Shallow Water Drowning

[Morlaf]

Shallow Water Drowning is (for the non scuba divers/snorklers among us) when you hyper-hyper ventilate before taking a deep breathe and submerging. What happens then is (this is the boring bit) that there is so little CO2 in your blood that your brain thinks (because it is climbing CO2 levels in your blood that makes you wanna breathe) that there is no need to breath. However, what keep you concious is the O2 in your blood. That drops to a level that causes you to loose conciousness before you are allerted of the need to breathe. Why, oh why (this is the interesting bit) is your need to inhale governed by CO2 levels when what keeps you from falling unconcious is the O2 level? there has to be a reason, that makes this counter-intuitive design have merrit. Please enlighten me!

thanks,
Morlaf

[canalon]

The answer is probably that the evolution of a sensor for CO2 concentration (probably linked to pH sensing too) preexisted that of an O2 sensor. And since in 99.9% of the case, CO2 levels are a good indicator of the need to breathe, there was no selective pressure for another system to appear.

[Morlaf]

Interesting!
thanks for your reply. Talk to me some more about pH sensing please (i know all about the chemistry aspect of pH and what it measures etc - just gimme the biological stuff - please).
but back to the CO2-O2 stuff, surely "Danger! Low O2 - please inhale NOW!" would predate all minor-importance CO2 level danger warning?!?

Morlaf

[Morlaf]

well? any1 bright ideas off ppl please? any1 to back back Canalon up or offer a different explanation? please?

[canalon]

I do not have much info on the subject, but you can start here:
http://en.wikipedia.org/wiki/Carbon_dio ... physiology
Basically CO2 in blood is converted in HCO3- and decreases pH. Changes in pH can quite easily cause changes in protein charges, which probably accounts for an easy way to sense changes of pH compared to the reference value. So it becomes an easy proxy for the detection of the concentration of CO2 in blood. On the other hand I am not aware of such an easy proxy for O2 biological detection. So the CO2 detection system is there, and considering the limited chances of dying of hypoxia with low level of O2, there was no strong pressure that would have helped a hugely complex system to evolve. If it is even at all possible.

[Morlaf]

wow! that's interesting....
like it!
thanks!

[Darby]

Another aspect is that the oxygen in your blood is isolated from monitoring because it's inside red blood cells. The bicarbonate is dissolved and easier to access.

[Morlaf]

interesting....
thanks guys....
now i might go and post on the animal-biology sub-forum the same qusation regarding all those animals that do likewise: Dolhins, whales, etc birds, and many other mammlas like otters, seals and whatnot....
They might have developed something a bit more robust, doubtful though that is....

thanks again!