# Urinary Bladder and Trigone > Urinary bladder anatomy — capacity, trigone, detrusor, urachus, nerve supply and rupture patterns for MBBS Anatomy exams. - Canonical URL: https://prepelephant.com/topics/mbbs/anatomy/urinary-bladder - Exam / course: MBBS · Subject: Anatomy - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Urinary Bladder and Trigone", PrepElephant, https://prepelephant.com/topics/mbbs/anatomy/urinary-bladder ## Direct answer A hollow reservoir of interlacing smooth muscle holding roughly 400 to 600 mL when full, the urinary bladder sits behind the pubis when empty and rises into the abdomen when distending, stripping the peritoneum off the anterior abdominal wall as it does. Its internal landmark is the trigone — the smooth-walled triangle between the two ureteric orifices and the internal urethral orifice — which is embryologically mesodermal, unlike the endodermal rest of the bladder, and which resists distension so the ureteric orifices keep their oblique, reflux-preventing entry. Whether a ruptured bladder leaks into the peritoneal cavity or the extraperitoneal tissues depends purely on whether it was full and struck above or crushed with the pelvis. ## What you must remember - **Parts:** apex with the urachus (median umbilical ligament), body, fundus or base (posteroinferior), and neck, the lowest and most fixed point, resting on the prostate in the male. - **Trigone:** limited by the two ureteric orifices and the internal urethral orifice, raised between the ureters as the interureteric crest; derived from the mesonephric ducts, hence mesodermal. - **Peritoneum:** covers only the superior surface and the upper posterior wall, so most of the distending bladder is extraperitoneal — the basis of safe suprapubic puncture. - **Male posterior relations:** seminal vesicles and ampullae of the vasa deferentia, then the rectum, separated by the fascia of Denonvilliers; the vasa cross above the ureters at the superolateral angles. - **Female relations:** the uterus and upper vagina behind, with the vesicouterine pouch between; the cervix and anterior vaginal wall related to the base. - **Nerve supply:** parasympathetic pelvic splanchnic nerves S2 to S4 contract the detrusor; sympathetic fibres from L1 and L2 close the bladder neck; the pudendal nerve supplies the voluntary external sphincter. - **Blood supply:** superior and inferior vesical arteries from the internal iliac; the superior vesical artery is the patent proximal part of the fetal umbilical artery, whose distal obliteration becomes the medial umbilical ligament. ## Two ruptures, two operations A drunk man strikes his lower abdomen in a road accident with a full bladder. The distended dome, the weakest and most peritonealised part, bursts intraperitoneally, and urine lavages the abdomen — uraemia with a rising creatinine and a sugar-positive ascitic tap, repaired with a two-layer closure and drainage. His sober counterpart in the next bed was crushed pelvis-first: the shearing pelvis or a bone fragment tears the bladder neck or anterolateral wall extraperitoneally, presenting with blood at the meatus, a palpable high prostate, and extravasation into the perivesical space of Retzius on imaging, managed by catheter drainage or repair. The same anatomy makes the elective procedures safe. A distended bladder rises above the pubis, carrying the peritoneal reflection with it, so a needle or trocar entering just above the symphysis traverses skin, fascia and bladder wall only — suprapubic cystostomy never enters the peritoneal cavity when the bladder is properly full. Inside, the cystoscopist reads the trigone first: the interureteric crest is the landmark between which the ureteric jets must be seen, and trigonal anatomy is what reimplantation surgery rebuilds when reflux defeats the oblique tunnel. ## Where students slip The urachus and the umbilical artery remnants are muddled: the median umbilical ligament is the urachus from the bladder apex, the medial umbilical ligaments are the obliterated umbilical arteries, and a patent urachus presents as urine dribbling from the umbilicus in a newborn boy. The trigone's smoothness is decorative in most answers; its function is mechanical — it does not distend with filling, keeping the ureteric orifices slit-like and closed. And the first desire to void arises at roughly 200 to 300 mL with urgency near full capacity — pain, however, is referred suprapubically and to the perineum through S2 to S4. ## Frequently asked questions ### What is the capacity of the bladder and when is the first urge felt? About 400 to 600 mL capacity, with the first desire to void around 200 to 300 mL and urgency as filling approaches capacity. ### What are the boundaries of the trigone and why is it special? The two ureteric orifices and the internal urethral orifice; it is mesodermal, derived from the mesonephric ducts, and remains smooth and undistended during filling. ### Why is suprapubic catheterisation safe in a full bladder? A distended bladder rises above the symphysis pubis and peels the peritoneal reflection upwards, so a needle just above the pubis enters the bladder without crossing the peritoneal cavity. ### What is the fascia of Denonvilliers? The fascial plane between the prostate and base of the bladder anteriorly and the rectum behind, the plane developed in prostatectomy and rectal excision. ### Which nerves empty the bladder? Parasympathetic pelvic splanchnic nerves from S2 to S4 contract the detrusor and relax the neck; sympathetic L1 to L2 fibres do the opposite; the pudendal nerve controls the voluntary external sphincter. ### How do intraperitoneal and extraperitoneal bladder ruptures differ? Intraperitoneal rupture tears the dome of a full bladder, leaking urine into the peritoneal cavity; extraperitoneal rupture accompanies pelvic fractures and dissects urine into the space of Retzius.