Good points are made here but they are overlooking the massive white elephant in the room. By the tone of your writing, it gives the appearance that you have never squirted. As a squirter I can assure you that whatever does come out of my body, the only resemblance it has to urine is that it is a liquid.
It is clear, odourless and with a faintly sweet taste. Entirely lacking of the normal acrid aroma of urine. Moreover, when I have covered liberally my partner in the ‘squirt’, it leaves a faint sheen on the skin. It also attracts cats.
The sensation to squirting is identical to peeing. So I can happily conclude that the majority of the liquid expelled comes from the bladder. Not all though.
I would fully expect liquid that comes for the bladder to have a similar composition to squirt. The other problem I have is the rate of refill of the bladder. So let’s get detailed here. As an experiment yesterday, after having an hour long session with my partner, I paused and went downstairs and emptied my bladder. The contents were distinctly yellow. I then went back upstairs and I was squirting — a lot — within 5 minutes. The colour and aroma of the squirt was not urine.
So the rate of refill appears to quicker during sexual arousal than under normal conditions. Interestingly, this very point was picked up on by the author of the 2014 study who decided not to pursue it further.
I would also argue that the seven studied cannot be statistically significant and without access to the full journal, I don’t know how he conducted the study and if it was inherently flawed (I am a physicist by trade). What I do know is that the matter is not clear. If you will pardon the pun.