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NBME 21 Answers

nbme21/Block 3/Question#33 (53.5 difficulty score)
Left radial arterial and venous blood samples ...
HCO3 transported in the plasma🔍

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 +36  upvote downvote
submitted by hayayah(990),
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het raitjmyo fo nraocb oeiddxi melusecol aer eirradc sa part of eht itcbebanroa feburf tesmys. nI sith e,ssymt boranc dodieix ifudsfse tnoi het s.RBC bocrnaCi yrhednasa AC)( iwhnit sRBC ckyluiq ectrvson the onabcr iddixeo onit racinocb cdia OHC3(.)2 icboarnC ciad is na lneabust taniedeiemrt euocleml atth detaelyimim esoitcasdsi onti eoicbrtaban soin -3H(CO) nda nyehdrog +)(H si.on

heT ylwne dstiyehsezn oabaitrnceb oni is astotpderrn uot fo eth CBR noti eht msaapl ni cagxenhe ofr a eroihdcl ion ;C()−l hsti si cdleal the irelcohd h.stif hnWe hte lodob eheasrc het gsnu,l eth ebctbaoarin oni si ordanettsrp bcka ntoi eth RCB ni chnxeeag rfo the cihoerld noi. hTe H+ nio esdtisacois fomr het boiheolgmn nda sdinb ot eht teaabbonric oin. iTsh uscepord hte ionbccra adic ia,tdemiteern hwihc si renvdetco kacb tino boracn eoddiix gthuohr eth cnztmyeia catoni fo AC. The ornabc eodiixd uepdodcr is llxpedee thhrgou the lusgn ruidgn .ieltxanoha

hungrybox  Amazing explanation. Thank you!! +1  
namira  in case anyone wants to visualize things... +4  
ergogenic22  CO2 is carried in the blood is bound to hemoglobin, known as carbaminohemoglobin (HbCO2) (5%), dissolved CO2 (5%), bicarb is 90% +3  
pg32  Nice explanation, but can anyone clarify how we know from the question that we are measuring HCO3 rather than dissolved CO2? +1  
qball  @pg32 This question is asking about what accounts for the LARGER amount of co2 and the HCO3 buffer is about 85% of this transport and dissolved C02 is about 5-7%. +1  
teepot123  fa 19 pg 656 +1  

 +2  upvote downvote
submitted by mcdumbass(10),

I thought thought measuring a gas in the blood only measures partial pressure of dissolved gas in the blood

schistosite  Yeah, me too...the fact that they expect us to assume that HCO3- is counted in their measurement for CO2 concentration is absolutely absurd. +1  
happyhib_  they tell you measured is around 24 and 22 or some bs but give "mM"; if you look at the lab sheet HCO3 has a units in mM and normal is 22-28 = they are not just looking at partial pressure's. +  

For RBS's:

At the lung: Cl- out, HCO3- in

At the tissues: Cl- in, HCO3- out