When you hyperventilate...

Discussion in 'Biology & Genetics' started by visceral_instinct, May 16, 2009.

  1. visceral_instinct Monkey see, monkey denigrate Valued Senior Member

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    Why do you get vasoconstriction of the brain?

    I looked this up on wikipedia:

    Why do we have such a mechanism in the first place, though? Why not just measure oxygen?
     
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  3. cosmictraveler Be kind to yourself always. Valued Senior Member

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    because we just were not built that way, There are many things humans have different about them that could be "improved" upon but alas as yet it is still hard to do.
     
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  5. Enmos Valued Senior Member

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    I suspect because it would be used up in order to measure it, or at least it costs energy to free them again

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  7. Idle Mind What the hell, man? Valued Senior Member

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    And also because O[sub]2[/sub] is bound to hemoglobin in the blood stream so it would be very difficult to measure. CO[sub]2[/sub] is a dissolved gas and is directly related to respiration (as a product).

    To answer the first question, I'd imagine that your brain has a sensitive range of O[sub]2[/sub] and CO[sub]2[/sub] concentrations in which it functions at its highest efficiency. Low CO[sub]2[/sub] will mean a higher concentration of O[sub]2[/sub], so it will close off blood vessels to limit blood flow to the brain (richer in O[sub]2[/sub] means less blood required).

    But, I'm just speculating.
     
  8. Roman Banned Banned

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    CO2 is measured indirectly in the blood stream by proteins changing conformation in response to changes in pH. Carbon dioxide dissolved in water forms carbonic acid, so as CO2 concentrations in your blood increase, your blood gets more acidic, which causes proteins to change shape (conformational change).

    This is probably a simpler and more efficient mechanism than having red blood cells releasing signals that their hemoglobin has been bound by oxygen.
     
  9. Enmos Valued Senior Member

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    It could also be that this is just how it evolved. Once the mechanism is in place it's probably unlikely to evolve into a complete opposite.
    If evolution was done all over again it might evolve the other method.
     
  10. Roman Banned Banned

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    That's true, but not a very popular form of evolutionary explanation.
     
  11. Enmos Valued Senior Member

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    I don't care about whether or not it's popular

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  12. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

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    There are two points, one very important, not yet noted:

    (1) Not so important is the fact (I think) that there is less CO2 than O2 dissolved in the blood so a dissolved concentration change of only 0.1% is a big relative change and easier to sense. I think also that a 0.1% increase in the CO2 also make the blood more acidic than even a 1% change in the O2 concentration can. Again easier to sense as many complex proteins fold differently with pH changes.

    (2) The really important reason is that the brain has different regions doing different tasks. Thus, when part of the brain is working on a math problem its needs to have increased O2 (and glucose etc.) supply coming to it than the part that recognizes faces does, etc. That means it needs more blood flow to it. As the heart has only one output of pressure vs. time curve that means somehow that active part of the brain needs to expand the diameter of the local blood vessels. Evolution has an optimum system for doing this. In general it is hard to improve on "mother nature's designs."

    As I worked a lot with JHU doctors, when two US navy surgeons came to APL, they were sent to talk to me. They wanted to know if APL could help develop an artificial blood. (APL has basically lived for ~70 years by solving many problems for the US Navy.) Fortunately, I was too busy already to help as my ideas about it were all exactly wrong. I thought it would be nice if the artificial blood could carry more O2, and had less viscosity. This is exactly what a company, now nearly bankrupted, thought too when they developed their artificial blood with these characteristics. (In the accident trauma trials with large blood loss, it was worse than simple volume replacement with saline solution!) The lower viscosity means it went thru the capillaries too fast to transfer the O2 to the cells. In the larger vessels, most of the molecules did not even contact the walls. The higher O2 per molecule was also wrong, but I forget why.

    Again: It usually dam near impossible to improve on what has evolved. There has been, however, an improvement in stored natural blood. - The part of the hemoglobin molecule that makes for the various bloods of different types can be stripped off so only one modified natural blood serves for all.

    ---------------
    In answer to thread's question: Prolonged hyperventulaltion can so lower the concentration of CO2 that the capillaries in the brain become abnormally small. I think you can lose consciousness as the blood flow in the entire brain is too reduced. Also note that that AFAIK, only the brain's smallest diameter blood vessels change diameter with the CO2 concentration. I.e. they are unique capillaries.
     
    Last edited by a moderator: May 16, 2009
  13. Roman Banned Banned

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    I'm not sure this answers VI's initial question- why detect CO2 instead of O2?
     
  14. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

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    No, but my point (1) gave two reasons.

    later by edit:
    Post 12 repeats and amplfies in part my point about pH change of CO2 being greater than for O2. I mentioned that by changing the shape of proteins this is easily detected as their properties is mainly due to the shape. I.e. at one pH they may bind to a selective site on a nerve and a slightly different pH not be able to do so. O2 has nothing like this effect so it takes a more extreme change to even be detectable.
     
    Last edited by a moderator: May 17, 2009
  15. Asguard Kiss my dark side Valued Senior Member

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    actually we measure both. central cemo receptors in the brain measure o2 but it has to fall quite a lot before it can be detected. i think its around 70 SpO2 but i would need to check that

    pH on the other hand can both pass the blood brain barrer and it can easerly be detected. further more its very important for the oxygen dissasoation curve amongst other things. as the resp system is the system most in charge of regulating pH the resp drive is very sensitive to it. this is why people who have diabetic acidosis hyperventalate as a coping mech to adjust it
     

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