Mechanics of the urethral closure (continence) and opening (mcturition). X-ray 1: resting closed “S” denotes slow twitch muscle contraction of the three directional striated muscle forces, forwards PCM, backwards LP, downwards LMA (arrows) which contract against pubourethral ligament (PUL) anteriorly and uterosacral ligament (USL) posteriorly to close urethra distally and at bladder neck (6) CX, cervix; U, urethra; LP = levator plate muscle; PCM, pubococcygeus; LMA, conjoint longitudinal muscle of the anus; V, vagina; R, rectum; PB, perineal body. Radioactive dye has been inserted into vagina, rectum and levator plate. White broken lines = vertical and horizontal bony co-ordinates. X-ray 2: urethral closure during effort (coughing or straining). Compared to X-ray 1 (same patient), the forward muscle force (arrow) stretches the distal vagina (V) forwards against PUL to close the distal urethra, “urethral closure mechanism”; the backwards and downwards muscle forces (arrows), stretch and angulate the upper vagina and urethra around PUL to “kink” the proximal urethra, “bladder neck closure mechanism.” X-ray 3: micturition X-ray superimposed on resting xray patient sitting. Vascular clips have been applied to the midurethra “1.” Bladder neck “2” and bladder base “3.” Radio-opaque dye has been injected into the levator plate “LP,” which has been angulated downwards during micturition, as has the rectum “R,” which has 10 mL of barium paste. Broken lines indicate position of organs during micturition. Subscript “m” indicates the position of rectum “R” and levator plate “LP” during micturition. Note: backward extension of clips “2” and “3” indicating opening out of bladder neck and proximal urethra by LP/LMA contraction, and downward and forward movement of clip “1” indicating opening out of distal urethra by lateral/downward contracrtion of ischiocavernosus and bulbocavernosus muscles [9]. X-ray 4: the difference from straining (X-ray 2) is that the forward vector (“S” resting closed) relaxes. The backward/downward vectors stretch the vagina backwards and downwards against USL to open out the posterior urethral wall. The enlarged outlet exponentially decreases the resistance to evacuation by detrusor contraction inversely by the 4th power (Poiseuille’s Law). Comparing resting X-ray 1 with micturition X-rays 3 and 4: the distal urethra is pulled backwards from the vertical bony co-ordinate indicating PCM relaxation; angulation of the anterior portion of LP by downward LMA contraction pulls down rectum “R” and bladder base to “funnel” the bladder base and open the posterior wall of urethra.