@boiga:
Thanks tamzilla, Silverblade, Piratemarimo, and Bartart for the kudos. I don't post often any more, but it's nice to be noticed when I do.
I don't think that life return is really necessary to explain what is going on here. That's a very specialized technique and I would be remiss to claim that luffy picked it up on his own.
The gomu gomu fruit allows Luffy to manipulate the elastic modulus (stretchiness) of the material of any part of his body at will. He relaxes his elasticity to shoots a limb out and then tightens it to have it snap back into place greater force than required to initially stretch it out. The fact that he is able to manipulate his stomach in this manner suggests that it is not limited areas of his body he can consciously control though skeletal muscles. Gear 2 has always relied on his ability to manipulate his blood vessels, otherwise they all would have hemorrhaged the first time he tried pumping the system with his legs.
Logically the amount of control it would require to relax and tighten his heart should not be greater than that for Buggy's ability to zip cubic inches of his abdomen around in the air or for the EL marine captain to turn specific parts of his torso into balls.
Ahh sweet, someone who knows what they're talking about. One thing to remember is that Luffy, being a rubber man, basically has a circulatory system made up of rubber hoses instead of standard arteries and veins. By pumping his heart he creates a pulse of high pressure blood that is going to move down the path of least resistance. Luffy should be able to manipulate his major blood vessels enough to direct the high pressure pulse where he wants it to go. The high pressure circulatory zone does get redirected back to the heart before it heads out again to the lungs, so that area of high pressure is still a closed circuit, starting and ending in the heart (although admittedly different ventricles therein)
However, you are absolutely right that the limitation on Gear 2 is that it becomes harder and harder to oxygenate the blood in the first place. You can see luffy panting heavily near the end of every protracted Gear 2 fight. This is also why he only attempts Gear 2 for short bursts. It's a highly stamina draining technique, an aspect which will hopefully be ameliorated by his new ability to localize the area affected.
And yes, luffy's devil fruit means that he is not reliant on hormone regulated flight or fight responses to make blood flow to the parts of the body that are more useful in a fight. That's one of the reasons he kicks ass.
And sorry, this stuff is fun for me…
I wish I could post like you. I think I should but I'm missing something…
Anyway, having the heart being the active pump for Gear 2nd is a good idea. What you've said almost completely makes sense, in context.
Also, there was a post that argues that pressure doesn't rise from blood vessel dilation. That is correct, but the blood volume will increase in the areas where blood vessels are dilated, and that is the key purpose of pumping blood.
And Luffy obviously is still using his ANS (at times his parasympathetic NS gets the better of him). It's just that his DF allows his somatic NS more control over his individual body parts.
However, I think when Luffy gets really serious, he'll still have a whole-body Gear 2nd, because a strong punch is an action that involves the hands, arms, shoulder, back, waist, butt, legs and feet.
But, theoretically, shouldn't Luffy be able to make any muscle in his body a pump? The spatial logic that applies to Buggy should apply to Luffy, so any muscle can perform gear 2nd, provided that it receives enough motor nerve impulses.