My feeling is that it's possible at least in some cases that pressure is not even required. Let's say certain tissues are already generally inflamed from some other problem. My case is good example, but it could reflect many others. That is, that because of the circumstance of vasectomy and then reversal, anti-bodies present in the system cause damage. This was even told to me by the urologist who did the reversal. The sperm anti-bodies as a result of the vasectomy would attack the sperm in the vas and kill them "and could even damage the microsurgery that reopened the vas".
So if that area is inflamed already, it wouldn't even require pressure to find a way into the bloodstream.
There are many other possibilities that could present the same conditions. So damaged tissues, from whatever, could present conditions of inflammation which could allow for transference of material across the blood barrier, even without pressure.
I'm pretty certain though that it's not just the urethra that is the sensitive part, partly because of the wide variety of symptomatic profiles. It could be in the epididymis, the vas the prostate etc etc. So perhaps the location of the "breach" effects which actions start the process.
