011 Physiology of the GI tract: Gastric secretion Flashcards
What are the 3 parts of the stomach?
Fundus, Body, Pyloric antrum
How much volume does the stomach hold up to during expansion and during fasting?
50ml during fasting, 2L during expansion
Where is the major area of exocrine secretion in the stomach?
Body of the stomach
Where is the major area of endocrine secretion in the stomach?
Pyloric antrum
How many gastric glands per mm2 of surface are there?
100 gastric glands per mm2 (approximately 50% of the surface)
How much gastric secretion is produced per day?
2L
What does gastric secretion contain?
HCl, Pepsin, intrinsic factor
Which glands are found in the body of the stomach? and what cells do they contain?
Gastric (oxyntic) glands - containing enterochromaffin cells, parietal cells, mucus neck cells, surface mucus cells, D cells, Chief cells and
Which glands are found in the pyloric antrum?
Pyloric glands - containing surface mucous cells, mucous neck cells, G cells and D cells
What do parietal cells secrete?
HCl, IF
What do enterochromaffin cells secrete?
Histamine
What do chief cells secrete?
Pepsinogen
What do G-cells secrete?
Gastrin
What do D-cells secrete?
Somatostatin
Where does vitamin B12 uptake occur?
In the lower ileum
How does HCl secretion work in the parietal cells?
During fasting state, lumen pH remains at 2. During ingestion, the acid is diluted to pH 4. This stimulates acid production in the parietal cells. CO2 diffuses into the parietal cells from blood plasma. This reacts with water to form H2CO3 which dissociates into H+ and HCO3-. The H+ is transported through the H+/K+ antiporter pump using hydrolysis of ATP. HCO3- is exchanged for Cl- and forms the alkaline tide returning to the blood. The Cl- moves through the parietal cells and is excreted into the lumen through Cl- channels. There is a Na+/K+ ATPase pump which exchanges 3Na+ ions out of the parietal cell into the blood plasma for every 2K+ into the cell. This generates a K+ gradient, as the K+ that has entered the parietal cell through both Na+/K+ pump and the H+/K+ pump is pumped out K+ channels into the lumen.
What is PPT Omeprazole
Drug used to inhibit the proton pump in parietal cells.
How are parietal cells adapted to increase HCl secretion?
They have tubulovesicles that fuse with the canniculus membrane during parietal cell activation. These fuse to form the canaliculus (secretory network), increasing surface area 50-100 fold. When cells are no longer activated, they undergo endocytosis back into resting state.
How is pepsinogen secreted?
Chief cells secrete pepsinogen, which is activated by presence of HCl to form the soluble form pepsin. Hence chief cells are located in oxyntic glands along with parietal glands.
How is IF secreted and what is its purpose?
IF is secreted by parietal cells. As vitamin B12 enters the stomach it binds to the intrinsic factor. This moves through the digestive tract. At the terminal ileum, IF binds to receptors, causing receptor-mediated endocytosis of the vitamin B12. IF is egested in feces while vitamin B12 binds to protein to protect it from degredation within the blood.
What is a consequence of absense of IF?
Pernicious anaemia
What is the role of gastrin?
Endocrine hormone produced in the pyloric glands that stimulate HCl production, pepsinogen release, and mucus release. It maintains mucus and increases motility of muscle.
What is the role of somatostatin?
Paracrine hormone produced by D cells when pH is low enough, having a direct inhibitory effect of G cells. It also indirectly reduces acid secretion by limiting release of other hormones.
What is the role of histamine?
A paracrine hormone produced by enterochromaffin like cells that act on H2 receptors on parietal cells and stimulates H+ secretions.
What is the role of secretin?
In the presence of acid, it is secreted to cause pancreas to release water and bicarbonate. It also inhibits secretion by the stomach.
What is the role of CCK?
Stimulated by fats and proteins in the duodenum. Its primary purpose is protein and fat digestion by releasing pancreatic enzymes and bile. It also inhibits gastric processes e.g. gastric emptying, acid secretion.
Where is secretin released?
S cells in the crypts of lieberkuhn of the duodenum.
Where is CCK released?
I cells in the duodenum.
What are the 3 phases of gastric secretion?
Cephalic phase (30-35% of secretion) Gastric phase (60% of total secretion) Intestinal phase (5-10% of total secretion)
How does the cephalic phase occur?
Vagal nerves stimulate gastric parietal cells, or there is an indirect release of gastrin by vagal nerve fibres that travel in the blood and activates parietal cells.
How is the gastric phase stimulated?
Distension, elevated pH, peptides and amino acids, alcohol and caffeine.
How is the intestinal phase activated?
Duodenal stretch, decreased pH, presence of lipids and CHO.
How does HCO3- act as a mucosal barrier?
The alkaline tide created by parietal cells travel by arterial supply to surface epithelium and secreted into the mucus. When H+ is secreted, it causes a back diffusion of H+ that HCO3- is able to neutralize before reaching gastric cells.
What is the role of prostaglandin? and how does aspirin cause gastric ulcers?
Prostaglandin enhances the effectiveness of the mucosal barrier. Aspirin causes reduced local prostaglandin release that causes user to be more prone to gastric ulcers.
What is Zollinger-Ellison Syndrome?
It is a rare gastrin secreting tumor on the non-B cells of the pancreatic islet. The uncontrolled release of gastrin causes multiple peptic ulcers mainly in the distal duodenum. Symptoms include abdominal pain, diarrhea, steatorrhea.
