - containing surface mucous cells, mucous neck cells, G cells and D cells

Question 10

What do parietal cells secrete?

Answer 10

HCl, IF

Question 11

What do enterochromaffin cells secrete?

Answer 11

Histamine

Question 12

What do chief cells secrete?

Answer 12

Pepsinogen

Question 13

What do G-cells secrete?

Answer 13

Gastrin

Question 14

What do D-cells secrete?

Answer 14

Somatostatin

Question 15

Where does vitamin B12 uptake occur?

Answer 15

In the lower ileum

Question 16

How does HCl secretion work in the parietal cells?

Answer 16

During fasting state, lumen pH remains at 2. During ingestion, the acid is diluted to pH 4. This stimulates acid production in the parietal cells. CO2 diffuses into the parietal cells from blood plasma. This reacts with water to form H2CO3 which dissociates into H+ and HCO3-. The H+ is transported through the H+/K+ antiporter pump using hydrolysis of ATP. HCO3- is exchanged for Cl- and forms the alkaline tide returning to the blood. The Cl- moves through the parietal cells and is excreted into the lumen through Cl- channels. There is a Na+/K+ ATPase pump which exchanges 3Na+ ions out of the parietal cell into the blood plasma for every 2K+ into the cell. This generates a K+ gradient, as the K+ that has entered the parietal cell through both Na+/K+ pump and the H+/K+ pump is pumped out K+ channels into the lumen.

Question 17

What is PPT Omeprazole

Answer 17

Drug used to inhibit the proton pump in parietal cells.

Question 18

How are parietal cells adapted to increase HCl secretion?

Answer 18

They have tubulovesicles that fuse with the canniculus membrane during parietal cell activation. These fuse to form the canaliculus (secretory network), increasing surface area 50-100 fold. When cells are no longer activated, they undergo endocytosis back into resting state.

Question 19

How is pepsinogen secreted?

Answer 19

Chief cells secrete pepsinogen, which is activated by presence of HCl to form the soluble form pepsin. Hence chief cells are located in oxyntic glands along with parietal glands.

Question 20

How is IF secreted and what is its purpose?

Answer 20

IF is secreted by parietal cells. As vitamin B12 enters the stomach it binds to the intrinsic factor. This moves through the digestive tract. At the terminal ileum, IF binds to receptors, causing receptor-mediated endocytosis of the vitamin B12. IF is egested in feces while vitamin B12 binds to protein to protect it from degredation within the blood.

Question 21

What is a consequence of absense of IF?

Answer 21

Pernicious anaemia

Question 22

What is the role of gastrin?

Answer 22

Endocrine hormone produced in the pyloric glands that stimulate HCl production, pepsinogen release, and mucus release. It maintains mucus and increases motility of muscle.

Question 23

What is the role of somatostatin?

Answer 23

Paracrine hormone produced by D cells when pH is low enough, having a direct inhibitory effect of G cells. It also indirectly reduces acid secretion by limiting release of other hormones.

Question 24

What is the role of histamine?

Answer 24

A paracrine hormone produced by enterochromaffin like cells that act on H2 receptors on parietal cells and stimulates H+ secretions.

Question 25

What is the role of secretin?

Answer 25

In the presence of acid, it is secreted to cause pancreas to release water and bicarbonate. It also inhibits secretion by the stomach.

Question 26

What is the role of CCK?

Answer 26

Stimulated by fats and proteins in the duodenum. Its primary purpose is protein and fat digestion by releasing pancreatic enzymes and bile. It also inhibits gastric processes e.g. gastric emptying, acid secretion.

Question 27

Where is secretin released?

Answer 27

S cells in the crypts of lieberkuhn of the duodenum.

Question 28

Where is CCK released?

Answer 28

I cells in the duodenum.

Question 29

What are the 3 phases of gastric secretion?

Answer 29

Cephalic phase (30-35% of secretion)
Gastric phase (60% of total secretion)
Intestinal phase (5-10% of total secretion)

Question 30

How does the cephalic phase occur?

Answer 30

Vagal nerves stimulate gastric parietal cells, or there is an indirect release of gastrin by vagal nerve fibres that travel in the blood and activates parietal cells.

Question 31

How is the gastric phase stimulated?

Answer 31

Distension, elevated pH, peptides and amino acids, alcohol and caffeine.

Question 32

How is the intestinal phase activated?

Answer 32

Duodenal stretch, decreased pH, presence of lipids and CHO.

Question 33

How does HCO3- act as a mucosal barrier?

Answer 33

The alkaline tide created by parietal cells travel by arterial supply to surface epithelium and secreted into the mucus. When H+ is secreted, it causes a back diffusion of H+ that HCO3- is able to neutralize before reaching gastric cells.

Question 34

What is the role of prostaglandin? and how does aspirin cause gastric ulcers?

Answer 34

Prostaglandin enhances the effectiveness of the mucosal barrier. Aspirin causes reduced local prostaglandin release that causes user to be more prone to gastric ulcers.

Question 35

What is Zollinger-Ellison Syndrome?

Answer 35

It is a rare gastrin secreting tumor on the non-B cells of the pancreatic islet. The uncontrolled release of gastrin causes multiple peptic ulcers mainly in the distal duodenum. Symptoms include abdominal pain, diarrhea, steatorrhea.