Gastric secretions
This cell type functions to acidify the stomach environment for pepsin activation, protein denaturation, and pathogen sterilization.
What is a parietal cell?
This peptide hormone is secreted by the stomach to increase appetite and hunger. (hint: it makes your stomach growl)
What is ghrelin?
• Increases with fasting + sleep deprivation
• Decreases with food intake
This hormone is produced by D cells and it downregulates GI secretions.
What is somatostatin?
• Somatostatic --> inhibits secretion of various hormones (D cells Downregulate)
Downstream effects:
• ↓ gastric acid, gastrin, pepsinogen, CCK, secretin, pancreatic secretions, GIP
• Inhibits gallbladder contraction
• ↓motility of stomach + intestine
• Splanchnic vasoconstriction
This secretion gets trapped in the mucus covering the gastric epithelium, which enables it to neutralize gastric acid.
What is bicarbonate?
• Secreted by mucosal cells (stomach, duodenum, salivary glands, pancreas)
• Increases by pancreatic + biliary secretion with secretin
Histamine is secreted by these cells in the stomach to increase gastric acid secretion by stimulating parietal cells.
What are enterochromaffin-like (ECL) cells?
• For context: Enterochromaffin (EC) cells are specialized enteroendocrine cells in the gastrointestinal mucosa that act as primary gut chemosensors, regulating digestion and signaling the nervous system.
• G cells release gastrin --> ECL activation --> histamine --> parietal cell activation --> HCl
Somatostatin secretion is inhibited by activation of this nerve.
What is the vagus nerve?
• Vagus nerve normally increases secretion + motility ("rest + digest!") --> decrease somatostatin
This zymogen is produced by chief cells and becomes active via cleavage in a low pH environment (pH < 5), where it can initiate protein breakdown.
What is pepsinogen?
Chief cells --> pepsinogen --> low pH --> pepsin (optimal 1.8-3.5)
This neuropeptide, rather than acetylcholine, is used by the vagus nerve to stimulate G cells.
What is gastrin-releasing peptide (GRP)?
• GRP is released from presynaptic terminals of postganglionic fibers
This hormone increases secretion of bile and pancreatic duct cell secretion of HCO3 to neutralize acidic chyme in the stomach.
What is secretin?
• S cells (duodenum) secrete secretin so pH is stable/not sour (acidic)
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Parietal cells secrete intrinsic factor to bind to vitamin B12 and enable its absorption in this portion of the GI tract.
What is the terminal ileum?
Clinical correlate: if a patient has a resection of the terminal ileum, they might have severe vitamin B12 malabsorption --> neurological complications + megaloblastic anemia
Secretion of this hormone increases with vagus nerve stimulation, distention of the stomach, amino acids and peptides, and chronic PPI administration.
What is gastrin?
Secreted by: G cells
Function: Stimulates parietal cell acid production + mucosal growth (i.e. indirect increase in HCl)
Cholecystokinin (CCK) is secreted by I cells in the small intestine and among its many functions it helps relax this sphincter.
What is the sphincter of Oddi?
• It's a muscular valve at the junction of the bile duct, pancreatic duct, and duodenum that regulates the flow of digestive juices