Stomach and vomiting Flashcards
In the stomach what cells secrete HCL?
Parietal cells
HCL secretion is promoted by three stimuli. What are they?
- Histaimine - stimulates H2 receptors the most important stimulas for gastric acid secretion
- Parasympathetic stimulation through vagus nerve - ACh acts at the neurotransmitter at muscarinic M3 receptors
- Gastrin - least important, triggers histamine release from neighbouring enterochromaffin-like cells (ECL)
PEPSINOGEN - what does it do? what secretes it
It starts the process to breack down protein.
PROENZYME - converted to PEPSIN by the acidic environment - which is secretes in the stomach lumen by cheif cells.
Chief cells are triggered to secrete pepsinogen by
1) Gastrin
2) Oarasympathetic nervous activity through the vagus nerve
Gastrin - secreted by what cells? In response to what (3 tthings)
Peptide hormone which is secretes by G CELLS in response to
1) Parasympathetic nervous actibvity
2) Distension of the stomach
3) Presecence of partially digested protein oin the stomach
Gastrin has three main roles. What are they?
1) Stimulation of parietal cells to secrete HCL both directly and through stimulation of histamine release by the ECL cells
2) Stimulation of chief cells to secrete pepsinogen
3) Stimulation of gastric motility
Intrinisc Factor is secreted by what cells?
Parietal cells
Intrinisic Factor (IF) has important role? Which vitamin does it work alongside with? How is this special vitamin absorbed within the GI tract?
Vitamin B12 absorption
- Vitamin B12 is released from ingested animal proteins as theu are broken down in the stomach
- In the low ph environment of the stomach - IF has low binding affinity for B12, so very little is bound. Released b12 is instead bound by HAPTOCORRIN, a b12 binding protein. This protects the acid-sensitive structure of Vitamin b12.
- In the duodenum, vitamin b12 is re-released as haptocorrin is digested by trypsin
- In the higher pH environment of the duoodenum, IF avidly binds vitamin b12.
- In the terminal ileum, IF receptions allow absoprtion of the IF -vitamin12 complex
Pernicious anaemic leads to destruction of what cells? What kind of deficiency can result in this form of anaemia?
Autoimmune destruction of parietal cells. Vitamin b12 cannot be absorbed therefore you get megaloblastic anaemia.
Mucous cells secrete …
HCO3- RICH mucus which covers the gastric mucosa.
Two main roles
-Protection of the gastric mucosa from highly acidic contents of the stomach lumen
-Lubrication of the stomach wall, protected it from frictional damage due to vigorous peristalissis, mixing of partially digested food.
What are the key features of parietal cells?
- Close proximity to ECL cells
- Extensive network of secretory canaliculi
- H/K+ - ATPase pump
- Three stimulatory receptors, histamine H2, ACh and gastrin
- One inhibitoary receptor: somatostatin
- CA within the cell cytoplasm
How does histamine act in the stomach?
Increases cAMP concentration within the parietal cells
Explain clearly the mechanism for gastric acid secretion.
- CO2 diffuses into the pareital cell from the blood
-CO2 reacts with water to give H2CO3 in a reaction catalysed by CA
-H2CO3 dissociates into H+ and HCO3-. These ions then go their separate ways:
====1) In the apical membrane: The H+/K+-ATPse actively pumps H+ into the secretory canaliculi in exchange for K+. The H+/K+ ATPAse is known as the proton pump
====2)In the basolateral membrane: HCO3- is exchange for Cl-. HCO3- enters the blood where it causes a measurable increase in blood PH whenever gastric acid secretion is stimulated, this is referred to as the aklaline tide.
-Cl- diffuses down its concentration gradient through a CL- channel to the secretory canaliculi
-K+ also diffuses down its electrochemical gradient back into the secretory canaliculi through a K+ channel.
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Overall effect is HCL secretion into the stomach lumen and NaHCO3 secretion into the bloodstream.
