NBME 20 Ductal ion reabsorption is more efficient at low flow rates (NBME Answers)

Retired NBME Answer Explanations

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Retired NBME 20 Answers

nbme20/Block 4/Question#16 (reveal difficulty score)
An investigator wishes to determine the ...
Ductal ion reabsorption is more efficient at low flow rates 🔍 / 📺 / 🌳 / 📖
tags: GI

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submitted by ∗masonkingcobra(408)

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https://drawittoknowit.com/pop-quizzes/anatomy-physiology/at-a-high-flow-rate-how-does-saliva-compare-to-plasma

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utap2001 Nice, a brilliant picture! +

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submitted by ∗sinforslide(63)

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Overview of Saliva production:

Acinar cells secrete initial saliva, which is isotonic to plasma.
Ductal cells modify initial saliva, causing reabsorption of Na+, Cl- and secretion of K+ and HCO-. -> higher K+ and HCO- concentrations, lower Na+ Cl- concentrations in saliva. In this process, MORE NaCl is reabsorbed, meaning that saliva ends up being hypotonic to plasma.

Ductal reabsorption process is hindered in high-flow states, which causes isotonic saliva

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https://www.researchgate.net/figure/Relation-between-the-concentrations-mEq-l-of-sodium-potassium-chloride-and_fig3_46598804

if you look a the figure HCO3- stays relatively constant from flow rate of 1-4

+2/- j44n(141)

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submitted by ∗usmleuser007(464)

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Salivary secretion 1. At low flow = High concentration of potassium; low concentrations of sodium, bicarb, & chloride 2. at high flow = low concentration of potassium; high concentrations of sodium, bicarb, & chloride

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sherry That's exactly what I was thinking when I was taking the test. But I was sidetracked by same HCO3 level. Can somebody explain this part to me?? +

smc213 @sherry https://pcwww.liv.ac.uk/~petesmif/petesmif/salivary%20secretion/bicarbonate%20secretion.htm +2

charcot_bouchard Because salivary duct removes Na & Cl while secrete K & Hco3 in lumen. In low flow rate HCO3 & K inc because duct is doing its thing for more time. At high flow rate K slightly dec (as cant be secrted as much) but HCO3 stays almost same. the reason is high flow indicates higher metabolism & higher bicarb production. +

cienfuegos Regarding the bicarb (via BRS Physiology, which explains flow rate as coming down to "contact time" where slow flow allows more reabsorption of NaCl): The only ion that does not “fit” this contact time explanation is HCO3−; HCO3− secretion is selectively stimulated when saliva secretion is stimulated. +3

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submitted by ∗chandlerbas(118)

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Pt 1 (takes citric acid)

Pt 2 (takes candy)

Pt 1 (2x saliva = high flow rate, 2xNaCl [irrelevant - ignore])

Pt 2 (1x saliva = low flow rate, 1xNaCL [ignore not imp])

Pt1 = citric acid stim* ductal cell to secrete intracellular ductal storage of bicarb. bicarb neutralizes citric acid. thus low bicarb.

Pt2 = low flow rate, thus more bicarb reabsorbed out of duct. thus low bicarb

why do they have the same bicarb, because low flow rate.

*via Ach signal transduction to increase intracellular ductal calcium release --> HCO3 stored inside ductal cell

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submitted by ∗stepwarrior(29)

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all fine and dandy except bicarbonate isn't constant with flow rate

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submitted by usmledelirium(0)

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I think everyone is focusing on Bicarb when you can just compare flow rate and NaCL.

High flow rate = 2x Nacl ; same bicarb Low flow rate = 1x Nacl; same bicarb

this shows that when flow is slow, you are able to reabsorb more.

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