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NBME 20 Answers

nbme20/Block 4/Question#16 (54.8 difficulty score)
An investigator wishes to determine the ...
Ductal ion reabsorption is more efficient at low flow rates🔍,📺

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submitted by sinforslide(54),
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vrOewvei fo lavaiS d:onpuriotc

  1. icAran elscl sctreee lntaiii alas,vi hhwci is isooitnc ot .pmlaas
  2. ltuDca slecl ifmoyd atlniii ,ilavas uaingsc pneaobrriost of ,+Na lC- nad cionsrete of +K dna .OHC- ;- gt& hhgrie +K and -HCO tecaos,tinnoncr lwoer +Na lC- otceatnnsrcnio ni lvsa.ia In itsh srocse,p MREO lCNa si bor,abdseer namnegi atth salavi esnd up beign npyctohio to a.lmpas

catlDu eropsotainbr psesrco is hdneiedr ni lohw-ghfi es,stta which sseacu sitooicn ilaavs

utap2001  Nice, a brilliant picture! +  

submitted by usmleuser007(419),
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ayrSalvi rseoceint .1 At owl fowl = giHh rcniontaeotnc of tupm;ssiao lwo noctcotasnienr fo ,iduosm biac,rb pm&a; lodhe.2icr ta hghi lwof = olw anncirettncoo fo msui;oastp hghi onntcictensora of ,omudsi bicb,ra a&;pm ldhciore

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 “fit” this contact time explanation is HCO3−; HCO3− secretion is selectively stimulated when saliva secretion is stimulated. +3  

submitted by chandlerbas(106),
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tP 1 etsak( riitcc caid)

tP 2 (ketsa c)dnya

Pt 1 (x2 lvaais = ghhi wlfo ,erat a2CNlx lv[ranreite - rongi)]e

tP 2 x1( ilaasv = low fwlo re,ta LaxNC1 genior[ otn ip)]m

Pt1 = rtiicc adci ts*mi daluct ellc to etscree eaatlinlcurrl datulc aetsogr fo barbci rszlintueae itircc cid.a utsh olw ab.cbir

2Pt = olw wlfo r,tae usht moer ribcab sobrreadeb uto fo suth wlo ibracb

why od tyhe evah hte emas ircb,ab auecbes olw wolf tear.

vi*a chA isnagl nudstcnatrio ot cenreais cltunlraliera aductl iacmucl earesle -;&g-t 3COH ostred ieisdn cdatul cell

submitted by stepwarrior(23),

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.