The Stomach Flashcards
Mucus neck cells secrete
bicarbonate and mucus
Parietal cells secrete
HCl
Intrinsic factor
ECL cells secrete
histamine
Chief cells secrete
pepsinogen
lipase
G cells secrete
gastrin
D cells secrete
somatostatin
Body of the stomach contains which cells
Parietal cells and chief cells mainly
Antrum and pylorus of the stomach contains
G & D cells
no parietal cells
What is pump via which parietal cells secrete HCl
H+/K+ pump
H+ out into stomach lumen, K+ into parietal cell (then gets recycled back out)
Describe the alkaline tide (where the H and Cl comes from in parietal cells)
The H+ used in HCl secretion comes from water.
Resulting OH reacts with CO2 to from HCO3 via carbonic andhydrase.
Bicarbonate/chloride exchanger - bicarbonate is moved into the interstitial space and then into the blood - the is the alkaline tide. Cl moves into parietal cell
What are the direct and indirect methods of stimulation of acid secretion in the stomach
Direct (to parietal cells)
Vagal nerve - Ach/M3 receptors
Histamine
Gastrin
Indirect:
Ach and gastrin mediated histamine release by ECL cells
The biggest signal to acid secretion is
histamine from ECL cells
How are G cells stimulated
- by the vagus which acts through gastrin receptor protein (GRP)-mediated gastrin release
- or by small peptides in the stomach (+ve feedback!)
gastrin stimulates parietal cells directly or indirectly via ECL cells
What triggers Ach release by the vagus
distension of the stomach
how can gastrin and somatostatin secreted by G and D cells in the antrum of the stomach act on parietal cells in the body of the stomach?
endocrine mechanism - they are hormones they go via the blood
D cells secrete somatostatin. What does somatostatin do?
Antagonises histamine-mediated acid secretion
- stops ECL cells making histamine and inhibits parietal cells from responding to it
Inhibits gastrin release from G cells
Where in the stomach is somatostatin produced and how does it act on parital cells in the body of the stomach from these locations?
- by D cells in the body of the stomach - paracrine action
- by D cells in the antrum of the stomach - endocrine action
Effect of vagal stimulation on D cells
Inhibits somatostatin secretion
No vagal stimulation - somatostatin is secreted inhibit acid secretion as no food in stomach
What is the negative feedback in the stomach which prevents pH getting too low?
When pH gets very low (below 2) it stimulates D cells to release somatostatin
How are prostagladins important in maintaining the mucosal barrier in the stomach?
inhibit acid secretion, stimulate bicarbonate and mucus secretion from mucus neck cells
What does intrinsic factor do?
Needed for vitamin B12 absorption - binds B12 in stomach and facilitates absorption in the ileum
The basal phase of acid secretion
stomach acid secretion follows circadian rhythm - low in morning, high in evening
The cephalic phase of acid secretion
smell, sight, thought, swallowing of food triggers secretion via the vagus
30% of total acid secretion
The gastric phase of acid secretion
Food present in the stomach causes secretion
50-60% of total secretion
The intestinal phase of acid secretion
Partially digested peptides/amino acids in the proximal portion of the small intestine stimulates duodenal G cells to secrete gastrin
(whether it is stimulatory or not is questionable)
Accounts for 5-10% of acid secretion
What does pepsinogen secreted by chief cells do?
converted to pepsin below pH 5
pepsin degrades proteins to peptides, but only has a small role in digestion
Inactivated in intestine above pH 7.2
