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1.In a steady state, the difference in CO2 content between the venous blood leaving a tissue and the arterial blood entering the tissue is determined by which of the following ratios?
(A) Alveolar ventilation to tissue blood flow
(B) Alveolar ventilation to tissue O2 consumption
(C) Tissue CO2 production to tissue blood flow
(D) Tissue CO2 production to tissue O2 consumption
(E) Tissue CO2 production to venous PCO2

This is from NBME sample question and the answer is C.

Can't it be B?

I thought if you consume more O2 then the venous CO2 content would rise.
And everyone knows that PaCO2 inversely correlate with Alveolar ventilation.

Why is tissue blood flow related with CO2 content?

Second one is as follows.

14. A 15-year-old girl limits her diet to carrots, tomatoes, green vegetables, bread, pasta, rice, and skim milk. She has an increased
risk for vitamin A deficiency because its absorption requires the presence of which of the following?
(A) Heme
(B) Intrinsic factor
(C) Phosphatidylcholine
(D) Protein
(E) Triglyceride

How does TG aids in the process of vitamin A absorption?
I heard that vitamin A has aromatic ring and thus would be degraded by different enzyme from TG which is lipase.

Correct me!!!:confused:
 

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1.In a steady state, the difference in CO2 content between the venous blood leaving a tissue and the arterial blood entering the tissue is determined by which of the following ratios?
(A) Alveolar ventilation to tissue blood flow
(B) Alveolar ventilation to tissue O2 consumption
(C) Tissue CO2 production to tissue blood flow
(D) Tissue CO2 production to tissue O2 consumption
(E) Tissue CO2 production to venous PCO2

This is from NBME sample question and the answer is C.

Can't it be B?

I thought if you consume more O2 then the venous CO2 content would rise.
And everyone knows that PaCO2 inversely correlate with Alveolar ventilation.

Why is tissue blood flow related with CO2 content?

Second one is as follows.

14. A 15-year-old girl limits her diet to carrots, tomatoes, green vegetables, bread, pasta, rice, and skim milk. She has an increased
risk for vitamin A deficiency because its absorption requires the presence of which of the following?
(A) Heme
(B) Intrinsic factor
(C) Phosphatidylcholine
(D) Protein
(E) Triglyceride

How does TG aids in the process of vitamin A absorption?
I heard that vitamin A has aromatic ring and thus would be degraded by different enzyme from TG which is lipase.

Correct me!!!:confused:
Question 1:

The answer is C.

I will try to break it down. So the question asks what determines the difference in CO2 content between the veins and arteries.

The correct answer is C - CO2 production/blood flow. If you increase CO2 production, the ratio increases and the A-V difference increases. If you increase blood flow, the ratio decreases and the A-V difference decreases because if the CO2 production stays the same, there is simply more blood for that CO2 to dissolve in, hence the concentration decreases.

So as the ratio increases, A-V difference increases. As the ratio decreases, A-V difference decreases.

A is incorrect because if you increase alveolar ventilation you will decrease both arterial and venous CO2 content and it will not have an effect on A-V difference.

B is incorrect because of the same reason. See, the thing is that when you increase alveolar ventilation, you do breathe out more CO2 but that same blood gets circulated through both venous and arterial sides of the circulation, so maybe for a few seconds there will be an increased difference but it will eventually equalize to the prior value.

D does not make any sense at all. O2 consumption and CO2 production are practically the same thing, there is no reason to be takin their ratio.

E also does not make any sense. They are directly proportional values. So as you increase CO2 production, venous PCO2 also increases, ratio stays the same. Does not give you anything.

Question 2:

Most likely the answer is E. Vitamin A is a fat-soluble vitamin. No fat=no absorption. I would not dig any deeper. I believe you are overthinking again. Simple, one-step questions are rare, answer them with simple answers and do not overthink them.

P.S. Those are just my guesses lol. I am not a physiologist and I have not seen the official explanations to those questions.
 

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Discussion Starter #3
Thanks a lot always. A little bit of investigation through Guyton 13th helped me out.

The thing was I didn't consider this was 'steady situation' which means we don't consider pulmonary compensation for change in tissue metabolism.

Question number 1 was just about guyton figure 41-7 which you can find by googling 'Effect of blood flow and metabolic rate on peripheral tissue PCO2' and the first image you see.

The question here wanted to ask 'do you know CO2 is perfusion limited gas because of its great diffusing ability'
Yes, alveolar ventilation really changes arterial CO2 content but it does so to keep alveolar CO2 rather constant around 40mmHg which 'steady state' was all about.

So the answer must be C not B.

I really begin to think that boards people really want us to read textbook itself, not just summarized books like HY or FA.

Guyton chapter 40, 41 was of great help, if you might be interested.

Thanks always Nodo.
 

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Thanks a lot always. A little bit of investigation through Guyton 13th helped me out.

The thing was I didn't consider this was 'steady situation' which means we don't consider pulmonary compensation for change in tissue metabolism.

Question number 1 was just about guyton figure 41-7 which you can find by googling 'Effect of blood flow and metabolic rate on peripheral tissue PCO2' and the first image you see.

The question here wanted to ask 'do you know CO2 is perfusion limited gas because of its great diffusing ability'
Yes, alveolar ventilation really changes arterial CO2 content but it does so to keep alveolar CO2 rather constant around 40mmHg which 'steady state' was all about.

So the answer must be C not B.

I really begin to think that boards people really want us to read textbook itself, not just summarized books like HY or FA.

Guyton chapter 40, 41 was of great help, if you might be interested.

Thanks always Nodo.
I do not know about big textbooks. I have only studied a select few chapter from Guyton and I do not remember anything I studied from there. I do think that high yield and First Aid are not enough for physiology. Something like a Costanzo textbook (approx. 500 pages, well written, easy to understand) combined with Kaplan lectures is perfectly fine. Guyton is overkill. There is way too much minutiae that you will never need on the test or real life practice. I tried to go through Guyton, I bought the textbook and after 100 pages I realized it was not working for me.
 
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