SPOILER... NS 3 B/B Q 16

[zhonghang]

Which of the following is least likely to be observed in a patient experiencing hyperventilation?

A. Hypoxia (correct answer)
B. Net exhalation of CO2
C. Increased blood pH
D. Increased hemoglobin O2 affinity

the answer is A

okay, so i get that hyperventilation causes CO2 levels to go down (net CO2 out due to expiration). i also understand that hyperventilation will increase O2 levels b/c inhalation. however, i feel like if hyperventilation causes increased hemoglobin O2 affinity, then that means O2 will bind to Hb more easily but it will be more difficult for the Hb to unload the O2 to the cells... therefore, hypoxia would occur. why or how is this not the case?

wikipedia even says, "This leftward shift indicates that the hemoglobin under study has an increased affinity for oxygen so that hemoglobin binds oxygen more easily, but unloads it more reluctantly." so yeah i feel like if answer D "Increased hemoglobin O2 affinity" is true, then answer A "Hypoxia" would be likely to occur.

can someone explain how increased Hb affinity for O2 would not cause hypoxia?

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[NextStepTutor_2]

Which of the following is least likely to be observed in a patient experiencing hyperventilation?

A. Hypoxia (correct answer)
B. Net exhalation of CO2
C. Increased blood pH
D. Increased hemoglobin O2 affinity

the answer is A

okay, so i get that hyperventilation causes CO2 levels to go down (net CO2 out due to expiration). i also understand that hyperventilation will increase O2 levels b/c inhalation. however, i feel like if hyperventilation causes increased hemoglobin O2 affinity, then that means O2 will bind to Hb more easily but it will be more difficult for the Hb to unload the O2 to the cells... therefore, hypoxia would occur. why or how is this not the case?

wikipedia even says, "This leftward shift indicates that the hemoglobin under study has an increased affinity for oxygen so that hemoglobin binds oxygen more easily, but unloads it more reluctantly." so yeah i feel like if answer D "Increased hemoglobin O2 affinity" is true, then answer A "Hypoxia" would be likely to occur.

can someone explain how increased Hb affinity for O2 would not cause hypoxia?

@NextStepTutor_1
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@NextStepTutor_3

Hi @zhonghang ! Be careful here, the question does not ask which of the following will NOT happen, but rather which of the following is LEAST LIKELY to be seen as a result of hyperventilation.

This lowering of PaCO2 and increase in the pH results in alkalosis within the body. This results in dilatation of the blood vessels and a fall in blood pressure. The lower the PaCO2, the worse are the symptoms. There are several causes that may lead to hyperventilation, including hypoxia. thus, it is likely that hypoxia will lead to hyperventilation, but much less likely that hyperventilation leads to hypoxia.

With hyperventilation you can increase O2 saturation of the arterial blood by 1-2% depending on its basal level. However, since arterial blood constitutes only ~25% of total blood volume this change is not significant. Saturation of the venous blood is another story. It is normally around 75% so there is some space for improvement. Hyperventilation markedly increases cardiac output which results in increased oxygen delivery to tissues. If oxygen demand remains constant this should theoretically lead to decreased oxygen extraction and increased saturation of the venous blood. However, things are a bit more complicated.

Yes there may be a small increase in Hb affinity for O2 during hyperventilation, Though alveolar P02 will increase if the patent is breathing deep enough to increase alveolar ventilation, and PO2 arterial will increase, the total amount of oxygen carried by your blood will not rise much, since Hb carries most of the oxygen, and is pretty much completely saturated at normal physiological conditions anyways. You will get a little more dissolve in blood though, since gas exchange is perfusion limited for most.

Of course there is also myoglobin, however, it is almost fully saturated at normal tissue pO2 so there is not much space for increase. The content of O2 physically dissolved in tissues can also increase if hyperventilation is sufficiently long. However, considering total body oxygen stores it is usually not significant.

You are correct in that eventually hypoxia could occur, but its typically only in the brain since a decrease in C02 causes vasoconstriction in brain arterioles. which leads to syncope (fainting). This, however, would occur much later and to a much lower extent than all the other direct effects of hyperventilation listed in the answers.

Hope this helps! Good luck

 

[Mariolee]

I have a problem with the same B/B section, except Passage 1.

Specifically, can someone explain to me what the signals represent? It doesn't make sense, it says that FISH trakcs signals for each X inactivated, and yet in Klinefelter syndrome where the chromosome is XaXaY and there are no inactive Xs, it obtains two signals? I don't understand how that works.

In addition, for B/B #6, it says that Akt levels begin at Small Intestine at Week 16, but don't they clearly begin at Week 14 in the Esophagus?

Thanks in advance!

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[zhonghang]

Hi @zhonghang ! Be careful here, the question does not ask which of the following will NOT happen, but rather which of the following is LEAST LIKELY to be seen as a result of hyperventilation.

This lowering of PaCO2 and increase in the pH results in alkalosis within the body. This results in dilatation of the blood vessels and a fall in blood pressure. The lower the PaCO2, the worse are the symptoms. There are several causes that may lead to hyperventilation, including hypoxia. thus, it is likely that hypoxia will lead to hyperventilation, but much less likely that hyperventilation leads to hypoxia.

With hyperventilation you can increase O2 saturation of the arterial blood by 1-2% depending on its basal level. However, since arterial blood constitutes only ~25% of total blood volume this change is not significant. Saturation of the venous blood is another story. It is normally around 75% so there is some space for improvement. Hyperventilation markedly increases cardiac output which results in increased oxygen delivery to tissues. If oxygen demand remains constant this should theoretically lead to decreased oxygen extraction and increased saturation of the venous blood. However, things are a bit more complicated.

Yes there may be a small increase in Hb affinity for O2 during hyperventilation, Though alveolar P02 will increase if the patent is breathing deep enough to increase alveolar ventilation, and PO2 arterial will increase, the total amount of oxygen carried by your blood will not rise much, since Hb carries most of the oxygen, and is pretty much completely saturated at normal physiological conditions anyways. You will get a little more dissolve in blood though, since gas exchange is perfusion limited for most.

Of course there is also myoglobin, however, it is almost fully saturated at normal tissue pO2 so there is not much space for increase. The content of O2 physically dissolved in tissues can also increase if hyperventilation is sufficiently long. However, considering total body oxygen stores it is usually not significant.

You are correct in that eventually hypoxia could occur, but its typically only in the brain since a decrease in C02 causes vasoconstriction in brain arterioles. which leads to syncope (fainting). This, however, would occur much later and to a much lower extent than all the other direct effects of hyperventilation listed in the answers.

Hope this helps! Good luck

thanks for the response!

another question for you... this is from NS FL 4 C/P #39

Which of the following statements correctly describe the methods used in the experiment?

I. The retention factor in a TLC procedure depends on the solvent system, temperature, and the adsorbent.
II. A polar compound will exhibit a smaller retention factor on a TLC plate than a less polar compound.
III. Anhydrous methanol has a greater eluting strength than pentane when used as solvents in a TLC procedure.

A. I only
B. I and II only (this is what I think the answer is)
C. II and III only
D. I, II, and III (correct answer)

Here is the only information from the passage about the TLC procedure (which you don't even really need to answer the question):

"In the TLC procedure, dichloromethane was used as the solvent system and three spots were placed on the TLC plate."

Can you explain to me why the answer is D? I thought that for TLC, the plate is always polar (e.g. silica gel) and the solvent is always nonpolar (e.g. pentane). This is why the Rf value for polar substances is much smaller than the Rf value for nonpolar substances. Polar substance will interact more with the stationary phase (polar silica plate) and the nonpolar substances will interact more with the mobile phase (nonpolar pentane solvent). So how would a polar methanol solvent have greater eluting strength than a nonpolar pentane solvent in a TLC procedure? I always thought that TLC by convention is polar plate with nonpolar solvent. Am I wrong? The set up of the experiment from the passage (see quote above) even has a nonpolar solvent being used to elute (dichloromethane)...

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[popopopop]

Hello, I wasn't trying to answer your question specifically, but I ended up reviewing the wiki for TLC after I saw your post. This may be useful to you, if you trust wiki.


"If the mobile phase is changed to a more polar solvent or mixture of solvents, it is more capable of dispelling solutes from the silica binding places, and all compounds on the TLC plate will move higher up the plate. It is commonly said that "strong" solvents (eluents) push the analyzed compounds up the plate, whereas "weak" eluents barely move them. The order of strength/weakness depends on the coating (stationary phase) of the TLC plate. For silica gel-coated TLC plates, the eluent strength increases in the following order: perfluoroalkane (weakest), hexane, pentane, carbon tetrachloride, benzene/toluene, dichloromethane, diethyl ether, ethyl acetate, acetonitrile, acetone, 2-propanol/n-butanol, water,methanol, triethylamine, acetic acid, formic acid (strongest)."

https://en.wikipedia.org/wiki/Thin-layer_chromatography

 

[zhonghang]

Hello, I wasn't trying to answer your question specifically, but I ended up reviewing the wiki for TLC after I saw your post. This may be useful to you, if you trust wiki.


"If the mobile phase is changed to a more polar solvent or mixture of solvents, it is more capable of dispelling solutes from the silica binding places, and all compounds on the TLC plate will move higher up the plate. It is commonly said that "strong" solvents (eluents) push the analyzed compounds up the plate, whereas "weak" eluents barely move them. The order of strength/weakness depends on the coating (stationary phase) of the TLC plate. For silica gel-coated TLC plates, the eluent strength increases in the following order: perfluoroalkane (weakest), hexane, pentane, carbon tetrachloride, benzene/toluene, dichloromethane, diethyl ether, ethyl acetate, acetonitrile, acetone, 2-propanol/n-butanol, water,methanol, triethylamine, acetic acid, formic acid (strongest)."

https://en.wikipedia.org/wiki/Thin-layer_chromatography

hey thanks for the response. i understand what i read from the wiki but i'm not understanding how using a polar ("stronger") eluent would help in a TLC procedure. maybe i'm just not understanding the concept behind TLC correctly. my understanding has always been that TLC is used to separate compounds based on polarity. if you're using a polar plate (silica) and using a polar solvent (methanol) then wouldn't the polar substances on the plate interact with both the plate (stationary phase) and solvent (mobile phase). how would this cause better elution? and then what about the less polar (or nonpolar substances), what happens to them when they are sitting on a polar plate and a polar solvent is used as the mobile phase?

furthermore, when looking at wiki it says, "The mobile phase has different properties than the stationary phase. For example, with silica gel, a very polar substance, non-polar mobile phases such as heptane are used." i feel like this directly contradicts what is said above (using a polar solvent with a is a stronger eluent). ugh i'm just so confused now. i honestly felt like i understood TLC before this question from NextStep.

i just don't know what i am missing here... if anyone can help clear up the confusion that would be great!

 

[popopopop]

I'm by no means an expert on TLC and I don't know the whole context of the question, but a quick google search led me to this.

http://www.umich.edu/~orgolab/Chroma/TLChow.html

This pdf is pretty good at explaining it too.

https://www.chem.wisc.edu/deptfiles/OrgLab/handouts/CHEM 344 TLC info.pdf

 

[NextStepTutor_3]

Hi @zhonghang ! First of all, you're correct that TLC typically uses a polar plate (often silica) and a nonpolar solvent, and that nonpolar compounds in such a procedure travel farther up the plate and have higher Rf values. The setup in the passage (dichloromethane solvent) matched this. However, Roman numeral III didn't reference this procedure - it referenced "a TLC procedure" in general. It also asked about eluting strength (also known as eluent strength).

To be more specific, Roman numeral III never stated that anhydrous methanol is a "better" solvent than pentane - for the sake of the MCAT, you can consider pentane and hexane as perfectly good solvents for TLC. Choice III just said the methanol has a higher "eluting strength." To evaluate this option, we need to know what eluting strength is. In virtually all types of chromatography, you'll often see the term "elute" used to mean, in short, "wash a substance from a column or other stationary phase." In other words, something that is a good eluent is good at displacing molecules that have adsorbed to our stationary phase, whatever that may be.

Now, we go back to the fact that our plate is polar. How can we best displace molecules that have adsorbed to our plate? By introducing another polar compound, our eluting solvent, that they will be attracted to instead. This is why, for TLC with a polar stationary phase, the more polar the solvent, the higher the eluent strength.

Remember, this doesn't mean that you always (or even often) want to use a more polar solvent! Eluent strength is simply one measurement; your choice of solvent depends on the particular circumstances. Similar to what you said, there are plenty of cases where you wouldn't want to use a solvent with high eluent strength because it'll displace ALL of our polar compounds, forcing them up the plate and making them harder to discern from each other. But if you use a solvent with very low eluent strength (pentane has an eluent strength of 0, for example), then our polar compounds won't be likely to move up the plate at all, since nothing is displacing them, and comparison between them will be more difficult.

In general, though, your understanding of TLC is fine! This was just a tricky question. Hope this helps 🙂