Female Urethra Anatomy
The female urethra is a fibromuscular tube 3–4 cm long and begins at the internal urethral meatus of the bladder. Embedded in the anterior wall of the vagina, it inclines downwards and forwards through the pelvic floor (Fig. 5.18) and terminates in the vestibule at the external meatus between the clitoris and the vaginal opening.
The urethra passes close to the posterior aspect of the pubic symphysis (Fig. 5.17), to which it is attached by the pubourethral ligaments. The middle third of the urethra is encircled by striated muscle fibres of the external urethral sphincter, whose tone is the principal factor in maintaining continence of urine. Occlusive force on the urethra is also provided by contractions of the levator ani muscles (Fig. 5.18). Micturition occurs when bladder pressure is higher than urethral pressure and is produced by contraction of the detrusor muscle of the bladder wall accompanied by relaxation of the external urethral sphincter. Stress incontinence (leakage of urine during everyday activities that raise intra-abdominal pressure) may occur following vaginal delivery due to weakening of pelvic floor muscles.
The arterial supply to the urethra is provided by the inferior vesical arteries, and venous drainage is to the vesical plexus of veins. The mucosa receives its sensory nerve supply from the pudendal nerve, derived from the second, third and fourth sacral segments, which also innervate the external urethral sphincter.