The Stomach Flashcards
(29 cards)
What is the stomach’s mechanical function?
Storage (stores food 3-4 hours) with aid of rugae, forms chyme partly through peristaltic movements
What is the stomach’s hematopoietic function?
Creates intrinsic factor which is necessary for Vit B12 absorption
What is the name of the condition where a deficiency in RBCs arises from a lack of B12?
Pernicious anemia
What is the stomach’s protective function?
Maintains a pH gradient that protects the gastric wall from pepsin and HCl via a thick mucus layer:
H+ ions (coming from the stomach) and HCO3- (produced by the surface cells) diffuse into the mucus layer and are neutralized: so the pH of the lumen can be maintained at ~1.5 but stays closer to 6.5 at the cell surface
What is the stomach’s digestive function?
Secretes gastric juice: HCl + pepsinogen + mucus and cellular secretions
Secretes proteolytic enzymes like pepsin and rennin/chymosin
What is the name of the glands in the stomach and what do they empty into?
Gastric glands that empty into the bottom of gastric pits
Describe the varying composition of the glands between different regions of the stomach
Cardia: mostly mucus-secreting glands (no enzymes)
Fundus and Body: Neck cells - mucus Parietal/Oxyntic: HCl Chief: pepsinogen Encodrine/G cells: Gastrin
Pyloric: mucus and G cells releasing gastrin
Which part of the stomach do gastric ulcers usually occur?
In the antrum and along the lesser curvature
What two things do neck cells secrete? What is the stimulus and the function of this secretion?
- Mucous: stimulated with irritation of the mucosa and provides a physical barrier between the lumen and epithelia
- Bicarbonate; secreted with mucus and buffers gastric acid to prevent damage to the epithelia
What two things do Parietal cells secrete? What is the stimulus and the function of this secretion?
HCl -> activates pepsin to kill bacteria
Intrinsic factor -> complexes with Vit B12 to permit absorption
Both are stimulated by Ach, histamine and gastrin
What does enterochromaffin-like cell secrete? What is the stimulus and the function of this secretion?
Histamine; stimulated by gastrin and Ach and stimulates gastric acid secretion
What two things do chief cells secrete? What is the stimulus and the function of this secretion?
Pepsionegn -> digests proteins
Gastric lipase -> digests fat
Stimulated by Ach, acid and secretin
What do D cells secrete, what is the stimulus and the function
Somatostatin; stimulated by acid in the stomach and it inhibits gastric acid secretion
What do G cells secrete, what is the stimulus and the function
Gastrin; stimulated by Ach, peptides and amino acids and stimulates gastric acid secretion
What does acid production in the parietal cell depend on the generation of? What enzyme helps generate this molecule?
Carbonic acid, carbonic anhydrase
What mechanism expels H+ against the concentration gradient in parietal cells and where are they located?
The Proton pumps expel H+ into the lumen of the gastric gland, they are within the canaliculi (invaginations of the cell wall)
What are the three primary stimuli for acid to be produced by the parietal cell?
Ach, Histamine and Gastrin
Describe how Ach, directly and indirectly, stimulates acid production
Released from post-ganglionic parasympathetic neurones (VAGUS):
Directly: stimulates parietal cells by acting on muscarinic receptors
Indirectly: stimulates ECL cells (histamine) and G cells (gastrin)
Describe how Histamine acts on the parietal cell. Why are “histamine antagonists” effective?
Binds to the H2 surface receptor on the parietal cell -> produces cAMP -> stimulates the proton pump -> more acid is produced
Antagonists against this mechanism are easily effective as H2 receptors are uncommon
List the three phases of acid secretion
- Cephalic phase
- Gastric phase
- Intestinal phase
What happens in the cephalic phase?
Detecting, sensing and ingesting food provides autonomic stimulation - stimulating Ach
What happens in the gastric phase (including what is stimulated as a result of it)?
- Once food reaches the stomach it buffers the stomach acid and pH rises:
a. This helps gastrin as HCl is also a stimulus for somatostatin (which inhibits further production of acid, -ve feedback). Therefore, less HCl means less somatostatin and less inhibition for gastrin - The stomach distends via the rugae, this also stimulates the intrinsic nerves for more Ach
- Initial digestion releases peptides, this stimulates more gastrin (and in turn more histamine)
What happens in the intestinal phase?
Once the stomach begins to empty…
- Chyme stimulates the release of hormones from DD that antagonize gastrin
- The accumulation of acid in an empty stomach also inhibits gastrin
What is the role of prostaglandins?
Protective:
- Stimulate mucus and alkali secretions
- Vasodilate nearby blood vessels -> more bloodflow aids epithelial growth and inhibits acid secretion
