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NBME 20 Answers

nbme20/Block 4/Question#16 (49.8 difficulty score)
An investigator wishes to determine the ...
Ductal ion reabsorption is more efficient at low flow ratesπŸ”

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submitted by sinforslide(49),
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wOrvviee of iSlava iroctp:nduo

  1. Acanir elslc esertec iniiatl ,laivas ichhw is soctonii ot p.laams
  2. Dcault elcls omyifd nilatii aa,livs cgausin nstopobriare fo Na,+ -Cl and estcrnoie of +K and .C-HO ;&-tg ehihgr +K dna OH-C eoricntnstoacn, lreow N+a -lC rnnoicatoenstc ni v.ailas In tish spcero,s MREO aNCl is ,eserbbdora nanmeig that lvaasi send up giben pcohiynto to psaaml.

lcaDut rpobsnoatire rcsepos is eerhdind in oi-hwhglf ,stsaet wihch ecsasu siocnito laiasv

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

+/- j44n(38),

submitted by usmleuser007(370),
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iylavraS ceosetirn . 1 At owl lfwo = igHh naonctretnico of tospsma;ui owl tornnnetoiascc of smio,du a,cbibr &;apm 2doi crleh. ta ihhg olwf = owl ncrteitancono of pts;iaumos hghi nrasocneintoct of iudmso, abcb,ir pa&m; ociedrlh

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?? +  
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 β€œfit” this contact time explanation is HCO3βˆ’; HCO3βˆ’ secretion is selectively stimulated when saliva secretion is stimulated. +3  

submitted by chandlerbas(95),
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Pt 1 esk(at iccrit i)dca

tP 2 (etask dyc)na

Pt 1 2(x slaavi = hihg lwof rte,a a2lxCN elrvenai[tr - ])gonier

tP 2 (x1 vsiaal = low owfl r,eta 1LxCNa gni[oer ont m)]ip

tP1 = circti diac *tims dtaucl elcl to tesreec acrllrialuten ulcadt trsageo fo bb.iarc bcairb zeelitrnuas irctci .cdia hsut owl

tP2 = wol wolf a,tre tuhs meor abicrb dasboerreb tou fo utdc. uhst olw rbcabi

hwy od teyh aehv teh ames ,ibarcb aeeusbc wlo flwo

via* hAc sngail acndrustoitn to cenaiesr rculelnraatil ultdca cmluiac laesree t;&--g O3HC dtroes ienids utdcal cell

submitted by stepwarrior(20),

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

submitted by usmledelirium(0),

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.