Lecture 4.1 - Functions of the Stomach Flashcards
Why must food be stored within the stomach temporarily?
We eat food faster than we digest it
What would happen if the food remained static within the stomach during storage?
Pressure within the stomach would rise, causing reflux
How is reflux avoided during storage of food in the stomach?
- Receptive relaxation: vagally mediated relaxation of the orad region of the stomach
- Rugae distend, increasing stomach volume
How are the stomach contents mixed?
Peristalsis - coordinated movements every 20s or so
Briefly explain how the stomach separates out the larger chunks of food (which need further digestion) from the liquid chyme which is ready to move on to the duodenum
Peristalsis propels food from proximal to distal.
This allows the stomach to act as a funnel, as it is larger proximally than distally –> separates out the liquid chyme from the larger chunks
Is the pyloric sphincter open during peristalsis? Explain
No - only opes when chunks of food are small enough to progress
What is achieved by the acidity of the stomach’s internal environment?
- Unravels proteins
- Activates zymogens such as proteases
- Disinfects stomach contents
What do the parietal cells produce?
HCl and intrinsic factor
Where are the parietal cells found?
Proximal part of stomach - fundus and body only
Why is intrinsic factor important?
Vital for vitamin B12 absorption
Where are the G cells found?
What do they secrete?
Found in antrum
Secrete gastrin
What do enterochromaffin-like cells produce?
Histamine
What do the chief cells secrete?
Pepsinogen
What do the D cells produce?
Somatostatin
List the important cell types found in the stomach
Parietal cells G cells D cells Chief cells Enterochromaffin-like cells Mucous cells
What substances does the stomach secrete?
HCl Intrinsic factor Mucus HCO3- Pepsinogen
Where does secretion take place within the stomach walls?
Secretion occurs from gastric pits which overly gastric glands.
Gastric pits appear as pin-prick like holes in the stomach wall
List the cell types lining the gastric pit and gland, from superficial to deep
Mucous cells - most superficial - line pit
Parietal cells
Chief cells
Enteroendocrine cells - deepest
Briefly outline the mechanisms of stomach acid secretion
1) Water splits into OH- and H+
2) H+ moves into stomach lumen via H+/K+-ATPase –> highly ATP dependent
3) At the same time, Cl- moves into the parietal cell from the ECF in exchange for HCO3-, which is produced from the combination of CO2 and OH-. HCO3- moves into the bloodstream
4) Cl- then moves into the stomach lumen where it combines with H+ to form HCl
Explain the phenomenon known as the “alkaline tide”
Just after eating, there is a transient increase in blood pH.
This is caused by the influx of HCO3- moving into the blood from parietal cells, in exchange for secretion of HCl into the stomach lumen.
What stimulates parietal cells to secrete HCl?
Gastrin
Histamine
ACh
What does histamine bind to on parietal cells to stimulate HCl secretion?
Does histamine have a major role in stimulation of secretion?
H2 receptors
No - fine-tuning only
What sort of receptors does ACh bind to on parietal cells to stimulate acid secretion?
What nerve controls this?
Muscarinic receptors
Vagus nerve
What sort of receptors does gastrin bind to on parietal cells?
CCK receptors
What sort of molecule is gastrin and hence how does it travel to the parietal cells from its site of synthesis?
Hormone - travels in blood
Where is gastrin made and by what cell type?
The antrum of the stomach, by G cells
What stimulates gastrin secretion?
Peptides/AAs in stomach lumen
Vagal stimulation - ACh and gastrin-releasing peptide
What inhibits gastrin secretion?
What effect does this have in the stomach?
Somatostatin
Decreases HCl secretion
How is HCl production inhibited once food has left the stomach?
- Lower pH as food no longer acting as buffer - stimulates D cells to produce somatostatin –> inhibits G cells –> less gastrin –> less parietal cell stimulation
- Reduced distention means reduced vagal activity, which reduces stimulation of G cells and parietal cells
List 2 classes of drug which reduce gastric acid secretion and give an example of each.
H2 receptor blockers - cimetidine
Proton pump inhibitors (PPIs) - omeprazole
Outline the mechanism by which PPIs work
Inhibit the H+/K+-ATPase so H+ cannot be pumped out of the parietal cell into the stomach lumen
What are the 3 phases of digestion?
What proportion of HCl is produced during each phase?
Cephalic - 30%
Gastric - 60%
Intestinal - 10%
What is the purpose of the cephalic phase of digestion?
Occurs before food reaches for stomach - aim is to prepare for that moment
What stimulates the cephalic phase of digestion?
Smell/taste/chewing/swallowing -> stimulate vagus nerve which acts on parietal and G cells
How is the gastric phase of digestion initiated?
Food reaches stomach and causes it to distend, stimulating the vagus nerve
What happens during the intestinal stage of digestion?
Initially chyme stimulates gastrin secretion, but soon G cells inhibited as food has now left stomach
Why is it important for the stomach to have mechanims of defence?
It is designed to digest organic material, including itself
What are the 3 major mechanisms of defence of the stomach?
Mucus/HCO3- secretion
High turnover of epithelial cells
Prostaglandins
How does mucous protect the stomach from damage?
Forms a thick layer that adheres to epithelium, keeping it at higher pH
How do prostaglandins defend the stomach?
Vasodilators - maintain mucosal blood flow, hence supplying epithelia with plenty of nutrients for renewal
What may breach the stomach defences, causing stomach complaints?
How?
Alcohol - dissolves mucus layer
H. pylori
NSAIDs - inhibit prostaglandins
What are the 3 basic functions of the stomach?
Receiving food from the oesophagus and storing it temporarily
Disrupting food mechanically
Continuing digestion
