Lol, I knew someone would chime in with that. But by design at least, there should be no flow from the outside in. To the extent that we can call evolution's craftsmanship 'design'.

The only thing I know about that is the scene from House where he catheters himself. To the extent we can trust the show's medical accuracy, his reaction during the insertion, and the number of pain pills he took beforehand, let's just say it seems highly unpleasant!