29.21 : Urinary Bladder

The urinary bladder is a hollow, muscular sac that temporarily stores urine before it is expelled from the body. It can hold approximately 600 mL of urine prior to micturition. The bladder is retroperitoneal and located behind the pubic symphysis in the pelvic floor.

In males, the bladder is situated in front of the rectum, while in females, it is positioned anterior to the vagina and uterus. The bladder floor contains an inverted triangular area called the trigone, defined by the two ureteric orifices and the internal urethral orifice, which leads to the urethra.

The bladder wall consists of three histological layers: the mucosa, muscularis, and adventitia.

1. The innermost mucosa features folds called rugae and is composed of transitional epithelium or urothelium. This specialized tissue allows the bladder to stretch as it fills with urine while maintaining a watertight barrier.
2. The muscularis layer is made up of a circular layer of smooth muscle fibers sandwiched between inner and outer longitudinal layers, collectively forming the detrusor muscle. This muscle plays a crucial role in regulating bladder volume and enabling the expulsion of urine.
3. The outermost adventitia is continuous with the ureters and covers the posterior and inferior surfaces of the bladder. On the superior surface, the bladder is covered by the peritoneum.