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Intestinal Water and Electrolyte Absorption: Q&A for Physiology Students, Exams of Biology

A series of questions and answers related to the physiological processes of water and electrolyte absorption in the gastrointestinal tract, focusing on the small and large intestines. It covers topics such as the roles of intestinal villi and crypts, the mechanisms of sodium and chloride absorption, the effects of hormones like aldosterone, and the absorption of vitamin b12. Designed to test understanding of these complex processes and their clinical relevance, such as in conditions like atrophic gastritis and pernicious anemia. It also touches on the causes of stool color changes like melena.

Typology: Exams

2024/2025

Available from 06/03/2025

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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 โœ”โœ” absorbing
nutrients/electrolytes and secreting electrolytes into the intestinal lumen, respectively
What do Paneth cells do? -- Answer โœ”โœ” 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 โœ”โœ” the small
intestine
Where does the fluid entering the intestines come from? -- Answer โœ”โœ” 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 โœ”โœ” net
absorption of sodium, potassium, chloride, and water; net secretion of bicarbonate
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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 โœ”โœ” absorbing nutrients/electrolytes and secreting electrolytes into the intestinal lumen, respectively What do Paneth cells do? -- Answer โœ”โœ” 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 โœ”โœ” the small intestine Where does the fluid entering the intestines come from? -- Answer โœ”โœ” 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 โœ”โœ” 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 โœ”โœ” 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 โœ”โœ” 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 โœ”โœ” 1) electrogenic diffusion into the cell through aqueous channels

  1. coupled with nutrient uptake (glucose, galactose, amino acids)
  2. electroneutral uptake of sodium and chloride through a Na+/H+ exchanger and Cl- /HCO3- exchanger
  3. 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 โœ”โœ” 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 โœ”โœ” the Na+/K+ pump What is the major mechanism for postprandial Na+ absorption and how does it occur? -
  • Answer โœ”โœ” sodium-nutrient coupled transport in the jejunum; involves electrogenic
  1. 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 โœ”โœ” 1) increase in osmotic pressure due to the presence of digested particles in the lumen
  2. active secretion of electrolytes from crypt cells, which leads to water secretion What drives water secretion into the lumen from crypt cells? -- Answer โœ”โœ” 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 โœ”โœ” absorption is passive in the SI and active in the colon How is K+ absorbed in the jejunum and ileum? -- Answer โœ”โœ” passively, most likely through solvent drag as water moves How is K+ secreted in the colon? -- Answer โœ”โœ” 1) passive paracellular movement of K+ across tight junctions, driven by negative luminal voltage (primary mechanism)
  3. 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 โœ”โœ” 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 โœ”โœ” 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 โœ”โœ” 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 โœ”โœ” 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 โœ”โœ” 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 โœ”โœ” 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 โœ”โœ” 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 โœ”โœ” 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 โœ”โœ” 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 โœ”โœ” 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 โœ”โœ” 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 โœ”โœ” 1) acidic pH and activated pepsin in the stomach release ingested B12 from dietary proteins and allow it to bind to haptocorrin

  1. parietal cells release IF into the stomach
  2. once chyme enters the duodenum, haptocorrin is digested by pancreatic proteases, releasing B12 and allowing it to bind to IF
  3. the B12-IF complex travels to the ileum and is endocytosed
  4. 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
  5. 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 โœ”โœ” because the receptor that mediates this endocytosis recognizes and binds to IF, not to B What is melena and why does it occur? -- Answer โœ”โœ” 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 โœ”โœ” 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 โœ”โœ” 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 โœ”โœ” fat malabsorption What are the four types of diarrhea? -- Answer โœ”โœ” secretory, osmotic, motility- related, and inflammatory What toxin classically causes secretory diarrhea? -- Answer โœ”โœ” cholera toxin What is osmotic diarrhea? -- Answer โœ”โœ” diarrhea occurring when there is an excess of osmotically active particles in the intestinal lumen

Why are oral rehydration solutions effective for cholera? -- Answer โœ”โœ” 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 โœ”โœ” glucose, sodium, chloride, potassium, and citrate What is the mechanism of cholera toxin? -- Answer โœ”โœ” 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 โœ”โœ” 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 โœ”โœ” because they have mutated CFTR channels, and cholera toxin cannot irreversibly open these channels in the mutated form