Gastric Physiology

Question 1

What are the functions of the stomach?

Answer 1

Store & mix food
Dissolve & continue digestion
Regulate emptying into the duodenum 
Kill microbes
Secrete protease 
Secrete intrinsic factor
Activate protease
Lubrication
Mucosal protection

Question 2

What are the key cell types in the stomach?

Answer 2

Mucous cells, parietal, chief, enteroendocrine, enterochromaffin-like (ECL) cell, G cells, D cells

Question 3

What do mucous cells do?

Answer 3

Produce mucous, at entrance to gland

Question 4

What do parietal cells do?

Answer 4

Produce gastric acid & intrinsic factor

Question 5

What do Chief cells do?

Answer 5

Produce pepsinogen

Question 6

What do enteroendocrine cells do?

Answer 6

Secrete gastrin

Question 7

What do enterochromaffin-like (ECL) cells do?

Answer 7

Release histamine

Question 8

What do G cells do?

Answer 8

Release gastrin

Question 9

What do D cells do?

Answer 9

Release somatostatin

Question 10

Describe the upper portion of the stomach and what is secreted?

Answer 10

Glands in The thin walled upper portion of the stomach secrete mucous, hydrochloric acid (parietal cells), enzyme precursor pepsinogen (chief cells)

Question 11

Describe the lower portion of the stomach and what is secreted?

Answer 11

Has a much thicker layer of smooth muscle, responsible for mixing & griffin the stomach contents-the glands secrete little acid, but contain the endocrine cell grate secrete the hormone gastrin (G cell)

Question 12

Where does gastric acid secretion occur from?

Answer 12

Parietal cells

Question 13

What type of acid is gastric acid?

Answer 13

Hydrochloric acid

Question 14

What’s its pH & strength?

Answer 14

pH 2
Very strong
[H+]>150mM

Question 15

How much gastric acid is secreted?

Answer 15

2 litres per day

Question 16

Does gastric acid secretion require energy and what is it regulated by?

Answer 16

Energy dependent

Neurohumoral regulation

Question 17

What is the first step of gastric acid secretion (H2O)?

Answer 17

The origin of the H+ ions is CO2
CO2 & H2O from respiration are converted to bicarbonate (H2CO3) via enzyme carbonic anhydrase
H2CO3 rapidly dissociates into HCO3- & H+
The H+ ions produced can then react with the OH- ions from the breakdown of H2O to regenerate H2O

Question 18

What is the second step in gastric secretion (K+ & H+)?

Answer 18

The H+ ions from the break down of H2O are then pumped into the stomach lumen via H+/K+ ATPase pumps in the luminal membrane of parietal cells
They pump 1 K+ ion into the parietal cell for every 1 H+ ion they pump out into the stomach (ensure no change in polarity of cell)
These pumps require ATP to function
The K+ ions pumped in can diffuse back out into the stomach via K+ channels on the plasma membrane of parietal cells

Question 19

What is the last step of gastric secretion? (Cl- & HCO3-)

Answer 19

The HCO3- from the breakdown of H2CO3 is secreted into the capillary for the exchange of Cl- ions
Cl- ions can enter the stomach by diffusing through Cl- channels in the plasma membrane of the parietal cells
Then in the stomach, the H+ and Cl- ions can react to form HCl

Question 20

What does the removal of the end products of this reaction enhance?

Answer 20

Enhances the forward rate of reaction- in this way, production and secretion of H+ are coupled

Question 21

What can increases acid secretion stimulated by factors result from? What other than factors can increase acid secretion?

Answer 21

The migration of the protein H+/K+ ATPase in the membranes of intracellular vesicles in the parietal cell to the plasma membrane by fusion of these vesicles with the membrane thereby increasing no of pump protein in plasma membrane meaning more H+ can be pumped in = more acid

More pumps= more H+ pumped in stomach= more acid

Question 22

What is turning on gastric acid secretion called/caused by?

Answer 22

Cephalic phase
(Also gastric phase)
Proteins in the stomach

Question 23

What is the cephalic phase initiated by?

Answer 23

During a meal

The sight, smell, taste of food & chewing

Question 24

What are impulses transmitted by and what’s released?

Answer 24

Parasympathetic nervous system

Acetyl choline is released

Question 25

What does ACh do?

Answer 25

Acts indirectly on parietal cells Triggers the release of GASTRIN (g cells in the pyloric antrum of stomach) & HISTAMINE (from enterchrimaffin-like (ECL) cells)

Question 26

What do gastrin & histamine do in the cephalic phase? | What does this cause?

Answer 26

Increase the number of H+/K+ ATPase pumps on plasma membrane of parietal cells Net effect= increase acid production

Question 27

What initiates the gastric phase of turning on gastric acid secretion?

Answer 27

Once food has reached the stomach By gastric distension form volume of ingested ,arterial & presence of peptides & amino acids (released by the digestion of luminal proteins)

Question 28

What is released as a result of the gastric phase on turning on secretion and what does it act on?

Answer 28

Gastrin is released which acts directly of parietal cells

Question 29

What does gastrin also trigger in the gastric phase of turning on gastric acid secretion?

Answer 29

Gastrin also triggers the release of histamine which also acts directly on parietal cells

Question 30

What do gastrin & histamine do in the gastric phase? | What does this cause?

Answer 30

Both gastrin & histamine increase the number of H+/K+ ATPases pumps on the plasma membrane of the parietal cell Net effect=increases acid production

Question 31

How do proteins turn on gastric secretion?

Answer 31

Direct stimulus for gastrin release Proteins of foods in the lumen, act as a buffer, thereby reducing amount of H+ ions, increases pH, results in decreased secretion of somatostatin (somatostatin=inhibits parietal cell activity)which results in more parietal cell activity = more acid production

Question 32

What are the 2 phases causing gastric acid secretion to turn off?

Answer 32

Gastric phase | Intestinal phase

Question 33

What happens In the gastric phase to cause the turn off gastric acid secretion?

Answer 33

Low luminal pH (high [H+]) directly inhibits gastrin secretion thereby indirectly inhibiting histamine release Low pH also stimulates somatostatin release which inhibits parietal cell activity

Question 34

What is the intestinal phase of the turning off of gastric acid secretion initiated by?

Answer 34

In the duodenum | Duodenal distension, low pH, hypertonic solutions, presence of amino acids and fatty acids

Question 35

What does the intestinal phase do to turn off acid secretion?

Answer 35

The release of locally produced chemical messengers called enterogastrones eg, SECRETIN (inhibits gastrin release and promotes somatostatin release) & CHOLECYSTOKININ (CKK) Also trigger short & long neural pathways which reduce ACh release Effect=reduce acid secretion

Question 36

What is the regulation of gastric acid secretion controlled by?

Answer 36

Brain, stomach & duodenum

Question 37

What are the 1 parasympathetic neurotransmitter, 1 hormone, 2 paracrine and 2 key enterogastrones?

Answer 37

Parasympathetic Neurotransmitter - acetylcholine (+) Hormone - gastrin (+) 2 paracrine (produced in stomach) - histamine (+) & somatostatin 2 key enterogastrones - secretin (-) & CKK (-)