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    I think the question is asking what happens if Po2 did fall
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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 makes more sense now...

    Before hyperventilation the stimulation for breathing would be very high - possibly because of high PCO2 levels.

    Whatever it was, it causes a massive stimulation of breathing because hyperventilation occurs straight afterwards.

    Hyperventilation would cause the PCO2 levels to decrease... i wiki-ed it

    "Deeper or quicker breaths exchange more of the alveolar gas with air and have the net effect of drawing more carbon dioxide out of the body, since the carbon dioxide concentration in normal air is very low."

    The lower PC02 causes reduced stimulation of breathing and so "stimulation of breathing would be less than before hyperventilation"

    "If carbon dioxide levels are high, the body assumes that oxygen levels are low, and accordingly the brain's blood vessels dilate, to assure sufficient blood flow and supply of oxygen"

    I think that the converse of this is true. If the body has low CO2 levels then the body assumes that the oxygen levels are normal. This means that in this case, when the PCO2 has been decreased by hyperventilation this would decrese the stimulation of breathing. The PO2 falling would stimulate breathing, but this is just a trick, as overall the stimulation is less than before.

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