Just thought I'd geek out, the process in which the blood stops going to your extremities eg skin is called Vasoconstriction, it stops blood from going to far out from the core so that heat energy isn't lost by evaporation, the opposite happens when you're hot, VasoDILATION - blood is forced to the extremities in order to cool down, hence you sweat more :D,(reply>), that's actually all over-simplified and absolutely not how it works. i'm assuming you got taught that at gcse?