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First of all, I am new here, and I apologize if this theory on the “Piss Shiver” has already been given.
Secondly, I’d like to say that I am not a doctor, and have had no medical training beyond owning an “Operation” game when I was 10 years old.
Now, it seems to me the act of holding one’s bladder closed over time requires the constant clenching of certain muscles (stop giggling, I’m trying to be serious here!) Said clenching would inevitably raise your blood pressure. Releasing those muscles during urination would result in a significant drop in blood pressure, and the “shiver” would be a by-product of this. I’ve occasionally had shivers so strong that my heart would palpitate.
One more point, doctors (real ones,) often prescibe diuretics to patients with HBP. Coincidence?