after Werner Schafer
The simultaneous solution of the Bladder Output relation (BOR) and the passive urethral resistance relation (PURR)
The turquoise parabolas are BORRs corresponding to different external power values.
This diagram demonstrates the difference between a rigid urethra and an elastic one. If A1 is the cross sectional area of a rigid urethra it needs no pressure to open it so the lower of the 2 red lines passes through the origin. An elastic urethra requires pressure Puo to open it to the same area. Thus for the same flow rate Q1, a higher pressure is required in the elastic case and therefore a higher power from the muscle.
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