Anatomy and physiology of the continence apparatus
The female urethra, typically developed by the 12th gestational week from the urogenital sinus, is a 4 cm tubular structure that begins at the bladder neck and terminates at the vaginal vestibule (5).
The striated external urethral sphincter (compressor urethrae) is in the distal two-thirds of the urethra (level 5-6) and is composed of type I (slow-twitch) muscle fibres. This sphincter is horseshoe-shaped and is deficient posteriorly. Distally, the sphincter fans out laterally along the inferior border of the pubic rami and is fixed against the anterior vaginal wall. This arrangement is critical for urinary continence. Near the vestibule (level 3- 4) lies the urethrovaginal sphincter, which contracts with the bulbospongiosus muscle and tightens the urogenital hiatus. Distally, the female urethra is suspended by the suspensory ligament of the clitoris and the pubovesical ligament. It is this hammock (level 1-2), or sling, of fascial attachments that suspends the urethra beneath the pubis (6) (Figure 57.1). The arterial supply to the female urethra comes via the internal pudendal, vaginal, and inferior vesical branches of the vaginal arteries. Venous drainage is via the internal pudendal veins.The female urethra is a multilayered tube lined by transitional cell epithelium proximally and by non-keratinizing stratified squamous epithelium distally. The highly vascular and oestrogen-dependent submucosa contributes a large percentage of the urethral closing pressure; accordingly, hormone withdrawal can lead to stress incontinence. The urinary bladder has an apex at the anterior end and the fundus as its posteroinferior triangular portion. When completely filled, the bladder can have a capacity of up to 500 mL. The bladder trigone is bounded by the two ureteral orifices and the internal urethral orifice. The bladder neck is where the fundus and the inferolateral surfaces come together, leading into the urethra. At the bladder neck, as opposed to the upper bladder, the detrusor muscle layers—transitional epithelium, lamina propria, and muscularis mucosa—are clearly separable. In females, the inner longitudinal fibres of the bladder neck converge radially to pass downward as the inner longitudinal layer of the urethra. The bladder and urethra are supported by the pubovesical ligament (Table 57.1).
Physiology of micturition
See Figure 57.2 (7).