User blog comment:Pacifista15/Changing Gears/@comment-4498721-20120102100718

Your logic really makes sense. I'm not really good with biology but let me try this.

Pumping more oxygen and accelerating cellular respiration should produce more energy. But for that kinda speed, his heart should be pumping a lot more faster. I think pumping that fast requires faster breathing to keep up, or it will be like pumping 10 times faster but 1/10 oxygen than normal. So I'm thinking Luffy should be breathing a lot faster.(maybe that was what Oda intended, but he seems to be breathing at normal speed in anime)

Cellular respiration is exothermic(outside heating) right? Which means reaction is releasing heat, and causing the steam. I was gonna say steam could be the water produced by reaction but if I'm correct cellular respiration uses water, the one producing water was photosynthesis, so forget that :D. Steam is probably the sweat evaporating(like you said).

Applying Gear Second to just one part is not seems biologically possible to me. Even though he pumps his blood from his legs or arm, blood stream goes through whole body. So when he uses Gear Second on one arm(as in Sabaody), the effect of G2nd should be applied at least from his arm to his heart through blood stream. Also to maintain the effect, the pumping have to be continous, and since he isn't pumping blood manually all the time in G2nd, it should be his heart doing the rest. And if the heart is doing the pumping effect should apply to whole body. Luffy's pumping that activates the G2nd is noting more than a "turbo button" for his heart.

It could be possible to improve G2nd with physical conditioning(as you said) or some other environmental effect, like people living at higher altitude having more red blood cells therefore holding more oxygen in their blood.

I hope that made sense, I'm not sure my English is good enough for that kind of scientific and complicated topics :D. Even though it made sense, that's just a fictional power we're talking about. So not think on it too much :D