# Lecture 5: Urinary Bladder and Urethra ## Unit 1.5: Human Gross Anatomy III - Pelvis and Head/Neck --- ## Learning Objectives By the end of this lecture, students will be able to: 1. Describe the anatomy of the urinary bladder including its surfaces, ligaments, and internal features 2. Compare the male and female urethra 3. Describe the mechanisms of urinary continence 4. Explain the blood supply, innervation, and lymphatic drainage of the bladder 5. Describe the micturition reflex and its neural control 6. Correlate anatomical features with clinical conditions --- ## Overview of the Lower Urinary Tract The lower urinary tract comprises the urinary bladder, the urethra, and their associated sphincter mechanisms, functioning together to store urine produced by the kidneys and eliminate it at appropriate times through coordinated muscular action and neural control. The bladder serves as a distensible reservoir, while the urethra provides the conduit for elimination under the control of both involuntary and voluntary sphincters. Significant anatomical differences exist between males and females in this region. In males, the bladder lies between the pubic symphysis anteriorly and the rectum posteriorly, with the prostate gland positioned at the bladder neck. The male urethra, approximately 20 centimeters long, traverses the prostate, passes through the external sphincter, and runs through the penis to the external meatus. In females, the bladder lies between the pubis and the vagina/uterus, and the considerably shorter female urethra (approximately 4 centimeters) descends through the pelvic floor to open into the vestibule anterior to the vaginal orifice. These differences have important clinical implications for urinary tract infections, incontinence patterns, and catheterization techniques. Panel A: Male pelvis sagittal section showing bladder behind pubic symphysis with prostate surrounding proximal urethra. Panel B: Male urethra passing through prostate, external sphincter, and penis to external meatus with 20 cm total length. Panel C: Female pelvis showing bladder anterior to vagina and uterus with short 4 cm urethra. Panel D: Female urethra descending through pelvic floor to vestibule anterior to vaginal orifice with anatomical landmarks. --- ## The Urinary Bladder: Structure and Position The urinary bladder is a hollow, highly distensible muscular organ that serves as the reservoir for urine between voidings. Its capacity varies, with a comfortable storage volume of 400-500 milliliters, though it may accommodate over 800 milliliters when overfilled. The sensation of fullness typically begins at 200-300 milliliters. When empty, the bladder lies entirely within the lesser pelvis, posterior to the pubic symphysis. As it fills with urine, the bladder ascends into the greater pelvis, eventually becoming an abdominal organ when significantly distended—a feature exploitable for suprapubic catheterization. The shape transforms from roughly tetrahedral (with four surfaces and four angles) when empty to ovoid when distended. The bladder possesses four anatomical surfaces with distinct relationships. The superior surface is covered by peritoneum and relates to the sigmoid colon and small intestinal loops; in females, the body of the uterus rests upon it. The base (posterior surface) is triangular and relatively fixed, not participating in bladder expansion; in males, it relates to the seminal vesicles, ampullae of the vasa deferentia, and rectum, while in females, it relates to the vagina and cervix. The two inferolateral surfaces contact the levator ani and obturator internus muscles, separated from the pubic bones by the retropubic space (space of Retzius), which contains fat and the vesical venous plexus. Four angles define the bladder's geometry: the apex lies anteriorly, where the median umbilical ligament (the obliterated urachus) attaches and extends toward the umbilicus; the neck lies inferiorly at the junction with the urethra; and the two lateral angles mark the points where the ureters enter. Panel A: Three-dimensional empty bladder with tetrahedral shape showing superior surface covered by translucent peritoneum with intestinal coils above. Panel B: Triangular posterior surface (base) as fixed region with inferolateral surfaces facing pelvic walls. Panel C: Four angles marked - apex with median umbilical ligament, neck at urethra, and lateral angles at ureteric entry points. Panel D: Shaded retropubic space with fat content and inset comparing empty tetrahedral versus distended ovoid shapes. --- ## Internal Features of the Bladder The interior of the bladder displays distinct anatomical features that reflect both its function and embryological development. The trigone is a smooth, triangular area on the internal surface of the base, defined by the two ureteric orifices at the superior angles and the internal urethral orifice at the inferior angle. Unlike the rest of the bladder mucosa, the trigone remains smooth even when the bladder is empty because the mucosa is firmly adherent to the underlying muscle. This developmental difference arises because the trigone derives from the absorbed mesonephric ducts, while the remainder of the bladder develops from the urogenital sinus. The interureteric ridge (interureteric crest) forms a raised fold between the ureteric orifices, created by an underlying bundle of muscle fibers that connects the two ureters. This ridge serves as a useful cystoscopic landmark. The uvula is a slight midline elevation behind the internal urethral orifice, more prominent in males where it overlies the median lobe of the prostate. The ureteric orifices themselves appear as slit-like openings that admit urine from the ureters. The oblique passage of each ureter through the bladder wall for 1-2 centimeters creates an antireflux mechanism: as bladder pressure rises during filling, the intramural ureter is compressed, preventing retrograde flow of urine toward the kidney. Failure of this mechanism causes vesicoureteral reflux. The internal urethral orifice marks the lowest point of the bladder cavity, positioned at the bladder neck. In males, smooth muscle fibers encircle this opening to form the internal urethral sphincter, which prevents retrograde ejaculation and contributes to continence. In females, these fibers loop around the urethra without forming a distinct sphincter. The body mucosa displays rugae (folds) when the bladder is empty, which flatten as the bladder distends. The entire bladder interior is lined by transitional epithelium (urothelium), a specialized stratified epithelium capable of stretching while maintaining an impermeable barrier. Panel A: Endoscopic cystoscopy view showing smooth triangular trigone occupying lower field with slit-like ureteric orifices and urine jets. Panel B: Internal urethral orifice at center bottom with interureteric ridge as horizontal fold connecting ureteric orifices. Panel C: Uvula as slight elevation above internal urethral orifice with surrounding bladder body showing mucosal rugae. Panel D: Anatomical diagram overlay identifying key cystoscopic landmarks for orientation. --- ## Bladder Wall Structure The bladder wall consists of four layers arranged from outside to inside. The outermost layer is adventitia over most of the bladder, with serosa (peritoneum) covering only the superior surface. The muscular layer, called the detrusor muscle, comprises the bulk of the wall as three interlacing layers of smooth muscle—inner longitudinal, middle circular, and outer longitudinal—though these layers are not distinctly separable and form a complex meshwork. The submucosa consists of connective tissue, and the innermost mucosa is lined by transitional epithelium. The detrusor muscle's unique arrangement allows the bladder to contract uniformly during voiding, compressing the contents from all directions. At the bladder neck, the smooth muscle behaves differently in males and females: in males, the circular fibers thicken to form the internal urethral sphincter, which closes the bladder neck during ejaculation to prevent retrograde flow of semen into the bladder. In females, the muscle loops around the urethra without forming a distinct sphincter, contributing to the somewhat lower resting urethral pressure in women compared to men. Panel A: Cross-section of bladder wall showing transitional epithelium with umbrella cells lining lumen and loose connective tissue submucosa. Panel B: Thick detrusor muscle with interlacing smooth muscle bundles in multiple orientations corresponding to three layers. Panel C: Outer surface showing adventitia over most regions and peritoneum over superior surface. Panel D: Inset comparing male bladder neck with thickened internal sphincter versus female with looping muscle fibers without distinct sphincter. --- ## Bladder Supports and Ligaments The bladder is maintained in position by a combination of true ligaments (condensations of pelvic fascia) and false ligaments (peritoneal folds). The true ligaments provide structural support: the pubovesical ligaments in females (or puboprostatic ligaments in males) extend from the bladder neck (or prostate) to the posterior surface of the pubic bones, the lateral ligaments connect the bladder to the lateral pelvic walls, and the posterior ligaments relate the bladder base to the rectum. The false ligaments are peritoneal reflections that contain obliterated embryonic structures. The median umbilical ligament extends from the bladder apex to the umbilicus, containing the urachus—the fibrous remnant of the allantois that connected the fetal bladder to the umbilicus. Persistence of a patent urachus allows urine to leak from the umbilicus. The paired medial umbilical ligaments flank the median ligament, containing the obliterated umbilical arteries. The pelvic floor, particularly the levator ani, provides critical muscular support for the bladder. Weakness of these structures contributes to cystocele (prolapse of the bladder into the anterior vaginal wall) in women. Panel A: Superior-anterior view of bladder with median umbilical ligament extending from apex to umbilicus containing obliterated urachus. Panel B: Paired medial umbilical ligaments flanking midline containing obliterated umbilical arteries. Panel C: Pubovesical ligaments extending from bladder neck to posterior pubis with lateral ligaments to pelvic sidewalls. Panel D: Levator ani forming muscular floor support with inset showing patent urachus draining urine from umbilicus. --- ## Bladder Blood Supply and Lymphatic Drainage The bladder receives arterial blood from several sources, all deriving from the internal iliac artery system. The superior vesical arteries arise from the patent proximal portion of the umbilical artery and supply the upper bladder and dome. In males, the inferior vesical artery (a separate branch of the internal iliac) supplies the bladder base, prostate, and lower ureter. In females, the vaginal artery replaces the inferior vesical artery, providing branches to the bladder base. Additional smaller contributions may come from the middle rectal and obturator arteries. Venous drainage occurs through the vesical venous plexus, an extensive network surrounding the bladder base and, in males, the prostate. This plexus drains into the internal iliac veins. The vesical plexus communicates freely with the prostatic plexus (receiving the deep dorsal vein of the penis) and, importantly, with Batson's vertebral venous plexus. This valveless communication with the vertebral system provides a direct route for metastatic spread of bladder cancer to the spine without passing through the lungs. Lymphatic drainage follows regional patterns: the superior and body portions drain primarily to external iliac nodes, the base drains to both external and internal iliac nodes, and the neck drains to internal iliac and sacral nodes. Understanding these drainage patterns is essential for staging bladder cancer and planning lymphadenectomy. Panel A: Superior vesical arteries arising from proximal patent umbilical artery supplying upper bladder. Panel B: Inferior vesical artery (male) or vaginal artery (female) supplying bladder base with arterial distribution. Panel C: Vesical venous plexus surrounding bladder and prostate draining to internal iliac veins with Batson's plexus connections. Panel D: Green lymphatic vessels showing drainage from superior bladder to external iliac, base to external and internal iliac, and neck to sacral nodes. --- ## Bladder Innervation The bladder receives both autonomic and somatic innervation, which together control the opposing functions of storage and voiding. Parasympathetic innervation derives from the pelvic splanchnic nerves (S2-S4), which provide motor fibers to the detrusor muscle causing contraction and simultaneously inhibit the internal sphincter. Parasympathetic activity therefore promotes micturition—the act of voiding urine. Sympathetic innervation reaches the bladder via the hypogastric nerves (originating from T11-L2). Sympathetic activity relaxes the detrusor muscle while contracting the internal sphincter (in males), thereby promoting urine storage. This is particularly important during ejaculation, when sympathetic discharge closes the internal sphincter to prevent retrograde ejaculation. Somatic innervation arrives via the pudendal nerve (S2-S4), which provides motor control to the external urethral sphincter—a skeletal muscle under voluntary control. Tonic activity of the external sphincter maintains continence, while voluntary relaxation permits voiding. Visceral afferent fibers travel with both autonomic pathways. Sensations of bladder fullness and the urge to void travel with parasympathetic fibers to S2-S4. Pain sensations from overdistension travel with sympathetic fibers to T11-L2, explaining the referred pain pattern to the suprapubic and perineal regions. Panel A: Green parasympathetic pathway from pelvic splanchnic nerves (S2-S4) activating detrusor contraction and inhibiting internal sphincter for voiding. Panel B: Red sympathetic pathway from hypogastric nerves (T11-L2) relaxing detrusor and contracting internal sphincter for storage. Panel C: Yellow somatic pathway from pudendal nerve (S2-S4) providing voluntary external sphincter control. Panel D: Afferent pathways traveling with both autonomic systems and summary table of effects on detrusor and sphincters. --- ## The Female Urethra The female urethra is a relatively short tube, approximately 4 centimeters in length and 6 millimeters in diameter, extending from the internal urethral orifice at the bladder neck to the external urethral orifice in the vestibule, where it opens approximately 2-3 centimeters posterior to the clitoris and anterior to the vaginal opening. The urethra is intimately embedded in the anterior vaginal wall throughout its course, which has clinical implications: anterior vaginal wall prolapse (cystocele) typically involves the urethra as well, and urethral pathology may be palpable on vaginal examination. The urethra passes through the pelvic floor at the urogenital hiatus, where it is surrounded by the external urethral sphincter. Regarding sphincter mechanisms, the internal sphincter in females is relatively poorly developed—the smooth muscle at the bladder neck loops around the proximal urethra without forming a distinct circular sphincter as in males. The external urethral sphincter, composed of striated muscle surrounding the mid-urethra, provides the primary mechanism for voluntary continence and is innervated by the pudendal nerve. The short length of the female urethra contributes to the higher incidence of urinary tract infections in women compared to men, as bacteria have a shorter distance to travel from the external environment to the bladder. Additionally, the proximity of the urethral meatus to the vaginal opening and anus increases exposure to potential pathogens. Blood supply derives from the internal pudendal and vaginal arteries. Sensory innervation is provided by the pudendal nerve. Panel A: Sagittal section of female urethra from bladder neck through pelvic floor to vestibule embedded in anterior vaginal wall. Panel B: External urethral sphincter with striated muscle fibers surrounding mid-urethra with 4 cm length indicated. Panel C: External urethral meatus opening in vestibule anterior to vaginal orifice. Panel D: Perineal view inset showing urethral and vaginal orifices with proximity emphasized for ascending infection risk. --- ## The Male Urethra The male urethra extends approximately 20 centimeters from the internal urethral orifice to the external meatus at the tip of the glans penis, serving as the common pathway for both urine and semen. It comprises three distinct segments with different anatomical characteristics and clinical significance. The prostatic urethra, approximately 3 centimeters long, passes through the prostate gland from the bladder neck to the superior surface of the external sphincter. This is the widest and most distensible segment. Its posterior wall features the urethral crest, a midline ridge that bears the seminal colliculus (verumontanum)—an elevation where the ejaculatory ducts open laterally and the prostatic utricle (a blind Müllerian duct remnant) opens in the midline. The prostatic sinuses, grooves on either side of the crest, receive the openings of the prostatic ducts. The membranous urethra, approximately 1 centimeter long, traverses the urogenital hiatus of the pelvic floor and the perineal membrane, passing through the external urethral sphincter. This is the shortest, narrowest, and least distensible segment, making it the most vulnerable to traumatic injury—particularly in association with pelvic fractures that disrupt the urethra at its fixation to the perineal membrane. The spongy (penile) urethra, approximately 15-16 centimeters long, traverses the corpus spongiosum from the bulb to the external meatus. The bulbourethral gland ducts open into the proximal bulbar segment. The intrabulbar portion is slightly dilated. The penile portion maintains a relatively uniform caliber, while the navicular fossa represents a terminal dilation within the glans. The external meatus, a vertical slit, is the narrowest fixed point of the entire male urethra. Two sphincter mechanisms control the male urethra. The internal urethral sphincter, composed of smooth muscle at the bladder neck and proximal prostatic urethra, is under involuntary sympathetic control and prevents retrograde ejaculation during orgasm. The external urethral sphincter, surrounding the membranous urethra, provides voluntary control of voiding. Panel A: Blue prostatic urethra (3 cm) through prostate with urethral crest, seminal colliculus, and prostatic sinuses labeled. Panel B: Red membranous urethra (1 cm) as "danger zone" surrounded by external sphincter with vulnerability to injury emphasized. Panel C: Yellow spongy urethra (15-16 cm) traversing corpus spongiosum with bulbourethral duct openings, navicular fossa, and external meatus. Panel D: Cross-sections comparing diameters at each level with internal and external sphincters highlighted. --- ## Mechanisms of Urinary Continence Urinary continence depends on multiple anatomical and physiological mechanisms working in concert. During the storage phase, the detrusor muscle remains relaxed under sympathetic influence, allowing the bladder to accommodate increasing volumes of urine at low pressure. The internal sphincter (in males) maintains tonic contraction under sympathetic control, while the external sphincter maintains voluntary tonic contraction via the pudendal nerve. Bladder pressure remains below urethral closure pressure, preventing urine leakage. Additional factors contribute to urethral closure. The urethral mucosa creates a watertight seal through mucosal coaptation—the walls of the collapsed urethra adhere to each other. Urethral smooth muscle maintains resting tone, adding to closure pressure. Pelvic floor support, particularly from the levator ani, maintains the proper position of the bladder neck and urethra. In women, proper transmission of abdominal pressure increases to the proximal urethra (above the external sphincter) helps maintain continence during coughing or straining. During the voiding phase, the detrusor contracts under parasympathetic stimulation while both sphincters relax—the internal sphincter through parasympathetic inhibition and the external sphincter through voluntary relaxation. Coordination of sphincter relaxation with detrusor contraction is essential for efficient voiding; dyssynergia (failure of this coordination) results in obstructed voiding. Panel A: Storage phase showing partially filled bladder with blue relaxed detrusor and contracted internal and external sphincters. Panel B: Pressure gradients indicating urethral pressure exceeding bladder pressure during storage. Panel C: Voiding phase showing red contracting detrusor with relaxed opening sphincters and arrows indicating urine flow. Panel D: Flow chart of neural control pathway from pontine micturition center coordinating autonomic and somatic signals. --- ## The Micturition Reflex The micturition reflex is a coordinated neural process that controls bladder emptying, involving both spinal reflexes and suprapontine modulation. Understanding this reflex explains both normal voiding and the patterns of dysfunction seen with neurological injury. The afferent limb begins with stretch receptors in the bladder wall that sense distension. As the bladder fills, these mechanoreceptors generate signals that travel via the pelvic splanchnic nerves to the sacral spinal cord (S2-S4), where they synapse in the sacral micturition center. Ascending pathways also carry this information to the pontine micturition center in the brainstem and ultimately to the cerebral cortex, where fullness is consciously perceived. The pontine micturition center (Barrington's nucleus) serves as the master coordinator of voiding. When activated, it simultaneously stimulates parasympathetic outflow to contract the detrusor and inhibits both the pudendal nerve (relaxing the external sphincter) and sympathetic outflow (relaxing the internal sphincter). This coordination ensures that sphincter relaxation and detrusor contraction occur together, preventing the inefficient voiding pattern of detrusor-sphincter dyssynergia. Voluntary control is exerted through cerebral cortical influence on the pontine center. Social training teaches humans to inhibit the micturition reflex until an appropriate time and place for voiding. The cortex can either facilitate or inhibit voiding, allowing us to delay urination despite a full bladder or to initiate voiding before the bladder is completely full. Panel A: Bladder with stretch receptors and green afferent pathways traveling via pelvic splanchnic nerves to sacral cord (S2-S4). Panel B: Ascending connections to pontine micturition center as coordination hub receiving cerebral cortex input. Panel C: Efferent pathways showing red parasympathetic to detrusor, blue sympathetic to internal sphincter, and yellow pudendal to external sphincter. Panel D: Coordinated voiding patterns with sidebar showing spinal cord lesion effects above versus below pontine center. --- ## Clinical Correlations ### Urinary Tract Infections Urinary tract infections occur more commonly in women due to the shorter urethra and proximity of the urethral meatus to vaginal and rectal bacteria. Cystitis (bladder infection) presents with dysuria, urinary frequency, urgency, and sometimes suprapubic pain. Ascending infection may lead to pyelonephritis if bacteria reach the kidneys via the ureters. ### Urinary Incontinence Urinary incontinence takes several forms based on the underlying mechanism. Stress incontinence results from weakness of the pelvic floor and urethral support, causing urine leakage during activities that increase intra-abdominal pressure such as coughing, sneezing, or exercise; it is most common in women after childbirth. Urge incontinence (overactive bladder) results from detrusor overactivity, causing sudden, uncontrollable urges to void. Overflow incontinence occurs when the bladder becomes overdistended (from outlet obstruction or detrusor weakness), leading to continuous dribbling of urine. Neurogenic incontinence results from disruption of the neural pathways controlling the bladder. ### Benign Prostatic Hyperplasia Benign prostatic hyperplasia (BPH) develops in the transition zone of the prostate surrounding the proximal urethra, causing progressive compression of the prostatic urethra. Symptoms include hesitancy, weak stream, frequency, nocturia, and incomplete emptying. If severe, BPH may cause acute urinary retention requiring catheterization. ### Urethral Stricture Urethral stricture is narrowing of the urethra due to scarring, most commonly affecting the bulbar urethra following trauma (such as straddle injury) or the meatus from repeated instrumentation. Strictures cause obstructive voiding symptoms and increase the risk of urinary tract infection. ### Neurogenic Bladder Neurogenic bladder results from disruption of neural control at various levels. Upper motor neuron lesions (above the sacral cord) produce a spastic bladder with small capacity and reflex, involuntary emptying (spinal reflex voiding). Lower motor neuron lesions (affecting the sacral cord or peripheral nerves) produce a flaccid, acontractile bladder with large capacity and overflow incontinence due to inability to generate effective detrusor contractions. Panel A: Stress incontinence showing weakened pelvic floor with downward bladder neck displacement during coughing and arrow indicating urine loss. Panel B: BPH with enlarged prostate compressing prostatic urethra and cystoscopic view inset. Panel C: Urethral stricture in bulbar urethra showing narrowed lumen location. Panel D: Neurogenic bladder contrasting small spastic thick-walled upper motor neuron versus large atonic thin-walled lower motor neuron patterns. --- ## Summary The urinary bladder is a distensible muscular reservoir with a smooth trigone at its base defined by the ureteric and urethral orifices. The detrusor muscle comprises the bladder wall and contracts for voiding under parasympathetic (S2-S4) control, while sympathetic innervation (T11-L2) promotes storage. The female urethra is short (approximately 4 cm) and embedded in the anterior vaginal wall, contributing to higher infection rates. The male urethra has three parts: prostatic (widest, most distensible), membranous (narrowest, most vulnerable to injury), and spongy (longest). The internal urethral sphincter (smooth muscle, involuntary) prevents retrograde ejaculation in males, while the external urethral sphincter (striated muscle, voluntary via pudendal nerve) provides conscious continence control in both sexes. The micturition reflex is coordinated by the pontine micturition center with cortical override for social appropriateness. The bladder receives blood from superior and inferior vesical arteries and drains via the vesical venous plexus to the internal iliac veins, with potential metastatic routes to the spine via Batson's plexus. --- ## Key Terms **Trigone**: The smooth triangular area on the internal bladder base between the two ureteric orifices and the internal urethral orifice, where the mucosa is firmly adherent and does not form rugae. **Detrusor muscle**: The three-layered smooth muscle of the bladder wall that contracts during voiding under parasympathetic control. **External urethral sphincter**: The voluntary striated muscle sphincter surrounding the membranous urethra (males) or mid-urethra (females), innervated by the pudendal nerve, providing conscious control of voiding. **Prostatic urethra**: The proximal segment of the male urethra (approximately 3 cm) passing through the prostate, featuring the urethral crest and seminal colliculus; the widest and most distensible portion. **Membranous urethra**: The short segment (approximately 1 cm) of the male urethra traversing the external sphincter and perineal membrane; the narrowest, least distensible, and most vulnerable segment to traumatic injury. **Micturition reflex**: The coordinated neural process controlling bladder emptying, integrating sacral spinal reflexes with pontine coordination and cortical modulation. --- *This content is subject to the [MIT License](https://opensource.org/licenses/MIT). © 2024–2026 Hibbert School of Medicine.*