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NBME 21 Answers

nbme21/Block 3/Question#33 (reveal difficulty score)
Left radial arterial and venous blood samples ...
HCO3โˆ’ transported in the plasma ๐Ÿ” / ๐Ÿ“บ / ๐ŸŒณ / ๐Ÿ“–

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submitted by โˆ—hayayah(1212)
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the majority of carbon dioxide molecules are carried as part of the bicarbonate buffer system. In this system, carbon dioxide diffuses into the RBCs. Carbonic anhydrase (CA) within RBCs quickly converts the carbon dioxide into carbonic acid (H2CO3). Carbonic acid is an unstable intermediate molecule that immediately dissociates into bicarbonate ions (HCO3-) and hydrogen (H+) ions.

The newly synthesized bicarbonate ion is transported out of the RBC into the plasma in exchange for a chloride ion (Clโˆ’); this is called the chloride shift. When the blood reaches the lungs, the bicarbonate ion is transported back into the RBC in exchange for the chloride ion. The H+ ion dissociates from the hemoglobin and binds to the bicarbonate ion. This produces the carbonic acid intermediate, which is converted back into carbon dioxide through the enzymatic action of CA. The carbon dioxide produced is expelled through the lungs during exhalation.

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hungrybox  Amazing explanation. Thank you!! +2
namira  in case anyone wants to visualize things... +6
ergogenic22  CO2 is carried in the blood is bound to hemoglobin, known as carbaminohemoglobin (HbCO2) (5%), dissolved CO2 (5%), bicarb is 90% +5
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%. +5
teepot123  fa 19 pg 656 +2
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. +
realnorthomfs  FA 2020, pg670 +

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submitted by โˆ—mcdumbass(28)
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I thought thought measuring a gas in the blood only measures partial pressure of dissolved gas in the blood

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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. +3
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. +2
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) +1

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submitted by mafoo_xtreme(12)
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For RBS's:

At the lung: Cl- out, HCO3- in

At the tissues: Cl- in, HCO3- out

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