Our intestines go side-to-side, not straight upwards, so the arm follows that pattern. Therefore, when doing full-arm fisting you're actually bending your arm sideways.

I don't remember feeling any other organs, I wasn't feeling around for that, haha. I don't think I would have gone near the heart, that would have been crazy. I also had a triple-layer of gloves (nitrile, thick plastic horse insemination gloves, nitrile) so feeling around for those details wouldn't be so easy. I was also so lost in the rush of "OH MY GOD!! MY FULL ARM IS IN A DUDE!!" that I wasn't focusing on feeling around for organs.