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    1) A man is running the London Marathon. His breathing is initiated mainly by...

    A) Peripheral Chemoreceptors
    B) Central Chemoreceptors
    C) Neither


    2) If a man doubles his tidal volume but maintains the same rate of breathing then his alveolar ventilation...

    A) Increases double to what it was initially
    B) Increases to more than double it was initially
    C) Increases to less than double it was initially
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    Question one - I reckon that if he is running his PCO2 will increase....PCO2 is detected weakly by peripherial chemoreceptors and strongly by central chemoreceptors (diffuses across BBB and dissociates to form H+ which is what the central receptors respond to).

    Also, increased blood lactic acid from the exercise decreases blood pH which peripherial chemoreceptors respond strongly too.

    I am not 100% sure on this, but from the revision I have done thus far on the subject, that would make sense...
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    Question Two - he would surely double it to what it was initially? Since tidal volume isn't taking into account the dead space within the lungs....?
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    (Original post by nikk)
    Question one - I reckon that if he is running his PCO2 will increase....PCO2 is detected weakly by peripherial chemoreceptors and strongly by central chemoreceptors (diffuses across CCC and dissociates to form H+ which is what the central receptors respond to).

    Also, increased blood lactic acid from the exercise decreases blood pH which peripherial chemoreceptors respond strongly too.

    I am not 100% sure on this, but from the revision I have done thus far on the subject, that would make sense...
    Yeah, but that's the confusing thing...

    When you run a marathon... CO2 would increase, H+ would increase and O2 would decrease... low 02 and high H+ and CO2 would all stimulate peripheral chemoreceptors but C02 would have a major effect on the cental chemoreceptor
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    (Original post by nikk)
    Question Two - he would surely double it to what it was initially? Since tidal volume isn't taking into account the dead space within the lungs....?
    Alveolar ventilation = Breathing rate x (Tidal Volume - Dead Space)

    So if tidal volume did double, you'd think alveloar ventilation more than doubled... but that was my answer last time and it was wrong...
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    (Original post by Revenged)
    Yeah, but that's the confusing thing...

    When you run a marathon... CO2 would increase, H+ would increase and O2 would decrease... low 02 and high H+ and CO2 would all stimulate peripheral chemoreceptors but C02 would have a major effect on the cental chemoreceptor
    I assume that since we both agree that, you can only choose one of the options? If so then it seems a bit of a nasty question!

    BTW, I was told that peripherial chemoreceptors only respond to O2 if PO2 drops below 60mmHg which is literally just a life-saving reflex and hence doesn't come into play in normal circumstances.
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    (Original post by nikk)
    I assume that since we both agree that, you can only choose one of the options? If so then it seems a bit of a nasty question!

    BTW, I was told that peripherial chemoreceptors only respond to O2 if PO2 drops below 60mmHg which is literally just a life-saving reflex and hence doesn't come into play in normal circumstances.
    Cheers... yeah, i can only chose one of the options... it's bound to be central receptors then... there isn't a massive pH change in exercise and so I doubt that [H+] would really have an impact... so it's only the pCO2 that would really change, which is detected mainly by the central chemoreceptors...
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    (Original post by Revenged)
    Cheers... yeah, i can only chose one of the options... it's bound to be central receptors then... there isn't a massive pH change in exercise and so I doubt that [H+] would really have an impact... so it's only the pCO2 that would really change, which is detected mainly by the central chemoreceptors...
    Yep I agree. I remember it being really stressed to us that PCO2 very strongly activates central chemoreceptors.

    As for the ventilation question, well that is just weird! As you say, from looking at the formula then it should definitely increase! But if that is definitely not correct then I would assume it would have to double, because I can't see how increasing the tidal volume would decrease the alveolar ventilation!
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    (Original post by nikk)
    Yep I agree. I remember it being really stressed to us that PCO2 very strongly activates central chemoreceptors.

    As for the ventilation question, well that is just weird! As you say, from looking at the formula then it should definitely increase! But if that is definitely not correct then I would assume it would have to double, because I can't see how increasing the tidal volume would decrease the alveolar ventilation!
    the third option is 'increase to less than double'... which means that alveolar ventilation would still increse but to less than double the original value - not that alveolar ventilation would decrease.
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    (Original post by Revenged)
    the third option is 'increase to less than double'... which means that alveolar ventilation would still increse but to less than double the original value - not that alveolar ventilation would decrease.
    Sorry that is what I meant. I was about to say that I still can't see how it could increase less than double. But thinking about it, would the amount of dead space increase as the tidal volume was doubled? Because different airways are going to become ventilated and so perhaps increase the dead space....if you see what I mean? Just an idea, not sure if it is along the right tracks.
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    (Original post by nikk)
    Sorry that is what I meant. I was about to say that I still can't see how it could increase less than double. But thinking about it, would the amount of dead space increase as the tidal volume was doubled? Because different airways are going to become ventilated and so perhaps increase the dead space....if you see what I mean? Just an idea, not sure if it is along the right tracks.
    I see what you mean... I still dunno the answer though
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    1) Presumbly it's about the onset of hyperventilation, in which case it's neither, some higher centre results in hyperventilation by some sort of neurogenic mechanism.
    2) More than doubled I think, assuming dead space ventilation is the same, which makes sense as the anatomical dead space volume should remain the same.
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    (Original post by darkenergy)
    2) More than doubled I think, assuming dead space ventilation is the same, which makes sense as the anatomical dead space volume should remain the same.
    And even if the dead space did increase, it would be surely anatomically impossible for it to double, or more than double (which would be required for VA to be either doubled or less than doubled)
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    (Original post by j00ni)
    And even if the dead space did increase, it would be surely anatomically impossible for it to double, or more than double (which would be required for VA to be either doubled or less than doubled)
    Yeah, you are both right... I just got mixed up with the questions...

    Doubling the breathing rate causes the alveolar ventilation to increase less than half because it causes the air that remains in the anatomical dead space to increase...

    Doubling the tidal volume (whilst keeping the breathing rate constant) causes the alveolar ventilation to increase more than half because this has no effect on the anatomical dead space...

    Sorted...

    There is one more question...

    Following hyperventilation a fall in arterial blood P02 of 50mmHg from its normal value will:

    1. Stimulate breathing less than before hyperventilation
    2. Stimulate breathing the same amount as before hyperventilation
    3. Stimulate breathing more than before hyperventilation

    In my exam I put 3, but that is wrong so the answer is 2 - but why isn't more breathing stimulated?
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    (Original post by Revenged)
    Following hyperventilation a fall in arterial blood P02 of 50mmHg from its normal value will:

    1. Stimulate breathing less than before hyperventilation
    2. Stimulate breathing the same amount as before hyperventilation
    3. Stimulate breathing more than before hyperventilation

    In my exam I put 3, but that is wrong so the answer is 2 - but why isn't more breathing stimulated?
    II would say 1....because peripherial chemoreceptors are very insensitve to oxygen levels so this probably won't play much of a part, and because of hyperventilation, PCO2 will be very low....so breathing rate will drop to a lower level.
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    Should be less, as alveolar Pco2 is decreased, resulting in a less powerful drive for ventilation.
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    I think the 'trick' lies in the question: it says 'before hyperventilation', during which breathing is normal, before hyperventilation and hypoxia. Perhaps then compared to normal conditions, the opposing effects of low Pco2, and low O2 cancel out?
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    (Original post by nikk)
    II would say 1....because peripherial chemoreceptors are very insensitve to oxygen levels so this probably won't play much of a part, and because of hyperventilation, PCO2 will be very low....so breathing rate will drop to a lower level.
    Why does hyperventilation cause the PO2 to fall and the PCO2 to fall as well?!
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    (Original post by darkenergy)
    I think the 'trick' lies in the question: it says 'before hyperventilation', during which breathing is normal, before hyperventilation and hypoxia. Perhaps then compared to normal conditions, the opposing effects of low Pco2, and low O2 cancel out?
    It all depends on exactly what level of PO2 is required for the peripherial chemoreceptors to kick in. In my notes it says 60mmHg and since arterial PO2 is 100mmHg then you might expect them to kick in and increas ventilation. Thing is, i'm not sure how accurate those figures are....
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    (Original post by Revenged)
    Why does hyperventilation cause the PO2 to fall and the PCO2 to fall as well?!
    I don't think it does. It is just that you question is saying 'if it did', then what would happen. Hyperventilation is going to increase PO2 surely?
 
 
 
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