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PHY 503: B12/WATER/ELECTROLYTE ABSORPTION AND
STOOL: TEST QUESTIONS WITH CORRECT ANSWERS
What are intestinal villi and crypts responsible for? -- Answer โโ absorbing nutrients/electrolytes and secreting electrolytes into the intestinal lumen, respectively What do Paneth cells do? -- Answer โโ sense bacteria in the small intestine through a MyD88-dependent TLR activation, triggering an antimicrobial immune response that limits the ability of the bacteria to penetrate the mucosal surface and enter the body Where in the GI tract does most fluid absorption occur? -- Answer โโ the small intestine Where does the fluid entering the intestines come from? -- Answer โโ 25% from diet, the rest is secreted into the GI tract from endogenous sources (salivary, gastric, pancreatic, biliary glands) What does the small intestine have net absorption and secretion of? -- Answer โโ net absorption of sodium, potassium, chloride, and water; net secretion of bicarbonate
What does the large intestine have a net absorption and secretion of? -- Answer โโ net absorption of sodium, chloride, and water; net secretion of potassium and bicarbonate What is the typical tonicity of chyme that leaves the duodenum and what would happen if the tonicity was different? -- Answer โโ isotonic to the interstitial fluid; if it were hypotonic to the interstitial fluid the intestine would absorb water, and if it were hypertonic water would be secreted into the lumen What are the four mechanisms of sodium absorption in the intestines? -- Answer โโ 1) electrogenic diffusion into the cell through aqueous channels
- coupled with nutrient uptake (glucose, galactose, amino acids)
- electroneutral uptake of sodium and chloride through a Na+/H+ exchanger and Cl- /HCO3- exchanger
- coupled with extrusion of H+ in the absence of any parallel Cl- movement in the duodenum and jejunum only (occurs when GI luminal pH is alkaline) How can chloride be absorbed in the intestines? -- Answer โโ can follow Na+ paracellularly along with water, or can move transcellularly through Cl- channels due to negative electrical potential in the gut lumen as a result of the Na+ uptake What is the mechanism of extrusion of Na+ across the enterocyte basolateral membrane? -- Answer โโ the Na+/K+ pump What is the major mechanism for postprandial Na+ absorption and how does it occur? -
- Answer โโ sodium-nutrient coupled transport in the jejunum; involves electrogenic
- exchange of Cl- and HCO3- without parallel movement of HCO3- and H+ (in the ileum and colon) What are the two processes that establish an osmotic gradient to draw water into the intestinal lumen? -- Answer โโ 1) increase in osmotic pressure due to the presence of digested particles in the lumen
- active secretion of electrolytes from crypt cells, which leads to water secretion What drives water secretion into the lumen from crypt cells? -- Answer โโ Cl- secretion by the apical CFTR channel, whose activity can be increased by any secretagogue that increases cAMP/cGMP/Ca2+ levels What is the difference between K+ absorption in the small intestine and in the colon? -- Answer โโ absorption is passive in the SI and active in the colon How is K+ absorbed in the jejunum and ileum? -- Answer โโ passively, most likely through solvent drag as water moves How is K+ secreted in the colon? -- Answer โโ 1) passive paracellular movement of K+ across tight junctions, driven by negative luminal voltage (primary mechanism)
- active secretion induced by aldosterone in surface epithelial cells and by cAMP in colonic crypt cells Where does the greatest passive K+ secretion rate occur and why? -- Answer โโ the distal colon because this is where the voltage difference is most negative
What happens if the basolateral K+ channel is more active than the apical K+ channel (BK channel)? -- Answer โโ the predominant process will be simple recycling of K+ across the basolateral side; this is what occurs in the basal state because normal apical BK activity is low How can activity of the apical BK channel be increased? -- Answer โโ by aldosterone in colonic surface epithelial cells and by cAMP in colonic crypt cells, and this will lead to active K+ secretion How does active absorption of K+ in the distal colon occur? -- Answer โโ the colonic H+/K+ pump mediates active K+ uptake from the lumen, and it then leaves across the basolateral side; this occurs when there is a depletion in dietary K+ How do ouabain and omeprazole affect H+/K+ pumps in the GI tract? -- Answer โโ ouabain partially inhibits the colonic H+/K+ pump because it has a 60% amino acid sequence homology with the Na+/K+ pump, but omeprazole does not inhibit the colonic H+/K+ pump like it does with the gastric H+/K+ pump In what parts of the intestine do macronutrient, bile acid, and cobalamin absorption occur? -- Answer โโ macronutrient in the proximal small intestine (duodenum and jejunum), bile acid in the distal jejunum, and cobalamin in the ileum What are folds of Keckring? -- Answer โโ circular extensions around most of the small intestine (especially duodenum and jejunum) that increase absorptive surface area What is the general structure of intestinal villi? -- Answer โโ surface is lined with epithelial cells (enterocytes), enteroendocrine cells, goblet cells, and stem cells; within each is the central lacteal and blood supply for the tissue; each is covered with microvilli
Can patients with atrophic gastritis still digest dietary proteins? -- Answer โโ yes, because even though pepsinogen cannot be activated due to lack of gastric acid secretion, patients do not have elevated stool nitrogen, indicating that protein breakdown through only pancreatic proteases is sufficient for digestion and absorption What is the primary function of cobalamin (B12)? -- Answer โโ coenzyme for homocysteine:methionine methyltransferase, which transfers methyl group from tetrahydrofolate to homocysteine, converting it to methionine How does histamine affect intrinsic factor synthesis and release? -- Answer โโ histamine stimulates the release of IF from parietal cells, but not the synthesis of IF, so IF secretion will be blocked by H2-receptor antagonists just like acid secretion What are the steps for processing intracellular B12? -- Answer โโ 1) acidic pH and activated pepsin in the stomach release ingested B12 from dietary proteins and allow it to bind to haptocorrin
- parietal cells release IF into the stomach
- once chyme enters the duodenum, haptocorrin is digested by pancreatic proteases, releasing B12 and allowing it to bind to IF
- the B12-IF complex travels to the ileum and is endocytosed
- once the complex is internalized, B12 and IF dissociate, and the free B12 binds to trans-cobalamin II and exits across the enterocyte basolateral membrane
- free B12 circulates, and any excess is delivered to the liver through the portal vein for secretion back into the bile through the enterohepatic circulation
Why does B12 have to complex with IF to allow absorption across the apical membrane of ileal enterocytes? -- Answer โโ because the receptor that mediates this endocytosis recognizes and binds to IF, not to B What is melena and why does it occur? -- Answer โโ stool that is black in color due to how HGB is altered by digestive enzymes and intestinal bacteria What can cause maroon-colored stool? -- Answer โโ incomplete digestion of blood (from a mid-small intestine or proximal colon GI bleed) that may be due to ulcers, tumors, or Crohn's disease What causes clay-colored stool? -- Answer โโ blockage in the main bile duct preventing bile from reaching the intestinal lumen, often due to liver disease What causes greasy yellow-colored stool? -- Answer โโ fat malabsorption What are the four types of diarrhea? -- Answer โโ secretory, osmotic, motility- related, and inflammatory What toxin classically causes secretory diarrhea? -- Answer โโ cholera toxin What is osmotic diarrhea? -- Answer โโ diarrhea occurring when there is an excess of osmotically active particles in the intestinal lumen
Why are oral rehydration solutions effective for cholera? -- Answer โโ because nutrient-coupled sodium and water absorption is unaffected by cholera toxin What does the WHO oral rehydration solution consist of from highest to lowest concentration? -- Answer โโ glucose, sodium, chloride, potassium, and citrate What is the mechanism of cholera toxin? -- Answer โโ interferes with G-alpha-s subunit ability to hydrolyze GTP and inactivate itself, triggering chronically high stimulation of adenylyl cyclase and increased cAMP What is the effect of chronically increased cAMP in cholera? -- Answer โโ it acts on apical CFTR channels to conduct more chloride into the intestinal lumen, which leads to secretion/paracellular movement of water and sodium that overwhelms any fluid/electrolyte absorption mechanisms Why might CF patients be protected from cholera toxin? -- Answer โโ because they have mutated CFTR channels, and cholera toxin cannot irreversibly open these channels in the mutated form
