Gastric Physiology Flashcards
What are the functions of the stomach?
Store & mix food Dissolve & continue digestion Regulate emptying into the duodenum Kill microbes Secrete protease Secrete intrinsic factor Activate protease Lubrication Mucosal protection
What are the key cell types in the stomach?
Mucous cells, parietal, chief, enteroendocrine, enterochromaffin-like (ECL) cell, G cells, D cells
What do mucous cells do?
Produce mucous, at entrance to gland
What do parietal cells do?
Produce gastric acid & intrinsic factor
What do Chief cells do?
Produce pepsinogen
What do enteroendocrine cells do?
Secrete gastrin
What do enterochromaffin-like (ECL) cells do?
Release histamine
What do G cells do?
Release gastrin
What do D cells do?
Release somatostatin
Describe the upper portion of the stomach and what is secreted?
Glands in The thin walled upper portion of the stomach secrete mucous, hydrochloric acid (parietal cells), enzyme precursor pepsinogen (chief cells)
Describe the lower portion of the stomach and what is secreted?
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)
Where does gastric acid secretion occur from?
Parietal cells
What type of acid is gastric acid?
Hydrochloric acid
What’s its pH & strength?
pH 2
Very strong
[H+]>150mM
How much gastric acid is secreted?
2 litres per day
Does gastric acid secretion require energy and what is it regulated by?
Energy dependent
Neurohumoral regulation
What is the first step of gastric acid secretion (H2O)?
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
What is the second step in gastric secretion (K+ & H+)?
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
What is the last step of gastric secretion? (Cl- & HCO3-)
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
What does the removal of the end products of this reaction enhance?
Enhances the forward rate of reaction- in this way, production and secretion of H+ are coupled
What can increases acid secretion stimulated by factors result from? What other than factors can increase acid secretion?
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
What is turning on gastric acid secretion called/caused by?
Cephalic phase
(Also gastric phase)
Proteins in the stomach
What is the cephalic phase initiated by?
During a meal
The sight, smell, taste of food & chewing
What are impulses transmitted by and what’s released?
Parasympathetic nervous system
Acetyl choline is released
What does ACh do?
Acts indirectly on parietal cells
Triggers the release of GASTRIN (g cells in the pyloric antrum of stomach) & HISTAMINE (from enterchrimaffin-like (ECL) cells)
What do gastrin & histamine do in the cephalic phase?
What does this cause?
Increase the number of H+/K+ ATPase pumps on plasma membrane of parietal cells
Net effect= increase acid production
What initiates the gastric phase of turning on gastric acid secretion?
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)
What is released as a result of the gastric phase on turning on secretion and what does it act on?
Gastrin is released which acts directly of parietal cells
What does gastrin also trigger in the gastric phase of turning on gastric acid secretion?
Gastrin also triggers the release of histamine which also acts directly on parietal cells
What do gastrin & histamine do in the gastric phase?
What does this cause?
Both gastrin & histamine increase the number of H+/K+ ATPases pumps on the plasma membrane of the parietal cell
Net effect=increases acid production
How do proteins turn on gastric secretion?
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
What are the 2 phases causing gastric acid secretion to turn off?
Gastric phase
Intestinal phase
What happens In the gastric phase to cause the turn off gastric acid secretion?
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
What is the intestinal phase of the turning off of gastric acid secretion initiated by?
In the duodenum
Duodenal distension, low pH, hypertonic solutions, presence of amino acids and fatty acids
What does the intestinal phase do to turn off acid secretion?
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
What is the regulation of gastric acid secretion controlled by?
Brain, stomach & duodenum
What are the 1 parasympathetic neurotransmitter, 1 hormone, 2 paracrine and 2 key enterogastrones?
Parasympathetic Neurotransmitter - acetylcholine (+)
Hormone - gastrin (+)
2 paracrine (produced in stomach) - histamine (+) & somatostatin
2 key enterogastrones - secretin (-) & CKK (-)
