We developed a theoretical model to understand the extrinsic factors of the bladder-urethral system involved in stress urinary incontinence and their relationships with cystocele or the protective effect of dorsal decubitus and with overfilled-bladder-related dysuria. The model is based on known physical rules (elasticity, pressure-volume relationships in a closed cavity - vagina and its supportive tissue, Retzius space) applied to anatomy. The anterior vaginal wall, divided in two parts, plays a major role in stress urinary incontinence by moderating the rise in the abdominal pressure on the bladder and also by increasing the pressure in the Retzius space that increases the resistance to flow in the urethra. The tent canvas hypothesis applied to the anterior portion of the vaginal wall under the urethra explains the relationship between defective vaginal wall resistance and mobility of the urethra-bladder junction.