Download Digestive Functions: Comprehensive Q&A on Gastric Secretion and Digestion and more Exams Biology in PDF only on Docsity!
PHY 503: DIGESTIVE FUNCTIONS TEST 1 QUESTIONS WITH
ACCURATE SOLUTIONS
What stimulates the parietal cells to secrete HCl? -- Answer โโ gastrin, histamine, or ACh How does the tonicity of gastric secretions compare to the tonicity of the plasma? -- Answer โโ they are always isotonic to the plasma, the secretion rate just changes up the relative concentrations of the ions What are the three major functions of gastric acid? -- Answer โโ stop bacterial growth, convert pepsinogen to pepsin, and begin protein digestion How does vagal stimulation of HCl secretion occur? -- Answer โโ by direct stimulation of gastric gland parietal cells, or by stimulation of pyloric gland G cells to release gastrin that stimulates HCl (indirect) How does atropine affect the secretion of gastric acid? -- Answer โโ it blocks the M muscarinic AChR on the parietal cells, but does not block G cell secretion of gastrin because ACh is not used to stimulate gastrin release, nor does it block ECL cell secretion of histamine What does somatostatin do? -- Answer โโ inhibits acid and gastrin release What does gastrin-releasing peptide (GRP) do? -- Answer โโ stimulates gastrin release from G cells Presence of which nutrients in the stomach can directly stimulate gastrin release? -- Answer โโ small peptides and amino acids
What stimulates somatostatin secretion from D cells? -- Answer โโ acidification of the antrum of the stomach (pH <3) What does histamine do in the stomach? -- Answer โโ binds H2 receptors on parietal cells to stimulate acid secretion What substances can stimulate histamine release? -- Answer โโ ACh and gastrin What are the physiological effects of elevated gastrin besides release of acid? -- Answer โโ increase ECL cell proliferation and histamine release What is a possible consequence of long-term therapy to reduce gastric acid secretion? -- Answer โโ hypergastrinemia; gastrin is normally inhibited by the presence of antral gastrin, so when acid secretion is continually suppressed, the serum gastrin will be chronically high since the feedback mechanism is not being properly engaged What happens to the degree of acid secretion when more than one parietal secretagogue is activated at a time? -- Answer โโ enhances effectiveness of acid secretion (increases it) What do H2 blockers (i.e. cimetidine) do? -- Answer โโ attenuate the secretory responses to both ACh and gastrin by blocking H2 on the basolateral membrane of parietal cells, which will reduce acid secretion but have no specific effect on direct activity of gastrin or ACh on acid secretion What happens when both gastrin and ACh bind their receptors on parietal cells? -- Answer โโ they initiate a GPCR response through the G-alpha-q cascade, resulting in elevated Ca2+ and activation of kinases to stimulate acid secretion What happens when histamine binds its receptor on parietal cells? -- Answer โโ it initiates a GPCR response through the G-alpha-s cascade, which activates adenylyl cyclase to elevate cAMP and stimulate PKA to phosphorylate the H+/K+ pump to cause acid secretion How do somatostatin and prostaglandins affect acid secretion? -- Answer โโ by opposing histamine activity to inhibit acid secretion
What is the purpose of the gastric diffusion barrier produced by the stomach? -- Answer โโ prevents luminal secretions from damaging the stomach wall What are the 3 components of the gastric diffusion barrier? -- Answer โโ 1) apical membrane and epithelial tight junctions in gastric glands, which prevent acid from traversing the stomach lining because they are relatively impermeable to it
- mucus layer provides a physical barrier between luminal contents and gastric epithelium
- HCO3- rich layer next to the epithelium keeps the local pH high to protect the stomach lining from damage from acid and degradation by pepsin What materials can irritate or damage the stomach lining? -- Answer โโ aspirin, ethanol, NSAIDs, bile salts refluxing from the SI, and H. pylori How does H. pylori cause gastritis/ulcers? -- Answer โโ it resides in the neutral pH mucus barrier adjacent to the gastric epithelium, which allows it to survive and damage the stomach lining What are the 3 main phases of digestion? -- Answer โโ cephalic, gastric, and intestinal What is the cephalic phase of digestion? -- Answer โโ first phase in which the external stimuli of food activates parasympathetic afferents to promote salivation and acid secretion (all occurs before food enters the stomach) What is the primary mediator of the cephalic phase of digestion? -- Answer โโ the vagus nerve What blocks the cephalic phase of digestion? -- Answer โโ vagotomy What are the 4 events that increase acid secretion in the stomach due to vagus stimulation? -- Answer โโ 1) vagal postganglionic muscarinic nerves in the corpus release ACh, which directly stimulates acid secretion
- ACh triggers histamine release from ECL cells in the lamina propria of the corpus to stimulate acid secretion
- peptidergic postganglionic vagal neurons and other enteric NS neurons release GRP, which stimulates gastrin release from antral G cells
- vagus inhibits SST release from D cells, which reduces any background inhibition of gastrin release in the corpus and antrum What is the gastric phase of digestion? -- Answer โโ second phase that happens once food enters the stomach and excites vagovagal reflexes from the stomach to the brain and back in order to elicit gastric secretions What are the different mechanisms for eliciting gastric secretions during the gastric phase of digestion? -- Answer โโ 1) local enteric reflexes and gastrin cause secretion of gastric juice during the several hours that food remains in the stomach after a meal
- parasympathetic vagal neurons release GRP to stimulate antral G cells
- both local enteric and vagovagal reflexes employ ACh to trigger gastric secretions
- presence of food activates stomach stretch receptors, and is a reflexive trigger for gastrin and acid secretion How does the cephalic phase affect pancreatic function before food enters the duodenum? -- Answer โโ sight, taste, or smell of food causes pancreatic secretions to increase, likely by stimulating acinar mAChRs How does the gastric phase affect pancreatic function before food enters the duodenum? -- Answer โโ food in the stomach modulates pancreatic secretion through release of gastrin and by stimulating neural pathways through ACh to affect both acinar and ductal cells What is the gastropancreatic reflex? -- Answer โโ vagovagal reflex in which gastric distension from food in the stomach stimulates low levels of pancreatic secretion What is the intestinal phase of digestion? -- Answer โโ third phase that happens once food is present in the duodenum, which continues to cause stomach secretion of small amounts of gastric juice because of the small amounts of big gastrin being released by duodenal mucosa Which two hormones account for nearly all hormonal stimulation of pancreatic secretions? -- Answer โโ secretin and CCK
What do salivary acinar cells do? -- Answer โโ produce salivary enzymes What are the characteristics of the primary salivary acinar cell secretion? -- Answer โโ it is similar to plasma, Cl- is the principal secreted ion (not Na+), and it contains ptyalin/mucus/ECF What do salivary ductal cells do? -- Answer โโ produce a buffering secretion; actively absorb Na+, passively absorb Cl-, actively secrete K+, and also secrete HCO3- How does increased salivary flow rate affect the ionic composition of saliva? -- Answer โโ as it increases, ions approach their plasma values because there is less time available for modification (osmolarity increases) Na+ and Cl- do not reach their equivalent plasma levels in saliva, but how do the tonicity and K+ concentration of saliva compare to those of plasma? -- Answer โโ saliva is always hypotonic to plasma, and salivary K+ concentration is always greater than plasma K+ concentration How does increased salivary flow rate affect concentration of HCO3- and why is this important? -- Answer โโ increased flow results in higher HCO3- concentration, which alkalinizes the saliva to help neutralize gastric acid that would normally reflux into the esophagus What are the general functions of the stomach? -- Answer โโ short-term storage reservoir so food doesn't enter SI too fast and overwhelm absorption machinery, coordinated contractions to mechanically digest large fragments and increase surface area for chemical digestion, slowly release food into the SI for further processing, secretion of intrinsic factor What are the four major segments of the stomach? -- Answer โโ 1) cardia --> immediately distal to the gastroesophageal junction, completely devoid of acid-secreting cells
- corpus --> largest portion (most of the proximal part is called the fundus) that contains acid-secreting cells
- antrum --> most distal region that leads into the pylorus
What does the proximal stomach secrete? -- Answer โโ acid, pepsinogen, mucus, HCO3-, and IF What does the distal stomach secrete? -- Answer โโ gastrin and somatostatin What are the different cells of the stomach lining and their functions? -- Answer โโ superficial epithelial cells secrete HCO3-, mucous neck cells secrete alkaline mucus, stem cells provide new cells when old ones slough off, parietal cells secrete acid and IF, chief cells secrete pepsinogen, and endocrine cells secrete GI regulatory hormones What do the gastric glands secrete? -- Answer โโ HCl, pepsinogen, mucus, intrinsic factor, and rennin/chymosin What do the pyloric glands secrete? -- Answer โโ G cells secrete gastrin, D cells secrete SST, and neck cells secrete mucus and some pepsinogen How does the pH of the stomach change when stimulated by a meal? -- Answer โโ drops from a basal pH of 4-6 to a pH of less than 2 Why is it important that the stomach pH drops after a meal? -- Answer โโ because pepsinogens need a pH less than 3 to be activated, so the pH drop activates pepsin and ensures that any consumed bacteria do not grow in the stomach What do the parietal cell tubovesicular structures contain in their membranes? -- Answer โโ H+/K+ pump for acid secretion How does the H+/K+ pump get inserted into the parietal cell canalicular membrane? -- Answer โโ cytoskeletal rearrangement causes tubovesicular structures to fuse with the canalicular membrane, increasing apical surface area and inserting the pump (along with K+ and Cl- channels) to promote acid secretion and maintain balance across the membrane What are the two subunits of the gastric H+/K+ pump? -- Answer โโ alpha subunit --> contains catalytic function and requires an Mg2+ cofactor beta subunit --> required for targeting the pump to the apical membrane surface
