Narrowed urethra causes “obstructed micturition”: raised resistance, emptying pressure, and low flow. Upper figure: detrusor pressure at 3 different flow rates as a function of the urethral diameter for a tube length of 4 cm: frictional component: dotted line; dynamic component: dashed line; total: solid line. Blue curve: resistance to urine flow of 50 mL/s in the urethra, is exponentially determined, by the diameter of the urethra. Note: even a 0.5 mm change from 3.7 mm to 4.2 mm in urethral diameter reduces the detrusor pressure required to urinate at 50 mL/s from 180 cm H2O to 80 cm H2O. Opening to 6.0 mm (2.3 mm change) reduces detrusor pressure required to 20 cm H2O. Horizontal axis, urethral diameter; vertical axis = detrusor pressure required to drive the urine through the urethra. Lower figure: flow chart in a woman with “obstructed micturition” and an EMG electrode in the posterior fornix of vagina, recording contraction of the posterior opening muscles (LP and LMA). The EMG is activated before urine flow commences. Note continuous activation of the pelvic floor muscles.
