With modern medicine, ER within driving distances and good over-the-counter drugs we find Urethral Play is on the upswing of modern-day sexual fetishes.  Part of that practice, at least for men, is entering the delicate prostate region.  For both the male and female, the other part is pushing through the bladder sphincter muscle and entering the bladder. The bladder wall contains nerve cells, vessels, and elastic muscles but over time can be fun to experiment with.  This sub community will deal with this kind of play and other indirect simulation of it.  We will look a natural and artificial bladder expansions using air, fluids, and tiny balls.  The introduction of toys in empty and full bladders.  And the rare fetish of letting long flexible items curl inside there for further fun and play.  This is a risky practice but one I find now becoming more common with the r/Peehole and r/Sounding subs many of us visit.  I started my bladder play before I was old enough to create good ejaculate and now am over 70, if that tells you anything about what I have learned over time.  Enjoy and be safe.