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NBME 21 Answers

nbme21/Block 3/Question#33 (60.7 difficulty score)
Left radial arterial and venous blood samples ...
HCO3βˆ’ transported in the plasmaπŸ”

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submitted by hayayah(1076),
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eht oyritjma fo obacrn xodiedi uoecellms ear icdrrae as rpat fo the reibbacoant rfeubf nI hsti my,sste cborna iexoddi siefsdfu tion eht C.RBs oCainbcr sahardyne (A)C htwnii CBsR kliycuq rosnctev the rocabn dxeiodi oint racbncio daci C23(.)OH niarbCco iadc si na lentbsau rtnidmeeaite eumlleco ahtt lmeimyidaet aedcstioiss ntio ncaatroeibb onsi H3)C(-O dan ngorheyd )+H( nos.i

heT ylnew izsnyeeshtd tnraabibcoe ino is rpntadestor uto fo eth CRB iotn teh aalspm in nhgacxee ofr a ecolrdih oni C)(βˆ’;l isth is laledc hte diorcleh is.hft neWh the olbdo eahcser hte sn,ugl teh eoancbrtiba nio is atrptrdeson bakc into teh CRB ni ecnaegxh ofr het iolhrdce ion. The +H oin iosiaestdcs mfor eht imlbhooeng dna snbdi to het brecbioaatn .ion hiTs copursde the rcobncia icda titmeia,nerde whcih is ondrevtce bcak oint nbcoar ddxoiie hguohtr the ntaiyczme otinca of C.A eTh ranobc dxiidoe uecprddo is ledeexpl uotrhgh eth nsgul irndgu enahailo.tx

hungrybox  Amazing explanation. Thank you!! +1  
namira  in case anyone wants to visualize things... +5  
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? +3  
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%. +3  
teepot123  fa 19 pg 656 +1  
surfergirl  "majority of blood CO2 is carried as HCO3- in the plasma." I guess that is all they're testing us on, just in a very convoluted way. +  

submitted by mcdumbass(21),

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. +2  
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. +1  
an_improved_me  Yea,.. Suprisingly in clinic, when you are measuring important parameters in the blood (think fish-bone diagram), you actually use HCO3- as an approximation for CO2 in the blood (so you use the value in CO2 when calculating like the anion gap or something) +  

submitted by mafoo_xtreme(10),

For RBS's:

At the lung: Cl- out, HCO3- in

At the tissues: Cl- in, HCO3- out