Stomach function Flashcards
What are the basic functions of the stomach?
Short-term storage of food
Disrupt food
Continue digestion of food
Disinfect the food
Why does the stomach need to store food?
Because we eat faster than we digest and absorb
How is the stomach adapted to storing food?
It has temporary folds called rugae
these can unfold, expanding the stomach
How else does the stomach store food, apart from unfolding rugae?
Receptive relaxation
-vagovagal reflex
What is the vagovagal reflex?
Mechanoreceptors in stomach wall detect stretch
vagal afferents carry this sensory information to medulla
vagal efferents to smooth muscle in stomach wall cause it to relax
stomach expands
What is the importance of the stomach expanding to store food?
Intra-gastric pressure is not increased
prevents reflux of gastric contents into oesophagus
How does the stomach disrupt the food?
Muscular contractions of its wall
mix contents
How is the stomach adapted to disrupting food?
Has an extra layer of smooth muscle in its muscularis externae
called the oblique layer
How does the stomach mix its contents?
Every 15-20 seconds
waves of peristalsis from proximal to distal parts of stomach
these get more intense in the distal stomach
Why do the waves of peristalsis get more intense in the distal stomach?
Because it has a thicker muscular wall than the proximal stomach
How does the stomach continue digestion of the food?
Chief cells secrete pepsinogen
which is activated to form pepsin
and that digests proteins
How is pepsinogen activated into pepsin?
By hydrocholoric acid in stomach
secreted by parietal cells
What is the function of hydrochloric acid in the stomach, apart from activating pepsinogen into pepsin?
Innate chemical barrier, kills bacteria
disinfects the food
How do parietal cells secrete hydrochloric acid?
H+ K+ ATPase on apical domain of parietal cell
actively transports one hydrogen ion out of cell into stomach lumen and one potassium ion into cell from stomach lumen
AE on basolateral domain of parietal cell
moves bicarbonate ion from cell into interstitial fluid, chloride ion from interstitial fluid into cell
Chloride on channel on apical domain of parietal cell
chloride ions move through from cell into stomach lumen
Where do the hydrogen and bicarbonate ions in parietal cells come from?
Carbon dioxide reacts with water
reaction is catalysed by carbonic anhydrase
produces hydrogen ion and bicarbonate ion
What is the effect of the AE on the basolateral domain of the parietal cell moving bicarbonate ions into the interstitial fluid?
They diffuse into the plasma
makes the plasma of venous blood leaving the stomach more alkaline
called the alkaline tide
What stimulates the secretion of hydrochloric acid from parietal cells?
Gastrin secreted by G cells
Histamine secreted by enterochromaffin cells
Acetylcholine from vagus nerve
How does histamine interact with parietal cells to stimulate them to secrete hydrochloric acid?
Binds to H2 receptors on parietal cells
How does gastrin interact with parietal cells to stimulate them to secrete hydrochloric acid?
Binds to CCK receptor on parietal cells
What stimulates the vagus nerve to release acetylcholine?
Vagovagal reflex
How does acetylcholine interact with parietal cells to stimulate them to secrete hydrochloric acid?
Binds to muscarinic receptors on parietal cells
What stimulates G cells to secrete gastrin?
Peptides or amino acids in stomach lumen
Acetylcholine from vagus nerve
Gastrin-releasing peptide from vagus nerve
How does acetylcholine interact with G cells to stimulae them to release gastrin?
Binds to muscarinic receptors on G cells
How does gastrin-releasing peptide interact with G cells to stimulate them to release gastrin?
Binds to GRP receptors on G cells
What inhibits the release of gastrin from G cells?
Somatostatin secreted by D cells
How does somatostatin interact with G cells to inhibit the release of gastrin?
Binds to somatostatin receptors on G cells
What stimulates D cells to secrete stomatostatin?
Low pH
When is there a low pH in the stomach?
When food leaves the stomach as chyme
because food present in the stomach lumen acts as a buffer of gastric acid
What are the phases of gastric secretion?
Cephalic
Gastric
Intestinal
When does the cephalic phase of gastric secretion occur?
Before the food has reached the stomach
What happens in the cephalic phase of gastric secretion?
Smell, taste, chewing, swallowing etc. are stimuli
vagal efferents to G cells, parietal cells
release acetylcholine
stimulates G cells to secrete gastrin, parietal cells to secrete hydrochloric acid
release gastrin-releasing peptide
stimulates G cells to secrete gastrin
When does the gastric phase of gastric secretion occur?
When the food is in the stomach
What happens in the gastric phase of gastric secretion?
Vagovagal reflex
releases acetylcholine, gastrin-releasing peptide…
stimulates G cells to secrete gastrin, parietal cells to secrete hydrochloric acid
Amino acids and small peptides in stomach lumen
stimulates G cells to secrete gastrin
Food in stomach buffers low pH
less somatostatin secreted from D cells
less inhibition of G cells secreting gastrin
When does the intestinal phase of gastric secretion occur?
When chyme enters the duodenum from the stomach
What happens in the intestinal phase of gastric secretion?
Partially digested proteins in chyme in duodenum
stimulates G cells in duodenum to secrete gastrin
stimulates parietal cells in the stomach to secrete hydrochloric acid
Reverse enterogastric reflex
Inhibitory hormone released
What stimulates the reverse enterogastric reflex?
Lipids in chyme in duodenum
What happens in the reverse enterogastric reflex?
Reduced vagus nerve stimulation
What inhibitory hormone is released in the later intestinal phase of gastric secretion?
Secretin
What are the functions of secretin?
Inhibits release of hydrochloric acid from parietal cells
Which phases of gastric secretion contribute most to the total secretion of hydrochloric acid from parietal cells?
Gastric phase
then cephalic phase
then intestinal phase
What protects the stomach epithelial from damage by hydrochloric acid?
Thick mucus layer acts as a physical barrier
also contains bicarbonate ions, neutralises acid
Epithelial cells proliferate
old ones replaced with new ones quite often
Prostaglandins
How do prostaglandins protect stomach epithelial cells?
Maintain mucosal blood flow
increased delivery of nutrients to epithelial cells
How does the stomach only allow chyme to move through the duodenum, and not larger parts of food?
Stomach contents accelerate as they move downwards
leaves larger parts behind
liquid chyme reaches pylorus
Pyloric sphincter controls what passes through from stomach to duodenum
only allows chyme to pass through it
How often does chyme move into the duodenum during gastric emptying?
3 times a minute
What are the different parts of the stomach mainly responsible for secreting?
Cardia
-mucus
Fundus and body
- mucus
- hydrochloric acid
- pepsinogen
Pyloris
- gastrin
- somatostatin
What inhibits the release of hydrochloric acid from parietal cells?
Somatostatin secreted by D cells
inhibit release of gastrin from G cells and histamine from enterochromaffin cells
which reduces the secretion of hydrochloric acid from parietal cells
