Chapter 73: Bladder, Prostate & Urethra

The bladder functions as a muscular reservoir, dramatically altering its size and position based on content, lying within the lesser pelvis when empty and expanding into the abdominal cavity when distended. Its fixed neck rests against the prostate in males, while the apex is connected superiorly to the umbilicus by the median umbilical ligament, the fibrous remnant of the urachus. Peritoneal reflections create important clinical spaces, such as the rectovesical pouch and the retropubic space (of Retzius), dictating that extraperitoneal injuries may be managed conservatively, whereas intraperitoneal injuries usually necessitate surgical repair. Internally, the mucosa is generally loosely attached, except over the trigone, a smooth triangular area defined by the internal urethral orifice and the two ureteric openings, which are connected by the interureteric crest. The bladder wall, or detrusor muscle (muscularis propria), consists of three indistinct smooth muscle layers. Urinary continence and voiding are managed by a complex interplay of the central nervous system (cortical, thalamic, and pontine centers, including the 'M' micturition center and 'L' storage center) and autonomic innervation. The sympathetic system (T12-L2) maintains continence and closes the smooth muscle internal urethral sphincter (especially important in males to prevent retrograde ejaculation), while the parasympathetic system (S2-S4) via the pelvic splanchnic nerves triggers detrusor contraction. The internal lining is urothelium, characterized by specialized umbrella cells that maintain a critical permeability barrier, supported by the lamina propria, which contains connective tissue and neurovascular components. Chronic urinary outflow obstruction leads to detrusor hypertrophy, resulting in a thick-walled 'trabeculated bladder'. The male urethra is approximately 18 to 20 cm long and is structurally divided into the posterior urethra (intramural, prostatic, membranous) and the anterior spongy urethra. The 3-4 cm long prostatic urethra contains the midline urethral crest and the seminal colliculus, where the ejaculatory ducts and the prostatic utricle open. The shortest, narrowest, and least dilatable segment is the membranous urethra, which is surrounded by the striated external urethral sphincter, the main somatic muscle structure for voluntary continence. The prostate gland is a fibromuscular structure surrounding the prostatic urethra, encased by layers of pelvic fascia, including the prostato-seminal fascia (Denonvilliers’ fascia). Pathologically, the prostate is defined by its zonal anatomy: the peripheral zone (70% of volume, site of most carcinomas) and the central and transition zones. Benign prostatic hyperplasia (BPH) invariably develops in the transition zone with age, causing compression of the urethra. The female urethra is significantly shorter, about 4 cm, embedded in the anterior vaginal wall, and its smooth muscle receives extensive presumptive cholinergic parasympathetic innervation. Continence in females relies heavily on the smooth and striated urethral muscle and support from the pubovesical ligaments and the pelvic diaphragm.