as you climb higher in the atmosphere, there are three things that happen to your body, one is the circulatory affect, when there is hardly any oxygen in the blood, your body will start to react to this change in supply and either shut down, ((your lungs will give out, and you will pass out because of the lack of O2 in the brain)) or you will become Carbon-drunk. ((your cells will began to literally explode or shrink because of the fact there is no way to filter out your cells of the used up oxygen))
the second thing is with respiratory, your breathing will slow down and it will become hard to breath. its true that oxygen is lighter than carbon-dioxide, but oxygen is heavier then nitrogen and helium. Your body will start to feel heavier than normal and you will become weak. you will start to feel light-headed and eventually pass out if this feeling is new to you.
Third is the carbon affect, your body will end up dying because of it "reusing" carbon cells, and you will end up going into a coma because of the brain damage from no oxygen and too much carbon
