Temperature extremes and shock

SomeDose

Right, it's tutorial time. The answer to this is probably out there in google-land but there's no fun in that...

Just a query for our resident physiology and emergency med boffins. Many people will be familiar with the experience of hopping into a really hot bath or sauna, only to make a sharp exit shortly afterwards due to getting light headed with wooziness, and your pulse shoots up. It happened me only last week. Now, I think I can make easy sense of this in basic terms - body gets too hot, attempts to lose heat rapidly from the body surface i.e. peripheral vasodilation, which dumps your BP (hence the dizziness), and then some tachycardia to maintain the cardiac output. No big deal. But then I remembered the one (and only) time I jumped into an ice plunge pool in a gym- no joke, it nearly knocked me out. Struggled to breath, went completely pale and barely avoided passing out. So what's the pathophysiology here in response to acute extreme cold? Logic tells me there should be peripheral vasoconstriction to protect the core organs, but why I am experiencing a near-blackout as if my BP has just tanked?

I'm a sucker for detail so feel free to give it in geeky terms of adrenoceptors, baroreceptors, frank-starling law, vasoactive mediators etc etc.

Traumadoc

Diving reflex?

http://en.wikipedia.org/wiki/Mammalian_diving_reflex