Case 2 - gastric acid production Flashcards
what is the pH of the stomach
1.55-3.5
what is HCl produced by in the stomach
the parietal cells
what is the first step to produce hydrochloric acid
water and carbon dioxide combine within the parietal cell cytoplasm to produce carbonic acid
what is the carbonic acid catalysed by
carbonic anhydrase
what happens to the carbonic acid after it has been catalysed
spontaneously dissociates into a hydrogen ion and a bicarbonate ion
where is the hydrogen ion transported to and via what?
transported into the stomach lumen via the H+/K+ ATPase ion pump.
what does this pump use for the exchange of ions
this pump uses ATP as an energy source to exchange potassium ions into the patietal cells of the stomach with H+ ions
how is the bicarbonate ion transported out of the cell into the blood
via a transporter pump called anion exchanger which transports the bicarbonate ion out of the cell in exchange for a chloride ion.
what happens to this chloride ion
it is then transported into the stomach lumen via a chloride channel
what is the result of the Cl going into the stomach lumen via a chloride channel
result in both the hydrogen and chloride ions being present within the stomach lumen.
their opposing charges leads to them associated with each other to form hydrochloric acid
how is the production of gastric acid controlled
as a result, the number of H+/K+ ATPases present within the parietal cell membrane is minimal. the rest are sequestered within tubulovesicles in the parietal cell
upon stimulation the vesicles fuse with the cell membrane, hence allowing for the increases movement of hydrogen ions into the stomach thus increasing acid production
explain the mechanism to increase gastric acid production via ACH
ACh is released form the vagus nerve.
this is released firstly during the cephalic phase of digestion, which is activated upon seeing or chewing food, leading to direct stimulation of parietal cells via the vagus nerve
it is also produced during the gastric phase of digestion when intrinsic nerves detect distension of the stomach, stimulating the production of ACh by the vagus nerve
explain how gastric acid production is increased via gastrin
the main regulation pathway involves the hormone gastrin which is secreted by G cells in the stomach.
G cells are activated by the vagus nerve, gastrin related peptide and by peptides in the stomach lumen produced via protein digestion.
activation of the G cells leads to the production of gastrin which is released into the blood and travels through the blood until it reaches the parietal cells.
Gastrin binds to CCK receptors on parietal cells which also elevates calcium levels causing increased vesicular fusion
how is gastric acid production increased via histamine
enterochromaffin like cells in the stomach secrete histamine which binds to H2 receptors on the parietal cells. these cells release histamine in response to the presence of gastrin and ACh. this leads to increased fusion however it is via the secondary messenger cAMP as opposed to calcium in the other methods
how is gastric acid production decreased - important and long but learn all of it
Via the accumulation of acid in the empty stomach between meals. This increase in acid leads to a lower pH within the stomach, which inhibits the secretion of gastrin, via the production of somatostatin from D cells.
Once food has been broke down into chyme, it passes into the duodenum, triggering the enterogastric reflex.
This reflex can be stimulated by distension of the small bowel, if there is excess acid in the upper intestine, the presence of protein breakdown products as well as excess irritation to the mucosa
Inhibitory signals are sent to the stomach via the enteric nervous system, as well as signals to medulla - reducing vagal stimulation of the stomach
The enterogastric reflex, is important in slowing down gastric emptying when the intestines are already filled
what does the presence of chyme within the duodenum also stimulate
stimulates entero-endocrine cells to release cholecystokinin and secretin, both of which play a variety of important roles in completing digestion, but also inhibit gastric acid secretion
what is secretin release by and when
released by the S cells of the duodenum when there is excessive acid production in the stomacb
what other hormones work to decrease acid production in the stomach
Other hormones including glucose dependent insulinotropic peptide and vasoactive intestinal polypeptide
what are the two mai drugs used to prevent excessive acid formation
H2 antagonists and protein pump inhibitors
how do H2 antagonists work and give an example
H2 antagonists such as ranitidine bind to the H2 receptors preventing the binding of histamine and thus reduce acid secretion