Urinary Bladder and Urethra

Parts:

• Apex: Points anteriorly toward the pubic symphysis, continuous with the median umbilical ligament (remnant of the fetal urachus), ascending toward the umbilicus along the internal surface of the anterior abdominal wall.
• Body: The main storage portion, between the apex and fundus.
• Fundus (base): The posteroinferior wall, relatively fixed in position (in contrast to the more mobile body/apex, which can distend upward and anteriorly as the bladder fills); receives the ureters.
• Neck: The most inferior, fixed part, continuous with the urethra, surrounded by the internal urethral sphincter and, in males, closely related to the prostate base.

Trigone:

• A smooth-walled triangular area on the internal posteroinferior surface (fundus), bounded by the two ureteric orifices (posterolaterally) and the internal urethral orifice (anteroinferiorly), with the interureteric ridge/bar connecting the two ureteric orifices as its posterior boundary.
• Embryologically derived from mesoderm (absorbed mesonephric duct tissue), distinct from the rest of the bladder (which is derived from endoderm), this embryological distinction explains the trigone's smooth, non-distensible mucosal surface (lacking the rugae/folds seen elsewhere in the bladder wall) and its relative resistance to the field-change carcinogenic effects that can affect the rest of the urothelium diffusely.
• The oblique course of the ureters through the bladder wall (entering obliquely, rather than perpendicularly) creates a natural flap-valve mechanism at the ureteric orifices, helping prevent vesicoureteric reflux as bladder pressure rises during filling/voiding.

Relations:

• Anterior: Pubic symphysis, retropubic (Retzius) space, anterior abdominal wall (when distended, rising above the pubic symphysis into the abdomen, where it becomes palpable/percussible and accessible for suprapubic catheterisation).
• Posterior: In males, rectum (via the rectovesical pouch superiorly, and directly via Denonvilliers' fascia inferiorly), seminal vesicles, ductus deferens (ampullae), and the prostate (at the neck). In females, uterus (via the vesicouterine pouch), and the anterior vaginal wall (directly, below the peritoneal reflection).
• Lateral: Obturator internus and levator ani (via the pelvic fascia), and (in males) the pelvic plexus/prostatic venous plexus.
• Superior: Peritoneum (small intestine loops, sigmoid colon when the bladder is empty).
• Inferior: Prostate (males) or pelvic floor/urogenital diaphragm structures directly related to the urethra (females).

Peritoneal relations:

• Only the superior surface of the bladder is covered by peritoneum; the rest of the bladder is extraperitoneal, surrounded by loose connective tissue and the vesical venous plexus.
• As the bladder fills and rises, it displaces the peritoneal reflection superiorly, progressively exposing more of the anterior bladder wall directly against the anterior abdominal wall without intervening peritoneum, the anatomical basis for safe suprapubic catheter insertion in a distended bladder (avoiding peritoneal cavity entry).

Ligaments:

• Puboprostatic ligaments (males) / pubovesical ligaments (females), condensations of pelvic fascia connecting the bladder neck/prostate to the pubic bones, providing anterior support.
• Lateral (true) ligaments of the bladder: fascial condensations connecting the bladder to the pelvic side walls.
• Median umbilical ligament: the obliterated urachus, from the bladder apex to the umbilicus.

Blood supply:

• Superior vesical artery (from the patent proximal umbilical artery), supplies the superior bladder.
• Inferior vesical artery (males), supplies the bladder base/neck, and also contributes to prostate and seminal vesicle supply.
• Vaginal artery (females), contributes branches to the bladder base, given the intimate anatomical relationship with the anterior vaginal wall, functionally substituting for a discrete "inferior vesical artery" in females.

Venous drainage:

• Vesical venous plexus, draining to the internal iliac vein (in males, continuous with/part of the prostatic plexus).

Lymphatics:

• Predominantly to the external iliac nodes (most of the bladder), with some drainage to the internal iliac nodes (bladder neck/fundus), reflecting the mixed drainage pattern seen at the boundary between more "somatic-adjacent" anterior structures and deeper pelvic viscera.

Innervation:

• Parasympathetic (pelvic splanchnic nerves, S2-S4), contracts the detrusor muscle, relaxes the internal urethral sphincter, promoting micturition.
• Sympathetic (hypogastric nerves, via the inferior hypogastric plexus), relaxes the detrusor, contracts the internal urethral sphincter (particularly relevant in males, where sympathetic-mediated bladder neck closure prevents retrograde ejaculation during emission), promoting storage/continence.
• Somatic (pudendal nerve), voluntary control of the external urethral sphincter.
• Sensory afferents (bladder distension/fullness) travel predominantly with the parasympathetic pelvic splanchnic nerves (below the pelvic pain line).

Male Urethra (approximately 18-20 cm, divided into four parts)

Preprostatic part:

• Very short, from the internal urethral orifice to the prostate base, surrounded by the internal urethral sphincter (a true, involuntary smooth muscle sphincter in males, critically important for preventing retrograde ejaculation, by closing the bladder neck during emission).

Prostatic part:

• Passes through the prostate gland, the widest and most dilatable part of the male urethra; receives the openings of the ejaculatory ducts (paired, on the seminal colliculus/verumontanum) and numerous prostatic ductules along the prostatic sinuses flanking the seminal colliculus.

Membranous part:

• The shortest, narrowest, and least distensible part, passing through the deep perineal pouch, surrounded by the external urethral sphincter (voluntary, skeletal muscle), also the most fixed part of the urethra, making it particularly vulnerable to shearing injury (see Clinical Anatomy).

Spongy (penile) part:

• The longest part, running through the corpus spongiosum of the penis, from the membranous urethra to the external urethral meatus at the glans tip; contains a slight dilatation near its origin (the bulbar urethra, within the bulb of the penis) and another near its termination (the fossa navicularis, within the glans).

Female Urethra

Course:

• Short (approximately 4 cm), running from the internal urethral orifice (bladder neck) through the pelvic floor and perineal membrane, embedded within the anterior vaginal wall for much of its course, to the external urethral orifice in the vestibule (anterior to the vaginal opening, posterior to the clitoris).

Relations:

• Closely applied to and partially embedded within the anterior vaginal wall throughout its course, a key anatomical relationship explaining the shared involvement of both structures in pelvic floor trauma/prolapse and the surgical approach to many urethral/anterior vaginal wall procedures.
• Passes through the deep and superficial perineal pouches, surrounded by the external urethral sphincter (and the compressor urethrae/urethrovaginal sphincter components, additional skeletal muscle contributing to continence, sometimes grouped together as the "sphincter urethrae" complex).

Opening:

• The external urethral orifice opens into the vestibule, a short distance anterior to the vaginal opening.

Internal urethral sphincter:

• Male: A true, distinct, involuntary smooth muscle sphincter at the bladder neck/preprostatic urethra, under sympathetic control, essential for preventing retrograde ejaculation.
• Female: Less anatomically distinct as a discrete sphincter (a matter of some ongoing anatomical debate/terminology variation), functionally, continence at the bladder neck relies more heavily on the surrounding detrusor muscle arrangement and the external sphincter mechanism than on a well-defined separate internal sphincter comparable to the male structure.

External urethral sphincter:

• Voluntary skeletal muscle, part of the deep perineal pouch's musculature, surrounding the membranous urethra (males) or the urethra as it passes through the perineal membrane (females); innervated by the pudendal nerve (somatic, voluntary control).

• Urethral catheterisation in males must navigate the urethra's natural curves (the S-shaped course: initially downward/posterior through the prostatic and membranous urethra, then anterior/upward through the bulbar urethra before straightening within the penile shaft), awareness of this course, and gentle steady pressure rather than force, avoids creating a false passage, particularly at the relatively fixed and narrow membranous urethra.
• Suprapubic catheterisation relies on the anatomical fact that a distended bladder displaces the peritoneal reflection superiorly, exposing a direct extraperitoneal route from the lower anterior abdominal wall (just above the pubic symphysis) into the bladder, safe only when the bladder is adequately distended (confirmed by palpation, percussion, or ultrasound) to ensure the peritoneal cavity is not inadvertently entered.
• Female urethral catheterisation is generally more straightforward given the urethra's short length and relatively straight course, though correct identification of the external urethral meatus (located between the clitoris and vaginal opening, and sometimes difficult to visualise clearly) is essential.