> Don’t you think it requires more energy to keep that extra mass at 98.6F?
Nope, not when the surrounding internal organs are also at 98.6 C. No thermal gradient from the interior of the bladder: no energy transfer from the interior of the bladder. Basic physics.
> And adding insult to injury, more energy is expended just by the brain telling you to go!
Ah ... I'll pass on that one. Unmeasurable (and extremely small). I wonder though how it compares with the energy expenditure from dreaming, or from snoring?